KoritheMan's WunderBlog

Tropical weather analysis - August 30, 2012

By: KoritheMan, 12:57 AM GMT on August 31, 2012

Kirk

I've been neglecting to mention it because of personal impacts from Isaac to my area, but former 97L became Kirk a couple days ago. Since that time, Kirk has quickly become a hurricane. As of the latest NHC advisory, the following has been posted on this storm:

Wind: 90 mph, with higher gusts
Location: 28.2°N 50.3°W
Movement: NNW at 13 mph
Pressure: 982 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Kirk is a small but well-organized hurricane, with a small eye embedded in the central dense overcast. There is some banding to the north, and the outflow pattern is well-established in all but the southern quadrant.



Figure 1. Latest infrared satellite image of Hurricane Kirk. Image credit: NOAA

There is some dry air in the near-storm environment. Since Kirk is a very small hurricane, it will be more prone than usual to subtle changes in the atmosphere. However, Kirk seems to have done fairly well for itself in terms of warding off any dry air. Given current trends, Kirk may be rapidly intensifying, and the cyclone is expected to become a major hurricane in about 24 hours, but it could occur sooner. Barring any dry air entrainment, which should be largely mitigated by the hurricane's acceleration, conditions appear favorable for continued strengthening until around 48 hours, at which point Kirk is forecast to lose its anticyclone, begin moving over cooler waters and into an environment of very strong southwesterly shear. The conglomeration of these factors should bring about weakening at those times. The global models suggest that Kirk will become extratropical in about four days, and become absorbed by a frontal zone thereafter.

Recent satellite fixes suggest that Kirk has turned to the north around the western periphery of the subtropical ridge over the central Atlantic as a deep-layer trough approaches the hurricane from the west. A gradual bend to the northeast with some acceleration is forecast as the small hurricane becomes fully embedded in the trough. The model consensus is in excellent agreement on this scenario, and the synoptic pattern over this part of the Atlantic is rather straightforward.

5-day intensity forecast

INITIAL 08/31 0000Z 80 KT 90 MPH
12 hour 08/31 1200Z 90 KT 105 MPH
24 hour 09/01 0000Z 100 KT 115 MPH
36 hour 09/01 1200Z 105 KT 120 MPH
48 hour 09/02 0000Z 100 KT 115 MPH
72 hour 09/03 0000Z 75 KT 85 MPH
96 hour 09/04 0000Z 50 KT 60 MPH...EXTRATROPICAL
120 hour 09/05 0000Z...ABSORBED BY FRONTAL ZONE



Leslie

Tropical Storm Leslie formed earlier today from a strong tropical wave. As of the latest NHC advisory, the following was posted on the tropical storm:

Wind: 45 mph, with higher gusts
Location: 14.4°N 45.3°W
Movement: W at 21 mph
Pressure: 1004 mb

Satellite images show that Leslie is a well-organized tropical storm, with a ball of very cold convection near the low-level center. In addition, a well-defined curved band is seen emanating to the west of the center.



Figure 2. Latest infrared satellite image of Tropical Storm Leslie. Image credit: NOAA

Earlier microwave data suggested that the low-level center was along the northeast edge of the convection. Since that time, Leslie's satellite signature has improved, but the deepest convection still appears to be just south of the low-level center. This is probably due to a little bit of easterly shear associated with the storm relative anticyclone, which appears to be slightly dislodged from Leslie. However, the GFS forecasts a very favorable upper-level environment ahead of the storm, and the anticyclone in the 200 mb upper wind forecast from that model look almost as large as the one that eventually manifested over Hurricane Isaac. In addition, Leslie is large enough that it should be better shielded against the dry air over the central Atlantic than Kirk is. In fact, the SHIPS and HWRF actually forecast Leslie to become a major hurricane by the end of the forecast period. Lacking any obvious reason to suggest otherwise, I will indicate this as well.

Leslie is south of the subtropical ridge, which appears to be rebuilding in the wake of accelerating Hurricane Kirk. However, Leslie is moving quick enough that it should feel the residual weakness left behind by Kirk and a trough amplifying over the central Atlantic. The cyclone should respond by turning west-northwestward over the next 12-24 hours, followed by slowly by recurvature into the mid-latitude westerlies. The model consensus is in good agreement on this, although the ECMWF suggests that Leslie could stall near the end of the forecast period as a shortwave trough bypasses it and a ridge begins to rebuild. However, that model shows another shortwave moving into the western Atlantic almost immediately subsequent to the stall, which would favor recurvature like the remainder of the models are suggesting. In addition, an upper low is forecast to develop near the Bahamas soon from the trough that is recurving Kirk. This low is forecast to be strong from 500-300 mb, which should help to keep a continued poleward component on Leslie. However, if the storm does not move too far to the north over the next couple of days, it may pose a long-range threat to the northern Leeward Islands. I consider this to be a remote possibility at the moment, and the odds of it happening are only about 5%. Due to the uncertainty in long-range forecast tracks, interests in Bermuda should monitor the progress of Leslie.

5-day intensity forecast

INITIAL 08/31 0000Z 40 KT 45 MPH
12 hour 08/31 1200Z 45 KT 50 MPH
24 hour 09/01 0000Z 55 KT 65 MPH
36 hour 09/01 1200Z 60 KT 70 MPH
48 hour 09/02 0000Z 70 KT 80 MPH
72 hour 09/03 0000Z 80 KT 90 MPH
96 hour 09/04 0000Z 90 KT 105 MPH
120 hour 09/05 0000Z 100 KT 115 MPH



Ileana

Hurricane Ileana is moving across the open waters of the Eastern Pacific. As of the latest NHC advisory, the following was posted on the hurricane:

Wind: 85 mph, with higher gusts
Location: 20.5°N 114.1°W
Movement: NW at 6 mph
Pressure: 979 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Earlier microwave data revealed that the hurricane maintains an impressive inner core, with a well-defined eye. An eye is not evident on conventional imagery, and the hurricane is slowly losing organization.



Figure 3. Latest infrared satellite image of Hurricane Ileana. Image credit: NOAA

While upper-level winds are forecast to remain favorable, AOML SST analyses show that the hurricane is approaching the 26C isotherm, which in coordination with a very stable airmass should result in weakening, which appears to already be underway. Ileana is forecast to become a remnant low in about three days.

Ileana remains south of a weak mid-level ridge. Consequently, the hurricane is moving northwest. While the track is fairly straightforward, there is some disagreement between the GFS and ECMWF after 48 hours. The former suggests that Ileana will recurve ahead of the trough, while the latter indicates that the system will move westward in the low-level flow. Given the unfavorable environment, which tends to quickly weaken tropical cyclones in this part of the Pacific, I will side with the ECMWF. In any case, it seems to be of little consequence, as the hurricane should be a remnant low by that time. I am in agreement with the National Hurricane Center forecast track.

5-day intensity forecast

INITIAL 08/31 0000Z 75 KT 85 MPH
12 hour 08/31 1200Z 65 KT 75 MPH
24 hour 09/01 0000Z 55 KT 65 MPH
36 hour 09/01 1200Z 45 KT 50 MPH
48 hour 09/02 0000Z 35 KT 40 MPH
72 hour 09/03 0000Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/04 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
120 hour 09/05 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW



Eastern Pacific disturbance

An area of disturbed weather located several hundred miles south of the Mexican coastline is currently disorganized. However, environmental conditions appear favorable for gradual development as it moves west-northwest at about 15 mph.

Probability of development in 48 hours: 30%

I will not be drawing track forecasts this evening, as I am staying over my dad's until the power comes back on at my residence. The whole family is here, which makes concentrating fully rather difficult. I will return to doing that when the power resumes where I live.

2012 Atlantic hurricane season 2012 East Pacific hurricane season Hurricane Kirk Tropical Storm Leslie Hurricane Ileana

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Tropical weather analysis - August 28, 2012

By: KoritheMan, 9:57 AM GMT on August 28, 2012

Isaac

Isaac finally intensified a little today, and appears poised to become a hurricane very soon. The following was posted on the storm on the 5 AM NHC advisory:

Wind: 70 mph, with higher gusts
Location: 27.5°N 88.1°W
Movement: NW at 12 mph
Pressure: 978 mb

Isaac has a rather low central pressure for a tropical storm, but this is not unusual for large storms, as the large outer core causes the pressure gradient to be spread out over a wide area. Hurricane Ike in 2008 experienced a similar evolution.

Satellite images show that Isaac is gradually becoming better organized. Although a warm spot is not yet evident using conventional imagery, convection appears to be wrapping a little better on the north side. Earlier microwave data indicated the presence of a large eye that was open to the north. Since that time, convection has improved, and I believe that Isaac is trying to build a more complete eyewall.



Figure 1. Latest infrared satellite image of Tropical Storm Isaac. Image credit: NOAA

The center is currently out of range of the New Orleans radar, but this will change over the next few hours as the cyclone continues toward the northern Gulf Coast.

Analysis from the University of Wisconsin CIMSS suggests that the northeasterly shear that has been afflicting the system is relaxing, as the much-advertised anticyclone finally becomes established over the low-level center. Upper-level outflow remains restricted in the northern quadrant, where Isaac has been having the most troubles, indicating that the anticyclone has not completely become established yet. However, it appears to be becoming better defined in that quadrant as well. All these signs point to a system that is slowly intensifying. Given the enormous circulation associated with the storm, it no longer appears likely that Isaac will become a major hurricane. A Category 2 still cannot be completely ruled out, but it would be something of a surprise at this point. A good analog for this storm is a careful blending of Hurricane Isidore in 2002 and Hurricane Ike in 2008. The former was also an exceptionally large system that never fully recovered from passage overland. Isidore made landfall with 65 mph winds, while Ike, a similar storm in terms of size, made landfall with winds of 110 mph. An average of those speeds would result in a mid to high-end Category 1 storm, which is what the National Hurricane Center is currently predicting. We are fortunate that Isaac was so large as to never be able to truly consolidate. It could have easily been a Category 3 or 4. However, the storm surge will be characteristic of a Category 2 or 3 hurricane because of the large wind field, which has been impacting the northern Gulf waters for the last several days. As Ike adequately demonstrated, large storms can generate storm surge directly contradictory to their strength. Category 3 surge is more likely if Isaac intensifies as forecast. Either way, I would be preparing for surge characteristic of a Category 2 along the southeast Louisiana and southern Mississippi coasts. Depending on high tide, surge values could easily plateau at 10 to 12 feet across coastal sections of southeast Louisiana and southern Mississippi. Areas west of Morgan City should receive mostly offshore flow, which will considerably lessen the surge threat in those areas. The National Hurricane Center has an excellent surge probability map, located here. However, storm surge varies greatly over short distances, so I would advise consulting your local National Weather Service for products and packages delineating specific threats to your area.

Tornadoes are also an issue, especially in the right-front quadrant, where most of the rotating bands typically occur. Assuming it follows my current track, this would put the New Orleans area at the greatest risk for tornadoes. It should be noted that tornadoes associated with tropical systems generally aren't as strong as those produced by mid-latitude weather systems, and are usually rather brief in duration, but the threats should still be taken very seriously. I expect tornado watches to go up later this morning as Isaac gets closer to the coast. These watches will likely stretch from Baton Rouge to Apalachicola.

Concerning rainfall potential: Isaac is forecast to slow down considerably after landfall, which would enhance the rainfall threat, especially to the east of the center. In fact, as much as 20 inches could fall in that particular quadrant over the next few days, with locally higher amounts possible. Areas farther west will see less but still significant amounts. As of now, here is the current 5-day rainfall prediction by the Hydrometeorological Prediction Center:



Figure 2. 5-day QPF (Quantitative Precipitation Forecast) by the Hydrometeorological Prediction Center valid as of 8:00 PM Monday (0z Tuesday August 28).

Water vapor imagery and 0z upper air data indicates that the shortwave trough over the southeast United States is lifting out, and is slowly being replaced by a ridge. In addition, UW-CIMSS shows that the weakness to the north of the storm over the Tennessee Valley is slowly weakening. Concurrently, in advance of another shortwave trough over the western portion of the country, a rather large ridge is building over the Rockies. The general result should be for Isaac to continue moving generally northwest. It's important to note that both the GFS and ECMWF have come into better agreement on Isaac's track, with both taking him near the mouth of the Mississippi River, followed by a slow turn to the west-northwest. However, I will not yet show this, and instead forecast a steady northwest motion after the storm moves inland, especially given the time of year, where the cold fronts start becoming more amplified.

5-day intensity forecast

INITIAL 08/28 0900Z 60 KT 70 MPH
12 hour 08/28 1800Z 70 KT 80 MPH
24 hour 08/29 0600Z 80 KT 90 MPH...ON THE LOUISIANA COAST
36 hour 08/29 1800Z 70 KT 80 MPH...INLAND
48 hour 08/30 0600Z 50 KT 60 MPH...INLAND
72 hour 08/31 0600Z 30 KT 35 MPH...INLAND
96 hour 09/01 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
120 hour 09/02 0600Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW

5-day track forecast



Figure 3. My 5-day forecast track for Isaac.

Watches and warnings


A HURRICANE WARNING IS IN EFFECT FOR...
* EAST OF MORGAN CITY LOUISIANA TO THE ALABAMA-FLORIDA BORDER...
INCLUDING METROPOLITAN NEW ORLEANS...LAKE PONTCHARTRAIN...AND LAKE
MAUREPAS

A HURRICANE WATCH IS IN EFFECT FOR...
* INTRACOASTAL CITY TO MORGAN CITY LOUISIANA

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE ALABAMA-FLORIDA BORDER TO THE AUCILLA RIVER
* MORGAN CITY TO CAMERON LOUISIANA

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* EAST OF HIGH ISLAND TEXAS TO JUST WEST OF CAMERON LOUISIANA

A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED
SOMEWHERE WITHIN THE WARNING AREA. PREPARATIONS TO PROTECT LIFE AND
PROPERTY SHOULD BE RUSHED TO COMPLETION.

A HURRICANE WATCH MEANS THAT HURRICANE CONDITIONS ARE POSSIBLE
WITHIN THE WATCH AREA.

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE FORECAST
OFFICE.

2012 Atlantic hurricane season Tropical Storm Isaac

Updated: 10:01 AM GMT on August 28, 2012

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Tropical weather analysis - August 27, 2012

By: KoritheMan, 5:26 AM GMT on August 27, 2012

Tropical Storm Isaac has entered the southeastern Gulf of Mexico. As of the latest NHC advisory, the following was posted on the storm:

Wind: 65 mph, with higher gusts
Location: 24.2°N 82.9°W
Movement: WNW at 14 mph
Pressure: 993 mb

Isaac is presenting mixed signals this evening. The cyclone looks well-organized on satellite images with banding features to the north, as well as strong central convection over what appears to be the low-level center. However, doppler radar data from Key West suggests that the cyclone still lacks an inner core, as the central feature is open to the south. In addition, earlier reconnaissance data fixed a center that was well-removed from the convection.



Figure 1. Latest infrared satellite image of Tropical Storm Isaac. Image credit: NOAA

This is probably attributable to some dry air and moderate southerly to southwesterly shear associated with an elongated upper trough/upper low feature extending from Vermilion Bay to the northern Yucatan Peninsula. This low appears to be slowly moving toward the west. I would like to emphasize the word "slowly" in that last sentence, by the way. Assuming this feature, which remains quite vigorous, ever moves out of the way, the GFS forecasts the upper-level environment to become much more favorable, with a large anticyclone centered atop the low-level center. Since current imagery suggests that outflow is well-established in all but the southwest quadrant, this forecast has some viability, but again, it will be contingent upon how quickly the aforementioned upper low moves away. Assuming it does not, or does so more slowly, Isaac would likely continue to battle the current hostilities, and in that scenario, would probably be nothing more than a Category 1 hurricane. Another possibility based on evening radar fixes and reconnaissance data is that a center reformation could occur in the deep convection to the north, which would put Isaac in a more favorable environment for strengthening, as the distance between the upper low and the tropical storm would be greater. The upper low notwithstanding, Isaac is a considerably large storm. Hurricane Ike in 2008 was a hurricane of comparable size, and was never quite able to get it together. Combine that with the dry air, shear, and earlier land interaction, and you get a storm that will likely not rapidly intensify.

However, I have seen enough tropical cyclones in the past intensify just before landfall, so one must always be conscious of that possibility with Isaac. There is still a chance that Isaac can become a major hurricane, but the odds of this appear to be no greater than 35% at this time. In light of current trends, the course of least regret is to forecast slow intensification for the first 12-24 hours, then present a steadier strengthening rate during the final day or two before landfall as the upper low elongates and moves westward. Because of the inherent uncertainty in intensity prediction schemes, as well as the enormity of the circulation envelope, this is a low confidence forecast. Residents living along the northern Gulf Coast should anticipate the possibility of anything from a strong tropical storm to a Category 3 hurricane. The margin of error for storms like Isaac is quite large.

Now comes the fun part. Isaac has defied us. Again. The center appears to have jumped northward this evening, but a slew of satellite, radar, and microwave fixes suggest that the overall trend remains between WNW/NW, with a little more weight given to the latter as of now. This could be related to the poor inner core structure, as the convection remains displaced to the north of the advisory center. Such convective patterns can often muddle the situation, making an accurate center fix difficult, and subject to speculation. It could also be attributable to a weakness over the western Atlantic between the the high over the Rockies and the Bermuda-Azores ridge. While this is supported pretty well by water vapor imagery, 0z upper air data over Florida and the northern Gulf Coast suggests that this trough is not particularly strong. In fact, I see very little evidence of one using that data. Arguing against data collected from a weather balloon is pretty difficult. Nevertheless, Isaac is quite clearly north of the most recent NHC forecast point. Based on the ridge building over the southeast behind said trough, I expect Isaac to bend back toward the west-northwest overnight, with a gradual bend to the northwest into a weakness over the central plains, which appears to come from a trough amplifying over the western United States.

The model consensus has continued to shift westward toward Louisiana. In fact, some of the GFS ensembles, as well as the actual dynamical models, are now forecasting a strike on the Texas/Louisiana border. Curiously, the Euro continues to be the eastern outlier, and is about 300 miles east of the model consensus. While it is hard to ignore this model, it is equally hard to ignore the consensus, especially backed by the state of the art GFS, which has clearly exceeded the Euro this year. Based on the consensus, my new forecast track is shifted significantly westward, and now shows a landfall along the coast of southeast Louisiana early Wednesday. However, given the possible center reformation, which could have some slight (but not enough to alter the overall consensus) implications on the landfall point, and the fact that the Euro refuses to join the fray, makes this forecast less certain than usual. Interests from Vermilion Bay to Pascagoula should closely monitor the progress of Isaac.

Hurricane warnings are in effect for a section of the northern Gulf Coast, and residents in the warned area should be taking the necessary precautions. The onset of tropical storm force winds should begin across the northern Gulf Coast tomorrow night or early Tuesday morning, with hurricane conditions beginning late Tuesday or early Wednesday. Since Isaac has a large wind field, these winds will spread quite far inland, even if the storm does not significantly intensify.

5-day intensity forecast

INITIAL 08/27 0600Z 55 KT 65 MPH
12 hour 08/27 1800Z 60 KT 70 MPH
24 hour 08/28 0600Z 65 KT 75 MPH
36 hour 08/28 1800Z 75 KT 85 MPH
48 hour 08/29 0600Z 85 KT 100 MPH...NEAR THE LOUISIANA COAST
72 hour 08/30 0600Z 70 KT 80 MPH...INLAND
96 hour 08/31 0600Z 35 KT 40 MPH...INLAND
120 hour 09/01 0600Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW

5-day track forecast



Figure 2. My 5-day forecast track for Isaac.

Watches and warnings

A HURRICANE WARNING IS IN EFFECT FOR...
* EAST OF MORGAN CITY LOUISIANA TO DESTIN FLORIDA...INCLUDING
METROPOLITAN NEW ORLEANS...LAKE PONTCHARTRAIN...AND LAKE MAUREPAS

A HURRICANE WATCH IS IN EFFECT FOR...
* EAST OF DESTIN TO INDIAN PASS FLORIDA

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE FLORIDA PENINSULA FROM JUPITER INLET SOUTHWARD ON THE
EAST COAST AND FROM TARPON SPRINGS SOUTHWARD ON THE WEST COAST.
* FLORIDA KEYS...INCLUDING THE DRY TORTUGAS AND FLORIDA BAY
* LAKE OKEECHOBEE
* EAST OF DESTIN FLORIDA TO THE SUWANNEE RIVER

A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED
SOMEWHERE WITHIN THE WARNING AREA. A WARNING IS TYPICALLY ISSUED
36 HOURS BEFORE THE ANTICIPATED FIRST OCCURRENCE OF
TROPICAL-STORM-FORCE WINDS...CONDITIONS THAT MAKE OUTSIDE
PREPARATIONS DIFFICULT OR DANGEROUS. PREPARATIONS TO PROTECT LIFE
AND PROPERTY SHOULD BE RUSHED TO COMPLETION.

A HURRICANE WATCH MEANS THAT HURRICANE CONDITIONS ARE POSSIBLE
WITHIN THE WATCH AREA. A WATCH IS TYPICALLY ISSUED 48 HOURS
BEFORE THE ANTICIPATED FIRST OCCURRENCE OF TROPICAL-STORM-FORCE
WINDS...CONDITIONS THAT MAKE OUTSIDE PREPARATIONS DIFFICULT OR
DANGEROUS.

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

WATCHES OR WARNINGS MAY BE EXTENDED WESTWARD ALONG THE COAST OF
LOUISIANA EARLY MONDAY.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...INCLUDING POSSIBLE
INLAND WATCHES AND WARNINGS...PLEASE MONITOR PRODUCTS ISSUED BY
YOUR LOCAL NATIONAL WEATHER SERVICE FORECAST OFFICE.



Invest 97L

A tropical wave in the eastern Atlantic between the Cape Verde Islands and the Lesser Antilles appears to have become much better organized today. The low-level center appears to be under the southern edge of a ball of very deep convection with small banding features to the north.



Figure 3. Latest infrared satellite image of Invest 97L. Image credit: NOAA

Water vapor imagery suggests some weak southerly shear, which would mesh well with the lack of outflow in that direction. The GFS suggests an upper trough over the central Atlantic could produce southwesterly shear over it. However, this trough appears fairly weak, and I am not fully of the expectation that it will do much to retard development potential. Conditions appear conducive for a tropical depression to form over the next day or two as the system moves west-northwest to northwest.

As of now, it seems quite likely that 97L recurves, possibly even east of Bermuda.

Probability of development in 48 hours: 60%



Invest 97E

A low pressure area centered about 300 miles southwest of Acapulco is showing signs of organization. While convection is limited to a small patch west of the center, the circulation is slowly becoming better defined. However, water vapor imagery shows strong southwesterly shear approaching the system from a larger disturbance to the west.



Figure 4. Latest infrared satellite image of Invest 97E. Image credit: NOAA

While this disturbance is not expected to become a tropical cyclone, it could still impart some shear to 97E in the near-term. The GFS failed to pick up on this shear, so I cannot rely on its short-term shear forecasts. I imagine it will decrease in about 12-24 hours. Conditions appear favorable for a tropical depression to form over the next couple of days as the low moves west to west-northwest at about 15 mph.

Probability of development in 48 hours: 60%

2012 Atlantic hurricane season Tropical Storm Isaac Invest 97L Invest 97E

Updated: 5:33 AM GMT on August 27, 2012

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Tropical weather analysis - August 26, 2012

By: KoritheMan, 6:34 AM GMT on August 26, 2012

Isaac

Tropical Storm Isaac continues to hug the north coast of Cuba, and the storm is moving roughly parallel to the country. As of the latest NHC advisory, the following was posted on the storm:

Wind: 60 mph, with higher gusts
Location: 22.9°N 78.0°W
Movement: NW at 17 mph
Pressure: 997 mb

Isaac is becoming better organized. Satellite imagery indicates a ball of deep convection has developed over the low-level center. In addition, a loose convective band lingers along the eastern semicircle, which could attempt to wrap cyclonically into the center overnight. This increase in organization is evident in the Camaguey radar as well, which shows increased banding, particularly west of the center. A 0200 UTC AMSU microwave pass also shows the improving inner core structure.



Figure 1. Latest infrared satellite image of Tropical Storm Isaac. Image credit: NOAA

Except for proximity to the Cuban coast, which doesn't appear to be affecting Isaac all that much, all parameters appear favorable for intensification. The GFS forecasts the upper-level wind pattern to improve, especially after 24 hours when a large anticyclone is forecast to become collocated with the tropical cyclone over the Gulf of Mexico. Concurrent with the likelihood of a mature inner core by that time, there seems to be little apparent reason why Isaac will not become a strong hurricane as it makes landfall along the northern Gulf Coast. Interests there should be prepared for a major hurricane, although it is difficult to pinpoint exactly how strong Isaac will be at US landfall, and the general rule of thumb is to anticipate anything two categories lower or higher. Looking at water vapor imagery over the Gulf, there is a little bit of dry air near the Yucatan Peninsula that is being generated by an upper low. While some of this dry air may occasionally intrude into the circulation over the next 24 hours, it should ultimately not have much of an affect on Isaac, especially with no signs of vertical shear to assist in ejecting that sort of airmass into the inner core. Although not explicitly shown at this time, rapid intensification is a distinct possibility over the Gulf given the high oceanic heat content and highly anticyclonic upper environment shown by the GFS. In summary, Isaac is expected to be a large and dangerous hurricane, probably the size of Ike, when it makes landfall along the northern Gulf Coast. I would not be surprised to see tropical storm force winds extend out to 300 miles from the center as the storm matures. This has the potential to be a very serious hurricane, and everyone along the Gulf Coast from New Orleans to Apalachicola needs to take it extremely seriously. Hurricane watches will likely be required for a portion of the northern Gulf Coast today, as the average lead time for a hurricane watch in this basin is 48 hours. I would wait until you are definitively under a hurricane watch before you begin making any sort of preparations, but it never hurts to start now, especially with the demand for non-perishable goods throughout grocery stores along the northern Gulf Coast. Know where your evacuate routes are. If your local emergency management officials tell you to evacuate, do so. There is always an underlying reason behind evacuation orders. Do not attempt to remain in a low-lying area. Relocate with relatives who live farther inland or in areas less prone to flooding, or use local emergency shelters. The vast majority of deaths in a tropical cyclone occur due to storm surge, not wind. As the saying goes "Run from the water, hide from the wind." I do not want to hear how a family died because they were too conceited to evacuate. I realize some people do not have the capacity to prepare or evacuate, but if you do, then please do. I can't stress this enough. Property can be replaced, lives cannot.

I do not mean to sound so dire, but Isaac truly does have the potential to be a significant hurricane.

It should be noted that based on NOAA's AOML division Tropical Cyclone Heat Potential (TCHP) map, the northern Gulf Coast has much higher heat content than the central Gulf does. So if Isaac tracks closer to Alabama/Mississippi/Louisiana, more significant intensification could result, especially since it will have more time over water.



Figure 2. Gulf of Mexico Tropical Cyclone Heat Potential (TCHP) as of August 23, 2012.

A couple of warm eddies are noted off the coast of Louisiana, but on the current forecast track (and even if the track changes and the models converge into agreement on a Louisiana/Mississippi landfall), Isaac should miss these eddies by a fairly wide margin. Still, total oceanic heat content is clearly higher in the northern Gulf than points farther east, providing more fuel for what is likely to be a large and intense hurricane. Due to the orientation of the loop eddies relative to Isaac's track, as well as comparison with previous years where we had a much stronger and more centralized eddy, Isaac is unlikely to attain Category 5 status at any point over its trajectory through the Gulf of Mexico.

Water vapor, 0z upper air data, and steering analyses from UW-CIMSS suggest that the trough over the western Atlantic is deamplifying. Isaac appears to have passed the longitude of the trough, and a ridge is noted building over the western Atlantic and southeastern United States in the wake of this feature. Based on careful analysis of this triple data, I expect a more west-northwestward heading to begin soon, or at least not strictly northwest. Low cloud lines on satellite imagery and doppler radar data from Camaguey over northern Cuba suggest that this could already be beginning. Isaac is expected to gradually turn north over the Gulf of Mexico as it begins to feel a weakness over the southern plains in association with a weak shortwave trough noted on water vapor imagery. However, the models have generally been showing a stronger ridge and a more shallow trough. The attendant upper low is all the way up in Manitoba, and the flow over the northern US is mostly zonal, which usually indicates a weak trough. Upper air data at 0z also showed that this trough was not particularly strong. Barring any significant amplification, this trough may not be sufficient to recurve Isaac east of the Mississippi River. Models have generally been trending in this direction over the last several cycles, and most of them now show a strike somewhere from southeast Louisiana to Mobile. Indeed, even the Euro, which was forecasting a landfall near Apalachicola at 12z, has joined the westward consensus at 0z, and now calls for a landfall near Mobile. However, I will not yet go as far west as the consensus, pending data from the models throughout the day. Additional westward shifts may be required tomorrow if the guidance persists in this direction. It should be noted that time is running out for a significant track shift, as landfall is only three to four days away.

It is important not to focus on the center in the forecast track, as tropical cyclones are not points. This is especially true in Isaac's case, which is already a much larger than average tropical cyclone that is only forecast to grow.

5-day intensity forecast

INITIAL 08/26 0600Z 50 KT 60 MPH
12 hour 08/26 1800Z 55 KT 65 MPH
24 hour 08/27 0600Z 65 KT 75 MPH
36 hour 08/28 1800Z 75 KT 85 MPH
48 hour 08/29 0600Z 85 KT 100 MPH
72 hour 08/30 0600Z 100 KT 115 MPH...AT THE COAST
96 hour 08/31 0600Z 50 KT 60 MPH...INLAND
120 hour 09/01 0600Z 30 KT 35 MPH...INLAND

5-day track forecast



Figure 3. My 5-day forecast track for Isaac.

Although Isaac is not expected to make landfall in southwest Florida, hurricane warnings are up for all of the Keys as well as portions of south Florida (see below). While I doubt hurricane force winds, at least sustained, will afflict Miami since the system is far to the west, sustained tropical storm force winds are likely across that area as well as much of southern and central Florida. Given that they will be in the right-front quadrant of a northwestward moving tropical cyclone, the potential exists for brief tornadoes. Heavy rainfall will also be a concern, especially across northern Florida where rains from Beryl and Debby earlier in the year have caused saturated ground. Weather across they Keys should begin to deteriorate late this morning, and will go downhill in south Florida shortly thereafter. The weather will gradually improve over a two day period from south to north as Isaac moves toward the northern Gulf Coast. Storm surge flooding should be relatively minor; 2 to 4 feet across the keys, with perhaps 3 to 5 feet across the Florida west coast.

Watches and warnings

A HURRICANE WARNING IS IN EFFECT FOR...
* THE FLORIDA KEYS INCLUDING THE DRY TORTUGAS
* THE WEST COAST OF FLORIDA FROM BONITA BEACH SOUTHWARD TO OCEAN
REEF
* FLORIDA BAY
* ANDROS ISLAND IN THE BAHAMAS

A HURRICANE WATCH IS IN EFFECT FOR...
* THE FLORIDA EAST COAST FROM GOLDEN BEACH SOUTHWARD TO OCEAN REEF

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* CUBAN PROVINCES OF CIEGO DE AVILA...SANCTI SPIRITUS...VILLA
CLARA...CAMAGUEY...LAS TUNAS...GRANMA...HOLGUIN...SANTIAGO DE
CUBA...AND GUANTANAMO
* THE BAHAMAS EXCLUDING ANDROS ISLAND
* TURKS AND CAICOS ISLANDS
* THE FLORIDA EAST COAST FROM SEBASTIAN INLET SOUTHWARD TO OCEAN
REEF
* LAKE OKEECHOBEE
* THE FLORIDA WEST COAST FROM NORTH OF BONITA BEACH TO TARPON
SPRINGS...INCLUDING TAMPA BAY

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* CUBAN PROVINCES OF MATANZAS AND CIENFUEGOS
* THE FLORIDA EAST COAST NORTH OF SEBASTIAN INLET TO FLAGLER BEACH
* THE FLORIDA WEST COAST AND THE FLORIDA PANHANDLE NORTH OF TARPON
SPRINGS TO INDIAN PASS

A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED
SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN THE NEXT 24
HOURS. PREPARATIONS TO PROTECT LIFE AND PROPERTY SHOULD BE RUSHED
TO COMPLETION.

A HURRICANE WATCH MEANS THAT HURRICANE CONDITIONS ARE POSSIBLE
WITHIN THE WATCH AREA...IN THIS CASE WITHIN THE NEXT 24 HOURS.

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.

INTERESTS IN THE REMAINDER OF CUBA...THE REMAINDER OF FLORIDA...AND
ELSEWHERE ALONG THE NORTHERN GULF COAST SHOULD MONITOR THE PROGRESS
OF ISAAC. A HURRICANE WATCH MAY BE REQUIRED FOR A PORTION OF THE
NORTHERN GULF COAST ON SUNDAY MORNING.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED STATES...
INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE FORECAST
OFFICE. FOR STORM INFORMATION SPECIFIC TO YOUR AREA OUTSIDE THE
UNITED STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR NATIONAL
METEOROLOGICAL SERVICE.



Invest 97L

A tropical wave over the eastern Atlantic about 525 miles west of the Cape Verde Islands has become better organized over the past 24 hours. Satellite imagery shows deep convection is attempting to wrap into the center, which is slowly becoming better defined.



Figure 4. Latest infrared satellite image of Invest 97L. Image credit: NOAA

The southwesterly shear I was expecting has not materialized, as evidenced by the outflow pattern. Aside from dry air to the west, conditions appear favorable for the system to become a tropical depression over the next couple of days as it moves generally west-northwestward. Because the system is organizing sooner than I thought, the threat to the Leeward Islands is lessening. It may end up recurving like Joyce.

Probability of development in 48 hours: 50%



Invest 97E

An area of low pressure continues moving well south of the Mexican coastline. Shortwave infrared satellite images and recent microwave data suggest that this system possesses a small but vigorous surface circulation, and the only impediment to development is the lack of persistent deep convection.



Figure 5. Latest infrared satellite image of Invest 97E. Image credit: NOAA

Some northerly shear is noted squashing the outflow pattern to the north, but it appears to weak to penetrate the inner core at this time. Upper-level winds are forecast to be favorable -- albeit not anticyclonic -- and this system could gradually develop over the next couple of days as it moves west to west-northwest.

Probability of development in 48 hours: 30%

2012 Atlantic hurricane season Tropical Storm Isaac Invest 97L Invest 97E

Updated: 6:51 AM GMT on August 26, 2012

Permalink

Tropical weather analysis - August 25, 2012

By: KoritheMan, 5:21 AM GMT on August 25, 2012

Isaac

After a long and difficult struggle, Isaac has finally managed to consolidate a well-defined center today, which has resulted in strengthening, and the storm is now approaching hurricane strength. As of the latest NHC advisory, the following was posted on this storm:

Wind: 70 mph, with higher gusts
Location: 17.7°N 72.5°W
Movement: NW at 14 mph
Pressure: 990 mb

Oh how I wish Hispaniola had a radar. Even without that though, it's rather obvious that Isaac is a much better organized storm than 24 hours ago. The cyclone now possesses an inner core, as evidenced by earlier microwave data suggesting the formation of an eye and partial eyewall. Satellite imagery shows convection wrapping around the eye, and Isaac could make a run at hurricane status prior to reaching Hispaniola in a few hours.



Figure 1. Latest infrared satellite image of Tropical Storm Isaac. Image credit: NOAA

Upper-level outflow is now well-established in all directions, indicating strong anticyclonic flow aloft. So barring land interaction, there appears to be little reason why Isaac should not intensify. As is often the case, the intensity forecast is vastly more difficult than the track, complicated even more so in this instance by interaction with the rugged terrain of Haiti and Cuba. Isaac is an extremely large storm, probably about twice the size of your average tropical cyclone, and I expect the size to prevent the storm from dissipating while traversing Cuba. However, Hurricane Gustav took a path across western Cuba four years ago and it weakened considerably after that point due to a disrupted inner core, and was never able to completely recover. Considering that western Cuba is far less mountainous than eastern Cuba -- the latter of which Isaac is forecast to traverse -- I would be prepared for the possibility of significant weakening, even if Isaac only spends a little bit of time over the nation as the models have been trending toward. It is always difficult to accurately predict inner core changes, since we still do not understand inner core dynamics all that well. For this reason, interests along the northern Gulf Coast should be prepared for a storm of varying strength, ranging from a tropical storm to a Category 3. Residents living in the Florida peninsula should anticipate the possibility of a Category 1 hurricane approaching them. There is a chance that Isaac could attempt to become a Category 2 prior to reaching Florida if it avoids spending too much time over Cuba, or if it misses it entirely. It's probably a good reminder to stress just how uncertain I am of the intensity forecast I've given below. There are still quite a lot of variables to be worked out, and mountainous terrain can play tricks on the mind. And the storm.

Isaac remains south of a weak mid-level ridge over the central Atlantic extending westward to the central Bahamas. Water vapor imagery shows an upper trough over the western Atlantic that is slowly lifting northward. Indeed, it appears as if this trough will be insufficient to recurve Isaac. Looking at the big picture over the lower 48, a developing shortwave trough emerging from the Rockies is moving eastward. While the models have varying depictions of the strength of this trough when Isaac makes US landfall, they do at least agree on it being there to some extent. The models shifted back to the west at 18z, and the GFS hardly budged as of 0z. Awaiting the other models, and accounting for the possibility of additional shifts in the guidance, especially after the center interacts with Haiti and Cuba, I will keep my forecast track similar to last night's for now, showing an eventual landfall on the central Gulf Coast. Interests from New Orleans to the southwest Florida peninsula should closely monitor the progress of Isaac. Implicit by what I just said, the landfall threat to peninsular Florida appears to be a little bit greater than I was indicating yesterday, as the models have continuously shifted east during the early portion of the forecast track. If I had to narrow down US landfall probabilities for Isaac's final landfall at this point, they would be as follows:

New Orleans to Pascagoula: 10%
Mobile to Panama City: 60%
East of Panama City to Apalachicola: 30%

In the meantime, southwest Florida has a roughly 40% chance of Isaac making landfall somewhere along that peninsula before it begins its march toward the Gulf Coast. Watches and warnings have already been posted for portions of the western and eastern peninsula of Florida, including a hurricane watch. which I will indicate below. That area is going to be dealing with a two to three-day period of heavy rain, tropical storm force winds (with the possibility of sustained hurricane force winds in the northeastern eyewall depending on how close Isaac comes to the coast), tornadoes, and dangerous sea conditions. Rainfall could be particularly hazardous in northern Florida and across the Tallahassee area, where rains from tropical storms Beryl and Debby caused flooding earlier in the season. Power outages will also be a significant issue.

5-day intensity forecast

INITIAL 08/25 0300Z 60 KT 70 MPH
12 hour 08/25 1200Z 55 KT 65 MPH
24 hour 08/26 0000Z 60 KT 70 MPH
36 hour 08/27 1200Z 65 KT 75 MPH
48 hour 08/28 0000Z 70 KT 80 MPH
72 hour 08/29 0000Z 80 KT 90 MPH
96 hour 08/30 0000Z 90 KT 105 MPH...AT THE CENTRAL GULF COAST
120 hour 08/31 0000Z 45 KT 50 MPH...INLAND

5-day track forecast



Figure 2. My 5-day forecast track for Isaac.

Watches and warnings

A HURRICANE WATCH AND A TROPICAL STORM WARNING HAVE BEEN ISSUED FOR
ALL OF THE FLORIDA KEYS...INCLUDING FLORIDA BAY...AND FOR THE COAST
OF THE SOUTHERN FLORIDA PENINSULA FROM OCEAN REEF ON THE EAST COAST
WESTWARD TO BONITA BEACH ON THE WEST COAST.

A TROPICAL STORM WARNING HAS BEEN ISSUED FOR THE SOUTHEAST FLORIDA
COAST FROM NORTH OF OCEAN REEF NORTHWARD TO JUPITER INLET...AND FOR
LAKE OKEECHOBEE.

THE GOVERNMENT OF THE BAHAMAS HAS ISSUED A TROPICAL STORM WARNING
FOR ALL OF THE NORTHWESTERN BAHAMAS.

A TROPICAL STORM WATCH HAS BEEN ISSUED FOR THE EAST-CENTRAL FLORIDA
COAST FROM NORTH OF JUPITER INLET TO SEBASTIAN INLET.

THE CAYMAN ISLANDS METEOROLOGICAL SERVICE HAS ISSUED A TROPICAL
STORM WATCH FOR THE CAYMAN ISLANDS.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A HURRICANE WATCH IS IN EFFECT FOR...
* HAITI
* FLORIDA KEYS INCLUDING THE DRY TORTUGAS
* FLORIDA BAY
* THE FLORIDA EAST COAST FROM OCEAN REEF SOUTHWARD
* THE FLORIDA WEST COAST FROM BONITA BEACH SOUTHWARD

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* DOMINICAN REPUBLIC
* HAITI
* CUBAN PROVINCES OF CIEGO DE AVILA...SANCTI SPIRITUS...VILLA
CLARA...CAMAGUEY...LAS TUNAS...GRANMA...HOLGUIN...SANTIAGO DE
CUBA...AND GUANTANAMO
* THE BAHAMAS
* TURKS AND CAICOS ISLANDS
* THE FLORIDA KEYS INCLUDING THE DRY TORTUGAS
* THE FLORIDA EAST COAST FROM JUPITER INLET SOUTHWARD
* THE FLORIDA WEST COAST FROM BONITA BEACH SOUTHWARD
* FLORIDA BAY AND LAKE OKEECHOBEE

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* CUBAN PROVINCES OF MATANZAS AND CIENFUEGOS
* JAMAICA
* THE FLORIDA EAST COAST NORTH OF JUPITER INLET TO SEBASTIAN INLET

A HURRICANE WATCH MEANS THAT HURRICANE CONDITIONS ARE POSSIBLE
WITHIN THE WATCH AREA...IN THIS CASE WITHIN THE NEXT 24 TO 36 HOURS.

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.

INTERESTS IN THE REMAINDER OF CUBA AND THE REMAINDER OF THE FLORIDA
PENINSULA SHOULD MONITOR THE PROGRESS OF ISAAC.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED
STATES...INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE
MONITOR PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE
FORECAST OFFICE. FOR STORM INFORMATION SPECIFIC TO YOUR AREA OUTSIDE
THE UNITED STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR NATIONAL
METEOROLOGICAL SERVICE.



Invest 97L

A tropical wave over the eastern Atlantic ("97L") is producing a narrow band of disorganized thunderstorms to the west of the wave axis. Upper-level winds are only marginally conducive for development, but some gradual development could still occur before upper-level winds become less favorable in a couple of days. In about four days, the GFS suggests the system could find itself under a more diffluent upper air pattern, which would of course favor some development. As of now it is too early to speculate on where 97L may go. A weaker system would tend to move more westward, posing a long-range risk to the Leeward Islands. A slight weakness in the subtropical ridge induced by the western Atlantic trough steering Isaac northwestward should not allow for a straight westward runner with this storm.

Probability of development in 48 hours: 30%



East Pacific disturbance

An area of disturbed weather several hundred miles south of Acapulco is producing disorganized showers and thunderstorms. While shower activity remains poorly-organized, the cloud pattern suggests that upper-level winds are becoming more favorable. Gradual development of this system is possible over the next few days as it moves generally westward.

Probability of development in 48 hours: 30%

2012 Atlantic hurricane season Tropical Storm Isaac Invest 97L

Updated: 5:23 AM GMT on August 25, 2012

Permalink

Tropical weather analysis - August 23, 2012

By: KoritheMan, 3:48 AM GMT on August 24, 2012

Isaac

Tropical Storm Isaac continues moving through the eastern Caribbean. As of the latest NHC intermediate advisory, the following was posted on the storm:

Wind: 45 mph, with higher gusts
Location: 16.7°N 68.7°W
Movement: WNW at 18 mph
Pressure: 1001 mb

Isaac still remains weak and relatively disorganized according to satellite and radar data from San Juan. However, the central pressure has come down a bit, and the winds have responded by coming up a little. There still appears to be some sort of mid-level center orbiting around to the south of the cyclone, but overall, based on convective trends, the northern center appears to be the dominant one. This would also agree well with aircraft reports and extrapolation of earlier microwave data.



Figure 1. Latest infrared satellite image of Tropical Storm Isaac. Image credit: NOAA

Having said that, the overall circulation of the tropical cyclone remains broad, and we saw it intensify about the same time last night, only for it to weaken again. This was also a common motif for Ernesto when it was in this area three weeks ago. Convection has resurged near the center this evening, and I dare say, again, that Isaac may finally be intensifying. However, dry air still appears to be interrupting the development process, and the storm will soon have Hispaniola to contend with.

Other than that, broad anticyclonic flow still appears to be established over Isaac. The SHIPS/GFS still call for weak shear throughout the forecast period, and since Isaac appears to be battling little vertical shear, this feels like a reasonable prognosis. In fact, the GFS maintains a large anticyclone above Isaac up until landfall on the United States Gulf Coast, which is a pattern considered highly conducive to intensification. So far, the biggest impediment to significant strengthening of Isaac has been a combination of dry air and the enormous size of the circulation envelope. Although not explicitly shown here, depending on how Isaac's inner core fares after tangling with Hispaniola and Cuba, rapid intensification is a possibility in the Gulf of Mexico, and Isaac could become a major hurricane at its United States landfall. Interests along the Gulf Coast from New Orleans to Panama City should be prepared for a hurricane landfall. Peninsular Florida may not see a landfall at this point, but because of the location of the storm relative to the coast, high winds, heavy rains, and tornadoes will pose a serious threat even as Isaac barrels west.

Synoptic data indicates that the trough over the eastern United States is relatively weak below 600 mb, but remains strong above that. In addition, water vapor imagery shows that an upper low has formed over northern Florida, which will finally serve as a forcing mechanism to move the trough east. This is already happening, and the western extent of the Atlantic ridge appears to be breaking down a little. As a result, Isaac is expected to turn northwestward soon, and should maintain this motion as it traverses the Gulf of Mexico. Model guidance is in reasonable agreement on this, especially for during the early portion of the forecast period.

The model consensus has continued to shift westward today, especially at longer ranges. This appears to be due to the purported weakness in the subtropical ridge being farther west than originally anticipated. Some of the models have a weakness over the central plains, which is a classical setup for a strike on Louisiana or Mississippi. Others have see the weakness being farther east, which of course favors a strike farther east. Regardless, it appears that Isaac will be a hurricane threat to some portion of the United States Gulf Coast next week, and residents living there should review their hurricane preparedness plans now.

However, it would be most premature not to focus on the immediate threat -- vulnerable Haiti, which is facing a very serious flood situation, especially given the sheer size of Isaac. As much as 10 to 20 inches of rain could fall over the impoverished mountain nation over the next couple of days, with locally higher amounts. Needless to say, this is likely to produce some serious flash flooding and mudslides, and unfortunately, death. In 2004, Hurricane Jeanne killed 3,000 people from floods. I don't anticipate that the death toll will be quite that high with Isaac, but the vast majority of residents are living in tents, especially following the earthquake in 2010, residents there are particularly susceptible to floods. Not to mention nearly all of the country is deforested, which allows water and mud to more easily flow down mountainsides.

Based on model trends, my forecast track has shifted considerably westward since yesterday, and now shows a long-range threat to the central Gulf Coast. However, there is still room for error here. The upcoming 0z models should have data from tonight's synoptic surveillance mission that was conducted by the NOAA G-IV jet. This should give us a much better idea on the track, and any shifts after that will likely only be within a very specific 100 mile threat area. I may update the track subsequent to the arrival of these models.

5-day intensity forecast

INITIAL 08/24 0300Z 40 KT 45 MPH
12 hour 08/24 1200Z 40 KT 50 MPH
24 hour 08/25 0000Z 40 KT 45 MPH
36 hour 08/26 0000Z 60 KT 70 MPH...INLAND OVER EASTERN CUBA
48 hour 08/27 0000Z 55 KT 65 MPH...OFF THE NORTH COAST OF CUBA
72 hour 08/28 0000Z 60 KT 70 MPH...OVER WATER
96 hour 08/29 0000Z 70 KT 80 MPH
120 hour 08/30 0000Z 85 KT 100 MPH

5-day track forecast



Figure 2. My 5-day forecast track for Isaac.

Watches and warnings


A HURRICANE WATCH IS IN EFFECT FOR...
* HAITI

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* DOMINICAN REPUBLIC
* HAITI
* CUBAN PROVINCES OF CAMAGUEY...LAS TUNAS...GRANMA...HOLGUIN...
SANTIAGO DE CUBA...AND GUANTANAMO
* SOUTHEASTERN BAHAMAS INCLUDING THE ACKLINS...CROOKED ISLAND...LONG
CAY...THE INAGUAS...MAYAGUANA...AND THE RAGGED ISLANDS
* TURKS AND CAICOS ISLANDS

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* CUBAN PROVINCES OF CIEGO DE AVILA...SANCTI SPIRITUS...AND VILLA
CLARA
* ANDROS ISLAND AND THE CENTRAL BAHAMAS INCLUDING CAT ISLAND...THE
EXUMAS...LONG ISLAND...RUM CAY...AND SAN SALVADOR

A HURRICANE WATCH MEANS THAT HURRICANE CONDITIONS ARE POSSIBLE
WITHIN THE WATCH AREA...IN THIS CASE WITHIN THE NEXT 24 TO 36 HOURS.

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.

INTERESTS IN JAMAICA...THE REMAINDER OF CUBA...THE REMAINDER OF THE
BAHAMAS...SOUTH FLORIDA...AND THE FLORIDA KEYS SHOULD MONITOR THE
PROGRESS OF ISAAC.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED STATES...
INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE FORECAST
OFFICE. FOR STORM INFORMATION SPECIFIC TO YOUR AREA OUTSIDE THE
UNITED STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR NATIONAL
METEOROLOGICAL SERVICE.



Joyce

Joyce formed today from what was Tropical Depression Ten. And now, Joyce is already back to a tropical depression as of the latest NHC advisory:

Wind: 35 mph, with higher gusts
Location: 16.3°N 43.7°W
Movement: WNW at 14 mph
Pressure: 1008 mb

Satellite and microwave fixes show a highly disorganized tropical cyclone. There is little convection near the center, which is displaced over 100 miles south of the mid-level center due to strong southerly shear associated with a persistent central Atlantic upper cold low.



Figure 3. Latest infrared satellite image of Tropical Depression Joyce. Image credit: NOAA

The upper low is forecast to loosen its grip in about two days, but there could be little left of Joyce at that time given the current environment and storm structure. Dry air is also forecast to afflict the system over the next few days, although I have a feeling the SHIPS is overdoing it a bit. Should it survive, Joyce has the potential to become a hurricane prior to or during recurvature, although I am not explicitly showing that at this time. My intensity forecast remains the same as that of the National Hurricane Center for now.

Joyce remains situated between a weak low- to mid-level ridge to the east and an upper-level trough to the west. The guidance suggests that this ridge will break down further over the next few days as the upper trough moves eastward. Both in a synoptic and teleconnection fashion, this would make sense, and it appears that Joyce will recurve. Guidance is very well clustered, which leads to greater confidence in the forecast. It should be noted that the four day forecast point brings a moderate tropical storm very near Bermuda, and interests there should monitor the progress of Joyce in case it survives the current hostilities.

5-day intensity forecast

INITIAL 08/24 0300Z 30 KT 35 MPH
12 hour 08/24 1200Z 30 KT 35 MPH
24 hour 08/25 0000Z 30 KT 35 MPH
36 hour 08/25 1200Z 30 KT 35 MPH
48 hour 08/26 0000Z 35 KT 40 MPH
72 hour 08/27 0000Z 40 KT 45 MPH
96 hour 08/28 0000Z 45 KT 50 MPH
120 hour 08/29 0000Z 50 KT 60 MPH

5-day track forecast



Figure 4. My 5-day forecast track for Joyce.



Eastern Atlantic tropical wave

A tropical wave, probably soon to be designated Invest 97L, is over the eastern Atlantic between Africa and the Cape Verde Islands. Shower activity has increased a bit over the last few hours, but remains displaced south of the wave axis. This system appears to be getting at least part of its convection from the Intertropical Convergence Zone (ITCZ), which suggests that it is not a well-organized entity yet. Upper-level winds are somewhat conducive to development of this system over the next couple of days, although there are still some indications that it could run into some southwesterly shear beyond that time, especially if it moves faster or loses the weak anticyclone the GFS suggests will build over it. I expect this wave to move generally westward as low-level ridging builds in behind Joyce.

Probability of development in 48 hours: 20%



Eastern Pacific disturbance

A tropical disturbance has developed in the Eastern Pacific several hundred miles south of the Gulf of Tehuantepec. Shower activity is poorly organized, and some northeasterly shear appears to be affecting the system. However, environmental conditions appear quite conducive for additional development, the current shear notwithstanding. This system should move west-northwestward, and southern Mexico may need to watch it over the next few days.

Probability of development in 48 hours: 20%

2012 Atlantic hurricane season Tropical Storm Isaac Tropical Depression Joyce

Updated: 3:57 AM GMT on August 24, 2012

Permalink

Tropical weather analysis - August 22, 2012

By: KoritheMan, 5:36 AM GMT on August 23, 2012

Isaac

Isaac has entered the eastern Caribbean Sea. As of the most recent NHC advisory, the following information was available on the cyclone:

Wind: 45 mph, with higher gusts
Location: 15.8°N 63.0°W
Movement: W at 20 mph
Pressure: 1003 mb

Satellite and aerial reconnaissance observations indicate that Isaac is still not well-organized, but the central pressure appears to be slowly dropping, which should soon correspond to an increase in wind speed if the current organization persists. However, false appearances of organization are common on nighttime imagery, and particularly with weak storms like Isaac, which often have broad centers rotating around a common gyre.



Figure 1. Latest infrared satellite image of Tropical Storm Isaac. Image credit: NOAA

The center still appears to be a little broad, and radar fixes from the Lesser Antilles suggest that there could be a secondary swirl to the south of the main convective mass, but I suspect this is probably a mid-level reflection. The northernmost gyre appears to be the dominant one at this time.

Despite Isaac's struggles, conditions are still expected to become more favorable for intensification through the next several days. Isaac is situated in a col region between an upper low along the north coast of Cuba, and another upper low over the central Atlantic dropping southward. This low appears to have been inducing some northerly to northeasterly shear over Isaac over the last couple of days, as the center has consistently been exposed along the north edge of the convection. However, the vertical shear that has plagued Isaac since birth may finally be diminishing, as the cyclone has developed a well-defined outflow pattern, particularly to the north and west. This suggests that the upper flow over the tropical storm is becoming anticyclonic, a pattern that is considered very favorable for strengthening. This upper air pattern is forecast to continue throughout the forecast period. In fact, the GFS suggests a rather favorable outflow regime will develop over the storm, with a large southwesterly jet streak blowing near the storm at longer ranges. This could result in a large and intense hurricane when Isaac makes US landfall. The one fly in the ointment continues to be interaction with the Greater Antilles. Should Isaac pass north or south of those islands, a much stronger hurricane would likely result than what is currently being forecast. However, for the sake of continuity, and to see how trends evolve, I will continue to be conservative.

One impediment to rapid intensification early in the period is the large size of the circulation. Such systems tend to take longer than average to spin up.

Isaac remains situated south of a well-established subtropical high pressure area, and continues on a slightly north of due west motion based on analysis of satellite imagery and radar fixes. Based on UW-CIMSS steering layers, the ridge appears to have weakened somewhat, enough to ensure that Isaac doesn't go due west. However, I note that this weakness is quite minuscule, and the western Atlantic ridge still appears to be intact until near 80W. Thus, I do not expect this weakness to cause a significant poleward bend in the track. 0z upper air data over Florida and the southeastern United States indicates that the trough lingers across the eastern portion of the country. However, the trough axis remains well north of south Florida, as denoted by the uniform southwesterly flow observed at Jacksonville, while locations downstream are generally reporting easterly to southerly winds. This indicates that a break in the ridge begins right near 80W, as I said earlier, close to the longitude of the far western Bahamas and extreme south Florida. So far, the flow over the eastern US has remained fairly amplified, which has kept the trough pretty much quasi-stationary for the last 24 hours. The global models don't really show much movement of this trough, and ultimately lift it out by the weekend. The GFS is a little bit faster though, and lifts it out by Saturday. Given that the trough appears to be fairly weak and positively tilted, I tend to side with the GFS on a trough that departs sooner. The ECMWF continues to show a landfall along the northern Gulf Coast. I don't think this is just because of Isaac being weaker in that model, as it also seems to be showing a more westward-bridging high. The GFS and the other models see another shortwave dropping out of Canada reinforcing the weakness in the ridge after the trough departs, in a similar synoptic situation to what occurred with Debby in June.

Given the top performance of the GFS this year relative to the ECMWF, as well as the still tightly-clustered model consensus, I am not shifting nearly as far west as the Euro. However, given Isaac's reluctance to strengthen thus far, as well as the Euro's consistently westward solution, I will keep Isaac's forecast track along the western edge of the guidance. A NOAA G-IV mission is scheduled to sample the environment ahead of Isaac today, which should provide much more valuable data to the models, and significantly improve the track forecasts.

Interests from Louisiana to the Carolinas should monitor the progress of Isaac. Subtle shifts notwithstanding, the greatest threat still lies with Florida, as I just don't think the large scale pattern favors a track toward the northern Gulf Coast or the east coast at this time.

5-day intensity forecast

INITIAL 08/23 0300Z 40 KT 45 MPH
12 hour 08/23 1200Z 45 KT 50 MPH
24 hour 08/24 0000Z 50 KT 60 MPH
36 hour 08/25 1200Z 65 KT 75 MPH...APPROACHING HISPANIOLA
48 hour 08/26 0000Z 55 KT 65 MPH...INLAND OVER HAITI
72 hour 08/27 0000Z 55 KT 65 MPH...INLAND OVER EASTERN CUBA
96 hour 08/28 0000Z 60 KT 70 MPH...OVER WATER
120 hour 08/29 0000Z 70 KT 80 MPH...OVER WATER

5-day track forecast



Figure 3. My 5-day forecast track for Isaac.

Watches and warnings


A HURRICANE WARNING IS IN EFFECT FOR...
* SOUTH COAST OF DOMINICAN REPUBLIC FROM ISLA SAONA WESTWARD TO THE
HAITI-DOMINICAN REPUBLIC SOUTHERN BORDER
* HAITI

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* DOMINICA
* ST. KITTS...NEVIS...ANTIGUA...BARBUDA...MONTSERRAT...A ND ANGUILLA
* SABA...ST. EUSTATIUS...AND ST. MAARTEN
* BRITISH VIRGIN ISLANDS
* PUERTO RICO...VIEQUES...CULEBRA...AND THE U.S. VIRGIN ISLANDS
* NORTH COAST OF DOMINICAN REPUBLIC FROM THE HAITI-DOMINICAN
REPUBLIC NORTHERN BORDER EASTWARD TO NORTH OF ISLA SAONA

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* SOUTHEASTERN BAHAMAS INCLUDING THE ACKLINS...CROOKED
ISLAND...LONG CAY...THE INAGUAS...MAYAGUANA...AND THE RAGGED
ISLANDS AND THE TURKS AND CAICOS ISLANDS

A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED
SOMEWHERE WITHIN THE WARNING AREA. PREPARATIONS TO PROTECT LIFE
AND PROPERTY SHOULD BE RUSHED TO COMPLETION.

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.

INTERESTS IN CUBA...JAMAICA...AND ELSEWHERE IN THE BAHAMAS SHOULD
MONITOR THE PROGRESS OF ISAAC.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED
STATES...INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE
MONITOR PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE
FORECAST OFFICE. FOR STORM INFORMATION SPECIFIC TO YOUR AREA OUTSIDE
THE UNITED STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR NATIONAL
METEOROLOGICAL SERVICE.



Tropical Depression Ten

Tropical Depression Ten formed over the central Atlantic on Wednesday from former Invest 96L. As of the latest NHC advisory, the following information was posted on the cyclone:

Wind: 35 mph, with higher gusts
Location: 13.8°N 39.5°W
Movement: WNW at 17 mph
Pressure: 1007 mb

A burst of fairly cold convection has returned to the previously void low-level center over the last several hours, but this convection lacks curvature.



Figure 3. Latest infrared satellite image of Tropical Depression Ten. Image credit: NOAA

The depression appears to be battling some southerly shear induced by an upper low situated between Tropical Storm Isaac and the depression. The low and the depression are forecast to more or less move in tandem until the low begins to fill in about three days. The troubles for the depression do not end there, I'm afraid -- the GFS/SHIPS shows northerly shear increasing over the tropical cyclone once the low retreats. This shear appears to emanate from Isaac, which is forecast to be a rather large system at those ranges. Barring an unexpected relaxation of the vertical shear, it appears most unlikely that TD10 will become a hurricane.

The cyclone is situated south of a mid-level ridge containing a weakness due to the presence of the nearby upper low. Since the cyclone is forecast to ride cyclonically around the low, a steady gain in latitude is expected throughout the forecast period. This system is likely to recurve out to sea, although Bermuda should closely monitor it, as both my forecast track and the National Hurricane Center's takes it about 100 miles west of that island on day five as a strong tropical storm.

5-day intensity forecast

INITIAL 08/23 0300Z 30 KT 35 MPH
12 hour 08/23 1200Z 35 KT 40 MPH
24 hour 08/24 0000Z 40 KT 45 MPH
36 hour 08/24 1200Z 40 KT 45 MPH
48 hour 08/25 0000Z 40 KT 45 MPH
72 hour 08/26 0000Z 45 KT 50 MPH
96 hour 08/27 0000Z 50 KT 60 MPH
120 hour 08/28 0000Z 50 KT 60 MPH

5-day track forecast



Figure 4. My 5-day forecast track for Tropical Depression Ten.



New African wave

A new tropical wave has emerged off the west coast of Africa. Upper-level winds appear favorable for slow development over the next couple of days, although the system could encounter southwesterly shear later in the period depending on how far northward it moves.

Probability of development in 48 hours: 10%

2012 Atlantic hurricane season Tropical Storm Isaac Tropical Depression Ten

Updated: 5:37 AM GMT on August 23, 2012

Permalink

Tropical weather analysis - August 21, 2012

By: KoritheMan, 3:22 AM GMT on August 22, 2012

Isaac

Tropical Storm Isaac formed today from former Invest 94L. As of the most recent NHC advisory, the following was posted on the tropical storm:

Wind: 40 mph, with higher gusts
Location: 15.6°N 55.6°W
Movement: W at 18 mph
Pressure: 1006 mb

The center remains a little exposed along the northern edge of the convection due to moderate northeasterly shear, but Isaac appears to be maintaining itself for now.



Figure 1. Latest infrared satellite image of Tropical Storm Isaac. Image credit: NOAA

The SHIPS shows the shear decreasing almost immediately, but water vapor imagery shows a well-defined upper tropospheric cold low over the central Atlantic dropping southward toward the tropical cyclone. The GFS keeps this small low moving rather swiftly toward the south or southwest. Given the relatively zonal flow over the north Atlantic at this time, this seems reasonable. This may act to keep some northerly to northwesterly shear over Isaac for the next day or so. This should prevent Isaac from rapidly intensifying before reaching the Lesser Antilles. More positively, Isaac appears to have mixed out most of the dry air. In about 24 hours, the upper-level environment is forecast to become more favorable -- anticyclonic actually -- and that should allow for some more robust strengthening. Later in the period, possible interaction with the Greater Antilles dictates the intensity forecast remain conservative. However, if Isaac avoids the Greater Antilles, either from south or north, it could easily become a major hurricane. Given the forecast track, I currently see little to prevent Isaac from being a hurricane prior to interaction with Hispaniola. Interests across the eastern Caribbean should carefully monitor the progress of Isaac over the next few days. Residents living in the Lesser Antilles should anticipate the onset of tropical storm conditions Wednesday evening. Interests in Puerto Rico and Hispaniola should also anticipate tropical storm conditions. They should also be preparing for the possibility of hurricane force winds, especially in Hispaniola.

Synoptic data indicates that Isaac is south of a well-established mid-level ridge, which is the steering the system on a generally westward path. This has been the general pattern over this portion of the basin this year. 0z upper air data and water vapor imagery indicates that a trough is undergoing significant amplification over the eastern seaboard and southeastern United States, while a ridge builds over the central plains. However, the data indicates that this trough is moving rather slowly, with a weakness in the ridge not evident until roughly 75W. The models disagree on the depth and amplitude of this trough, as well as how amplified the flow over the central United States becomes over the next few days. Obviously, a more amplified regime would tend to keep the trough in place, while a more progressive flow would tend to prevent the trough from sticking around in any one place for too long. I tend to side with the GFS, which lifts the trough out by Saturday. Just when and where Isaac makes the turn will be critical in determining where US landfall will occur. Looking at the global model fields, a weaker storm will tend to move more westward, spending a longer duration over the Caribbean, while a stronger storm will tend to move poleward, more attuned to the weakness in the western Atlantic ridge. An AMSU microwave overpass taken just after 0z hinted that the cyclone was farther north, closer to 16N. However, this seems to contrast with conventional satellite trends. Probably the best thing for now is to blend those fixes.

While the 0z dynamical guidance has made a significant northward shift, the guidance has shifted in either direction for the last several days. It is unwise to shift one's forecast track in any given direction based on a couple of model shifts. By far the most interesting aspect to note for today is the significant westward shift shown by the ECMWF at 12z. The reason for this drastic shift is not quite clear, but looking at the sea level pressure fields on that model, it appears to forecast a much stronger western Atlantic ridge than the GFS, and a generally more progressive upper air pattern. It also significantly weakens Isaac after it enters the Caribbean Sea. Offshore buoy data over the central and eastern Caribbean Sea shows that the low-level trade winds are still rather strong, perhaps unclimatlogically so for this time of year. This is to be expected during El Nino years, where the Bermuda-Azores ridge tends to be stronger. This would also make sense when one considers the depth of the trough currently over the western Atlantic. It is this sort of synoptic regime that kept Ernesto weak, and that literally obliterated Helene. Perhaps this is why the Euro keeps Isaac weak. For now, I am considering this model to be an outlier, although if it continues with the westward trend, I will need to shift westward as well.

Interests along the southeastern United States coastline should review their hurricane preparedness plans, as Isaac has the potential to be a significant hurricane for that area if interaction with the Greater Antilles is limited.

5-day intensity forecast

INITIAL 08/22 0300Z 35 KT 40 MPH
12 hour 08/22 1200Z 35 KT 40 MPH
24 hour 08/23 0000Z 40 KT 45 MPH
36 hour 08/24 1200Z 45 KT 50 MPH
48 hour 08/25 0000Z 60 KT 70 MPH
72 hour 08/26 0000Z 75 KT 85 MPH...APPROACHING THE COAST OF HAITI
96 hour 08/27 0000Z 65 KT 75 MPH...INLAND OVER EASTERN CUBA
120 hour 08/28 0000Z 65 KT 75 MPH...OVER WATER

5-day track forecast



Figure 2. My 5-day forecast track for Isaac.

Watches and warnings

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* MARTINIQUE
* DOMINICA
* GUADELOUPE AND THE SURROUNDING ISLANDS...AND ST. MARTIN
* ST. KITTS...NEVIS...ANTIGUA...BARBUDA...MONTSERRAT...A ND ANGUILLA
* SABA...ST. EUSTATIUS...AND ST. MAARTEN
* BRITISH VIRGIN ISLANDS
* PUERTO RICO...VIEQUES...CULEBRA...AND THE U.S. VIRGIN ISLANDS

A HURRICANE WATCH IS IN EFFECT FOR...
* PUERTO RICO...VIEQUES...CULEBRA...AND THE U.S. VIRGIN ISLANDS

A HURRICANE WATCH MEANS THAT HURRICANE CONDITIONS ARE POSSIBLE
WITHIN THE WATCH AREA. A WATCH IS TYPICALLY ISSUED 48 HOURS
BEFORE THE ANTICIPATED FIRST OCCURRENCE OF TROPICAL-STORM-FORCE
WINDS...CONDITIONS THAT MAKE OUTSIDE PREPARATIONS DIFFICULT OR
DANGEROUS.

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

INTERESTS IN HISPANIOLA AND CUBA SHOULD MONITOR THE PROGRESS OF
ISAAC.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED
STATES...INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE
MONITOR PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE
FORECAST OFFICE. FOR STORM INFORMATION SPECIFIC TO YOUR AREA OUTSIDE
THE UNITED STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR NATIONAL
METEOROLOGICAL SERVICE.



Invest 95L

A weak area of low pressure in the western Gulf of Mexico has not become any better organized. Satellite images suggest that the system has moved inland, and development is not anticipated.

Probability of development in 48 hours: Near 0%



Invest 96L

A tropical wave over the eastern Atlantic ("96L") centered about 600 miles west-southwest of the Cape Verde Islands the potential to become a tropical depression over the next couple of days. Satellite images suggest that the cloud pattern is gradually becoming better organized, although there are no banding features yet.



Figure 3. Latest infrared satellite image of Invest 96L. Image credit: NOAA

I don't really want to speculate too much on 96L right now since there are still a lot of variables with a system so far out at sea. It could become a future player in the steering of Tropical Storm Isaac, but it is far too early to put on any sort of weight on this for now.

Looking at water vapor imagery and satellite, the easterly shear that was hammering the system yesterday does appear to be decreasing, as convection has managed to develop closer to the center relative to yesterday. However, the low-level center still remains on the eastern edge of the convection, which is reminiscent of some residual easterly shear. An anticyclone is forecast to build atop the system over the next several days, and conditions remain favorable for tropical cyclone formation in this area.

Large scale steering analyses suggests that a weakness is present to the north of Invest 96L, probably induced by the upper low currently shearing Isaac. There may be a more west-northwest motion over the next day or so as the low moves off to the southwest. Thereafter, the low is forecast to weaken and fill, and 96L should respond by stabilizing in a more westward trajectory.

Probability of development in 48 hours: 70%

2012 Atlantic hurricane season Tropical Storm Isaac Invest 95L Invest 96L

Updated: 3:28 AM GMT on August 22, 2012

Permalink

Tropical weather analysis - August 21, 2012

By: KoritheMan, 7:48 AM GMT on August 21, 2012

Invest 94L

A vigorous tropical wave and associated low pressure system is located in the central Atlantic located about 700 miles east of the Lesser Antilles continues to show signs of organization. Satellite images show a small area of increasing convection near the center, with banding features becoming evident to the west.



Figure 1. Latest infrared satellite image of Invest 94L. Image credit: NOAA

Since a closed surface circulation already exists, it would not take much additional organization to denote the formation of a tropical depression. I suspect we will see this at the 5AM advisory from the National Hurricane Center.

Environmental conditions appear conducive for additional strengthening, and the system is likely to become a tropical storm later today or on Wednesday. There is still some dry air to contend with, but with the GFS adamant about a large anticyclone moving in tandem with the system throughout the next several days, my hunch is that 94L will be able to ward off most of the dry air. Steady development prior to reaching the Lesser Antilles seems likely, with the biggest impediment being the fast forward speed. This particular affliction has plagued many otherwise promising tropical cyclones, namely Ernesto and Helene. However, 94L is large enough so that it will be less susceptible to hostilities than either of the two aforementioned storms were at this stage. I do not anticipate a hurricane moving through the Leeward Islands, but residents there should anticipate a strengthening tropical storm, probably in the 45 to 50 kt range. Watches and warnings will likely be required for a large portion of those islands by the National Hurricane Center later this morning, and interests across those islands should carefully monitor the progress of this system over the next couple of days.

Water vapor and UW-CIMSS steering data show a strong mid-level ridge is in place to the north. The motion of the clouds across the eastern Caribbean are uniformly east-southeast, indicating a slight weakness in the subtropical ridge, undoubtedly caused by the western Atlantic trough. Based on global model forecasts at 500 mb, as well as the current progression of the trough, I expect a turn to the west-northwest as the system moves across the eastern Caribbean. There shouldn't be a sharp poleward component of motion until the system approaches the longitude of Hispaniola. The western Atlantic trough is forecast to lift out, but leave behind enough residual cyclonic flow to maintain a general weakness in the ridge. This weakness is forecast to become reinforced by the amplification of another trough along the eastern US by Thursday. This should result in the system gradually turning more northwestward. As the system gets west of Hispaniola there are two possibilities: the first is for it to remain relatively weak, bypass the trough, and move into the Gulf of Mexico. The second, and more likely scenario, is for 94L to gradually curve into the weakness off the eastern seaboard and move northward somewhere over Cuba. This track would place the southeastern United States under the gun.

The model consensus has shifted considerably farther east today relative to yesterday, which gives me more confidence in the forecast track. One fly in the ointment will be the likelihood of interaction with the mountains of Hispaniola. If the system moves slow enough and is significantly disrupted by passage over that island, we could see a center reformation of some sort, potentially leading to a large track shift. I consider this an outlier scenario for now, though.

I prefer the southern edge of the model consensus at this time.

Interests in the Lesser and Greater Antilles should monitor the progress of this system over the next several days, as they will have to contend with it long before all of us here in the United States.

Probability of development in 48 hours: Near 100%



Invest 95L

An elongated area of low pressure continues over the western Gulf of Mexico. This system still possesses a broad circulation, but it is not well organized. After an earlier eruption of convection Monday afternoon, dry air and northwesterly shear haven taken their toll on 95L. Looking at water vapor images, the airmass over the western Gulf of Mexico is very dry, owing to the passage of an unseasonably deep cold front ushering in an unseasonably dry airmass, and yes, even a bit of cold air advection, as denoted by surrounding temperature data over the southern US.



Figure 2. Latest infrared satellite image of Invest 95L. Image credit: NOAA

Synoptic data indicates that the cold front appears to be ebbing away. Some shallow convection may attempt to ignite in the eastern portion of the circulation, where the ambient airmass is a little more unstable. But overall, the upper air pattern over the system is not one that I consider to be conducive to tropical cyclogenesis -- although UW-CIMSS shear data isn't working for some reason, one can get a rough idea that the shear over the northwestern Gulf is probably on the order of 20 or so kt at this time. While the GFS shows this shear abating somewhat in the next 24 hours -- which would be expected subsequent to a frontal passage -- the upper flow in that model still does not look all that anticyclonic, or even diffluent. In addition, it always takes awhile for airmass modification within the boundary layer to return to normal following the passage of such intense cold fronts. Hence, dry air will still be an issue even if the shear relaxes a little. Given this, I am less enthusiastic about development potential than the National Hurricane Center is.

Synoptic upper air data suggests that this system will move slowly westward, ultimately ending up back in Mexico over the next day or two.

Probability of development in 48 hours: 10%



Invest 96L

The tropical wave that emerged from Africa yesterday is showing signs of organization, and it appears we could be talking about another tropical cyclone over the next few days. Satellite imagery indicates that the center is along the eastern side of the gyre, away from the deep convection. This is due to moderate easterly shear. The GFS forecasts the current shear to abate, and the system may find itself situated beneath a small anticyclone in about 48 hours. I mentioned yesterday that outflow from 94L could hamper development of 96L in the long range. A more thorough analysis today suggests that 96L is moving slower than 94L. So even if 94L begins to develop an appreciable outflow channel in that direction, it is unlikely to have a significant affect on 96L.

Having said that, environmental conditions are favorable for steady development over the next couple of days, and this system could become a tropical depression over the next 48 hours. I have no idea where this system will go, but initial model prognostications indicate yet another possible threat to land down the road. Stay tuned.

Probability of development in 48 hours: 60%

2012 Atlantic hurricane season Invest 94L Invest 95L Invest 96L

Updated: 7:53 AM GMT on August 21, 2012

Permalink

Tropical weather analysis - August 20, 2012

By: KoritheMan, 5:05 AM GMT on August 20, 2012

Gordon

Gordon continues to weaken as it approaches the Azores. As of the latest NHC advisory, the following was posted on the hurricane:

Wind: 80 mph, with higher gusts
Location: 36.7°N 25.8°W
Movement: ENE at 20 mph
Pressure: 980 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

In an amusing turn of events, Gordon has moved far enough east as to be effectively out of the NHC floater range. Thus, I have had to resort to using RAMMB imagery, rather than NOAA's Satellite Services Division (SSD).

Gordon still has the cloud signature of a hurricane on satellite images, but deep convection is confined to the northern semicircle due to about 40 knots of southwesterly shear. This shear is also injecting dry air into the cyclone circulation.



Figure 1. Latest infrared satellite image of Hurricane Gordon. Image credit: RAMMB

Conditions are rapidly deteriorating on Sao Miguel and Santa Maria islands. At midnight Azores Time, the latter reported a sustained wind of 34 mph, along with southeast winds. Since that time, the barometric pressure has continued to fall as the center of Gordon approaches. Sustained hurricane force winds are not far offshore the island, based on the earlier wind report and extrapolation of the National Hurricane Center's estimated wind field.



Figure 2. Hurricane Gordon's wind field as of the 8:00 PM AST intermediate advisory from the National Hurricane Center.

Since Gordon is a fast mover, the period of hurricane force winds on the island will be very small, perhaps no more than two hours. However, wind gusts could exceed 100 mph in some of the higher elevations, particularly given Gordon's quick forward progress. As I understand it, the Azores have strict building codes in place so that wind damage is never really an issue. Also, the Azores are a volcanic archipelago, which obviously means increased elevation with height. This allows for an easy route to avoid incoming storm surge. At most, it appears that Gordon will deliver a brief period of damaging winds -- hurricane force winds on Santa Maria -- on Sao Miguel and Santa Maria Island as it accelerates toward the east-northeast. These winds will likely cause considerable damage to the power grid in these areas, and residents should anticipate some power outages. Power should be restored relatively quickly once Gordon moves away.

In agreement with the various global models, Gordon is forecast to become extratropical in about 36 hours. Given the extreme baroclinicity of the surrounding environment, I would not be surprised if this occurred somewhat sooner.

Gordon remains well-embedded within a powerful north Atlantic storm system. That pattern should result in a continued east-northeast movement over the next day or so, followed by a turn to the east after that. Gordon is forecast to dissipate by late Tuesday, though it is possible it could occur a little sooner.

It now appears unlikely that Gordon's post-tropical remnant will deliver any appreciable impact to western Europe, as it is forecast to be overtaken by a larger extratropical low near the British Isles.

5-day intensity forecast

INITIAL 08/20 0300Z 70 KT 80 MPH
12 hour 08/21 1200Z 55 KT 65 MPH
24 hour 08/22 0000Z 45 KT 50 MPH
36 hour 08/22 1200Z 35 KT 40 MPH...EXTRATROPICAL
48 hour 08/23 0000Z 30 KT 35 MPH...EXTRATROPICAL
72 hour 08/24 0000Z...DISSIPATED

5-day forecast track



Figure 3. My 5-day forecast track for Gordon.

Watches and warnings

A HURRICANE WARNING IS IN EFFECT FOR...
* THE EASTERN AZORES

A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED
SOMEWHERE WITHIN THE WARNING AREA. PREPARATIONS TO PROTECT LIFE AND
PROPERTY SHOULD HAVE BEEN COMPLETED.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.



Invest 94L

A well-defined tropical wave located in the central Atlantic about 1300 miles east of the Lesser Antilles ("94L") continues to show signs of organization, and is a threat to become a tropical depression or tropical storm over the next day or two. Should it attain storm status, it would receive the name "Isaac". Buoy data in the area shows a broad area of cyclonic turning is accompanying the wave at the surface. Nighttime satellite imagery suggests two vorticity maximums -- one near 15.5N 42.7W, and another farther south, near 15.0N 41.5W. The latter feature appears to be becoming more dominant, and this is what ATCF sees as well.



Figure 4. Latest infrared satellite image of Invest 94L. Image credit: NOAA

On the large scale, the environment ahead of the system is seemingly favorable, with warm SSTs and little vertical shear. However, there is some dry air to the west of the system, and I am waiting to see if 94L ingests some of this dry air as its center consolidates. So far, 94L appears to be doing well in warding off the subsident airmass; water vapor imagery and TPW data from UW-CIMSS shows that 94L is pushing the wall of dry air steadily westward. The tropical wave in front of Invest 94L may also be assisting in this endeavor. One potential negative is the area of convection lying to the south of 94L, which appears to be distinct from the tropical wave circulation. This activity appears to a direct appendage of the Intertropical Convergence Zone (ITCZ). Convection within this band continues to pulse, while 94L continues to struggle. Experience has shown that this type of setup isn't generally considered to be a conducive one for wave amplification/intensification, as the active convection acts as a completing influence to vorticity and storm relative inflow. Given this and the close proximity to dry air, I am not expecting 94L to rapidly develop overnight. I mentioned yesterday that a possibly more viable alternative is for the system to withhold on developing until west of 50W.

The long-term prospect remains highly uncertain, with the global models having a difficult time resolving the evolution of the synoptic pattern at longer ranges. At this time, I am not going to speculate, as I have enough to deal with already. In about three or four days, the models are suggesting that a trough will begin to amplify over the eastern United States that will be capable of turning the system northward. Where it turns northward remains to be seen. Alternatively, the trough could remain weak enough and shallow so that it bypasses 94L, allowing it to continue westward into the Gulf of Mexico. Given the persistent ridging pattern across the Caribbean Sea this year, I tend to favor the latter solution. The GFS has been rather insistent on moving 94L in a swift fashion similar to Ernesto and Helene, which further alludes to a continuation of that thematic pattern.

Regardless of whether this system becomes a tropical cyclone, now is a good time to remind everyone living along the United States coastline to review and replenish their hurricane preparedness plans, as 94L does have long-term potential to impact the United States anywhere from the Gulf Coast to New England.

A NOAA G-IV jet is scheduled to fly a synoptic environment surveillance mission in the path of the tropical wave on Thursday, which will be critical in resolving the model issues with the eastern US trough.

Probability of development in 48 hours: 80%



Invest 95L

An area of disturbed weather has formed over the southwestern Gulf of Mexico in association with the remnants of Helene. Satellite images show a small but well-defined cyclonic circulation centered off the coast of Mexico near 23N 97W, but convection remains minimal around this gyre. Inland and oceanic surface observations indicate that 95L possesses a rather vigorous surface circulation.



Figure 5. Latest infrared satellite image of Invest 95L. Image credit: NOAA

Moderate northwesterly upper flow along the back side of the unseasonably deep longwave trough stretching from the Texas coast to Atlantic Canada appears to be impacting the system, which is limiting development at this time. Looking at the GFS, an anticyclone is forecast to develop over the system as it moves into Mexico. Strong northerly to northwesterly flow is seen covering much of the Gulf of Mexico afterward. I theorize that this is because as 95L moves inland, it loses its anticyclone, allowing for strong shear to penetrate the Gulf waters. If 95L moves more poleward, which is a possibility given the current upper air pattern over the northern Gulf Coast, the anticyclone would likely follow it, although some residual shear would likely prevent rapid strengthening.

Based on a homogeneous comparison of the 850 and 500 mb pressure fields within the global models, as well as 0z upper air data over the northern Gulf Coast, a weaker system would tend to follow the low-level flow back into Mexico, while a stronger system with more convection would tend to move more poleward. In the meantime, the synoptic pattern is still in the process of amplifying over the Gulf -- the longwave trough previously alluded to has barely moved, while low-level ridging remains in place to the north of 95L. The combination of these features will likely result in a slow and erratic motion for the next day or so, and 95L is likely to move in fits and wobbles until a more definitive steering trend is established.

Interests along the western and northern Gulf Coasts should monitor the progress of this system.

It should be noted that even if 95L develops, it could still be rather slow moving based on prior runs of the global models that took it more poleward. This may create more of an opportunity for 95L to erode the western Atlantic ridge and allow Invest 94L to move more northward in the longer term.

Probability of development in 48 hours: 20%



Eastern Atlantic tropical wave

A tropical wave has emerged off the west coast of Africa. This wave has not yet been dubbed an invest. Upper-level winds are not currently too prohibitive, but it is possible that as 94L matures, its associated anticyclone could inflict northerly shear on the system, limiting development potential at longer ranges.

Probability of development in 48 hours: 10%



Invest 96E

A small area of low pressure located along the southwest coast of Mexico ("96E") is producing disorganized showers and thunderstorms.



Figure 6. Latest infrared satellite image of Invest 96E. Image credit: NOAA

The system is experiencing strong easterly shear, which is forecast to persist for at least the next 48 hours, and possibly longer. Surrounding steering currents are rather weak, and the system is forecast to move only slowly northwestward or northward. It could move inland over southern Baja or southwest Mexico over the next few days. Dry air also awaits it the farther north it gets. Development of this system is not currently anticipated, but it could produce locally heavy rainfall over portions of southwest Mexico over the next day or two.

Probability of development in 48 hours: 10%

2012 Atlantic hurricane season 2012 East Pacific hurricane season Hurricane Gordon Invest 94L Invest 95L Invest 96E

Updated: 5:11 AM GMT on August 20, 2012

Permalink

Tropical weather analysis - August 18, 2012

By: KoritheMan, 4:33 AM GMT on August 19, 2012

Gordon

Hurricane Gordon has rapidly intensified tonight, and is now just shy of being a major hurricane. As of the latest NHC advisory, the following was available:

Wind: 110 mph, with higher gusts
Location: 34.5°N 33.9°W
Movement: E at 22 mph
Pressure: 965 mb
Category: 2 (Saffir-Simpson Hurricane Scale)

An SSMIS microwave pass back near 2300 UTC showed that Gordon possessed a well-defined eye and an eyewall that was open to the south. Since that time, Gordon has maintained a well-organized satellite presentation, with the south side of the eye no longer exposed. The eye is also quite visible using conventional satellite pictures.



Figure 1. Latest infrared satellite image of Hurricane Gordon. Image credit: NOAA

I'm not easily impressed, but Gordon's abrupt intensification episode today was of great surprise. Let's look at a couple things here: first, it is sitting under SSTs of only around 26C. It is possible that these waters are a little warmer than the SHIPS sea surface temperature algorithm would suggest, as water temperatures of barely 80F are not typically thought to be major hurricane breeding grounds. On the other hand, waters warmer than 26C this far east is equally unlikely, and I lack direct evidence either way. One could also hypothesize that cold upper tropospheric temperatures are helping to produce a stronger vertical instability gradient and counteracting the otherwise negative effects of the cool waters, which seems fairly supported by earlier AMSU data. A similar synoptic situation occurred when Chris became a hurricane over the cold north Atlantic near 40°N back in June. Although such an event makes sense meteorologically, it is rather difficult to determine if these cold troposphere/lukewarm SSTs intensification sprees will actually occur. In many cases they don't.

Gordon has likely peaked its intensity, as it is quickly heading toward cooler water and high wind shear. However, given the cold upper-level temperatures alluded to on the AMSU pass, I have to assume the hurricane will maintain considerable strength as it approaches the Azores. In light of the strengthening trend that was observed today, hurricane warnings have been issued for the central and eastern Azores. Although Gordon is forecast to remain well south of Sao Miguel, tropical storm force winds will probably still be experienced there, especially if Gordon more northward than anticipated. In addition, it is forecast to pass close enough so that it could deliver a brief period of hurricane force winds could occur on Saint Maria. Rainfall will potential remains limited given the hurricane's fast forward motion.

The dynamical and statistical models -- in other words the whole shebang -- are in agreement that Gordon will dissipate by Wednesday or Thursday. It is still possible that Gordon will bring some remnant shower activity to sections of Portugal and Spain after it dissipates, although how much residual energy remains with post-tropical Gordon at that time remains to be seen.

It should be noted that any slight deviation to the north of the current forecast track could place the core of the hurricane over Santa Maria island.

Synoptic data indicates that Gordon continues to accelerate eastward, embedded within a well-defined westerly steering current between a strong low near the British Isles and the northern periphery of the Atlantic subtropical ridge. The combination of these features is expected to result in a turn to the east-northeast tomorrow. The global models remain in excellent agreement on this. As such, there has been no significant change to the synoptic reasoning or forecast track. Gordon is forecast to pass through the Azores during the early hours of Sunday morning as a minimal hurricane. Gordon is a rare hurricane threat for those islands, and preparations to protect life and property, particularly in the hurricane warning area, should be promptly completed. Winds could be especially dangerous in areas of elevated terrain. Near the end of the period, the hurricane is forecast to slow and turn toward the southeast as it comes under the influence of a building low- to mid-level ridge over the north Atlantic. My forecast agrees rather strongly with the National Hurricane Center's, based on multi-layer steering analyses as well as real-time synoptic trends.

5-day intensity forecast

INITIAL 08/19 0300Z 95 KT 110 MPH
12 hour 08/19 1200Z 90 KT 105 MPH
24 hour 08/20 0000Z 80 KT 90 MPH
36 hour 08/20 1200Z 65 KT 75 MPH
48 hour 08/21 0000Z 50 KT 60 MPH
72 hour 08/22 0000Z 35 KT 40 MPH...EXTRATROPICAL
96 hour 08/23 0000Z 25 KT 30 MPH...EXTRATROPICAL
120 hour 08/24 0000Z...DISSIPATED

5-day track forecast



Figure 2. My 5-day forecast track for Gordon.



Helene

Helene has dissipated inland over central Mexico. Helene is quickly approaching the rugged Sierra Madre mountain range, and some erratic motion is possible. The global models are less enthusiastic with regards to regeneration today, and atmospheric conditions aren't expected to be particularly conducive either, with a consistent flurry of northwesterly shear forecast over the northwestern Gulf of Mexico by the GFS. While it is still possible that Helene moves back over water in about three days, even the GFS which previously regenerated the system and meandered it off the Texas coast, now pulls it back into Mexico, providing it little opportunity to strengthen. The remainder of the global models are equally unenthusiastic, and some don't even show Helene moving back over water. Some residual mid-level energy and associated cloudiness could spread over portions of the northern Gulf Coast over the next five days, which could perhaps enhance the rainfall potential in those areas, but overall, Helene appears less likely to redevelop as the center surface center slides back into Mexico behind the advancing mid-level center.

Probability of development in 48 hours: Near 0%



94L

A vigorous tropical wave over the eastern Atlantic centered about 600 miles west of the Cape Verde Islands is showing signs of organization. The low-level center appears to be consolidating along the south side of the convection, which has been showing some banding.



Figure 3. Latest infrared satellite image of Invest 94L. Image credit: NOAA

In the near-term, atmospheric conditions appear favorable for development. Underlying sea surface temperatures are warm, vertical shear is seemingly weak, and the overall environment is moist. However, I see an unfavorable environment -- at least as far as thermodynamics are concerned -- ahead of 94L. Water vapor imagery shows a large expanse of dry air covering the central Atlantic from 40W to 60W. The SHIPS responds to this by forecast ambient relative humidity values to decrease by 48 hours. Nevertheless, the upper-level wind pattern over the system is forecast to be anticyclonic, so 94L will probably not dissipate. I am calling for continued gradual development, and 94L could become a tropical depression over the next day or two. An alternate scenario is that it doesn't develop until at or west of 50W. This is probably the more realistic scenario given the pattern this year, but we shall see.

Regardless, this system poses a long-range threat to the Lesser Antilles, and interests there should be anticipating the possibility of a tropical storm affecting them in about 5 - 6 days. This system could either pass north of the Leeward Islands, or enter the eastern Caribbean. Given recent model depictions, I tend to side with the latter camp for now. It is uncertain if 94L would be able to make full transit of the Caribbean before recurving. Perhaps if it stays weak, or if Hispaniola significantly disrupts it. Its ultimate track also hinges heavily on the evolution of the synoptic pattern. The models have been trending back and forth between a strong ridge and strong troughing. I still think the pattern favors an eventual recurvature, but when and where remains to be seen.

Probability of development in 48 hours: 60%

2012 Atlantic hurricane season Hurricane Gordon Invest 94L Helene

Updated: 4:33 AM GMT on August 19, 2012

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Tropical weather analysis - August 17, 2012

By: KoritheMan, 3:41 AM GMT on August 18, 2012

Gordon

Tropical Storm Gordon continues racing eastward toward the Azores. As of the latest NHC advisory, the following was posted on the storm:

Wind: 70 mph, with higher gusts
Location: 34.2°N 42.1°W
Movement: E at 18 mph
Pressure: 990 mb

Gordon is not quite as ragged on satellite imagery as it was 12 hours ago. There is more deep convection encompassing the center, and recent microwave data suggested a better-defined inner core structure, though the mid-level circulation remains displaced slightly to the northeast, probably due to continued westerly shear.



Figure 1. Latest infrared satellite image of Tropical Storm Gordon. Image credit: NOAA

Gordon still has about another day over waters of 26C. Since the strongest of the shear appears to be avoiding Gordon for the moment, some intensification is possible in the short-term. Gordon is forecast to become a hurricane before reaching cooler waters and encountering a sharp increase in westerly shear. Looking at the cyclone phase diagrams on FSU, the global models are in general agreement that Gordon will lose tropical characteristics shortly after passing through the Azores during the wee hours of Monday morning. No reintensification is expected once the transition is complete, as Gordon will be well to the south of a strong baroclinic low over the north Atlantic. Instead, a gradual decay is forecast while Gordon moves southeastward under the nose of the subtropical ridge.

There isn't much to say track wise. Gordon is well-embedded within the westerlies, and is moving toward the east. A turn to the east-northeast is forecast over the next 24 hours, followed by a gradual bend to the east-southeast with a decrease in forward speed subsequent to crossing the Azores. Model guidance remains in good agreement with this scenario. It is possible that Gordon could eventually bring some locally heavy showers and gusty winds to portions of Portugal and Spain late next week.

Although Gordon is forecast to be weakening by the time it approaches the Azores, some rather strong winds are possible in the archipelago, especially in areas of rugged terrain. Because of the forecast track, the government of Portugal has issued a tropical storm watch for portions of the island chain. Tropical storm warnings will likely be required for a portion of these islands on Saturday, since the average lead time for such warnings is 36 hours.

5-day intensity forecast

INITIAL 08/18 0300Z 55 KT 65 MPH
12 hour 08/18 1200Z 60 KT 70 MPH
24 hour 08/19 0000Z 65 KT 75 MPH
36 hour 08/19 1200Z 60 KT 70 MPH
48 hour 08/20 0000Z 55 KT 65 MPH
72 hour 08/21 0000Z 45 KT 50 MPH...EXTRATROPICAL
96 hour 08/22 0000Z 35 KT 40 MPH...EXTRATROPICAL
120 hour 08/23 0000Z 25 KT 30 MPH...EXTRATROPICAL

5-day track forecast



Figure 2. My 5-day forecast track for Gordon.

Watches and warnings


A TROPICAL STORM WATCH IS IN EFFECT FOR...
* THE CENTRAL AND EASTERN AZORES

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.



Helene

Tropical Storm Helene formed over the southwestern Gulf of Mexico today from the remnants of Tropical Depression Seven. As of the latest NHC advisory, the following was provided:

Wind: 40 mph, with higher gusts
Location: 21.0°N 96.4°W
Movement: N at 2 mph
Pressure: 1006 mb

Helene is looking rather sickly this evening, to say the least. There is virtually no deep convection anywhere within the center. Instead, the only convection is confined to shallow bands over southern Mexico. Satellite images suggest that these bands are diminishing, which could be an indication that Helene will soon begin to reestablish some convection over the center.



Figure 3. Latest infrared satellite image of Tropical Storm Helene. Image credit: NOAA

The center of Helene is difficult to locate, but reconnaissance observations and surface data suggest that it remains offshore. Earlier microwave data acquired just after 0000 UTC also indicated this. Sometimes satellite and radar data can offer a different perspective, but alas, nothing beats state of the art real-time observations.

Atmospheric conditions are seemingly favorable for strengthening, with the only obvious impediment being the limited amount of time over water that Helene has. After moving inland, Helene should quickly dissipate over the rugged terrain of Mexico, but since it is not forecast to penetrate too far inland, I have held the winds a little higher through the next two days than the National Hurricane Center, as the close proximity to the Gulf of Mexico could allow for convection to linger over the open water. That convection could help to maintain some strong winds even after the system moves inland. Regardless, Helene is forecast to dissipate in three days or less. Although Helene possesses a rather vigorous circulation as denoted by surface data, the current lack of convection and limited time over water argues against it becoming a hurricane. However, the funneling effect of Mexico's mountains, especially in this location, could help to locally enhance winds in any significant convective bursts. Thus, it would not be surprising to see Helene become a little stronger than forecast before it moves inland.

Water vapor and 0z upper air data suggest that Helene is south of a low- to mid-level ridge over the northern Gulf of Mexico, while a trough is amplifying over the east coast. The global models gradually weaken this feature as a well-defined trough begins to manifest over the central and southern United States. Although Helene is forecast to be inland by the time the trough begins to exist, the sheer scope of the trough as depicted in the models suggests that the cyclone will not get very far inland before it begins to move more poleward. Indeed, the global models actually take Helene back over the Gulf of Mexico in about three to four days. The CMC suggests an eventual threat to the northern Gulf Coast, while the GFS suggests a slow northward track into Corpus Christi. The ECMWF model quickly pulls Helene back into Mexico under a building ridge, which doesn't seem entirely realistic given the trough. While I doubt Helene moves as far northward as the CMC suggests, given the models and the strength of the trough, it seems a bit unlikely that Helene will continue moving toward the west after landfall. It appears that Helene will slow significantly in the event it moves back over water.

5-day intensity forecast

INITIAL 08/18 0300Z 35 KT 40 MPH
12 hour 08/18 1200Z 40 KT 45 MPH
24 hour 08/19 0000Z 45 KT 50 MPH...ON THE COAST
36 hour 08/19 1200Z 30 KT 35 MPH...INLAND
48 hour 08/20 0000Z 30 KT 35 MPH...INLAND
72 hour 08/21 0000Z...DISSIPATED

5-day track forecast



Figure 4. My 5-day forecast track for Helene.

Watches and warnings

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE COAST OF MEXICO FROM BARRA DE NAUTLA TO LA CRUZ

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN THE
NEXT 24 HOURS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA ...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.



Invest 94L

A tropical wave in the far eastern Atlantic about 300 miles southwest of the southern Cape Verde Islands ("94L") is producing limited shower activity, mainly to the west of the wave axis due to easterly shear. This is not uncommon for westward-propagating tropical waves in this part of the Atlantic.



Figure 5. Latest infrared satellite image of Invest 94L. Image credit: RAMMB

Conditions should become more favorable over the next 24 hours, and the GFS suggests that an anticyclone will accompany 94L for the next five days. Indeed, the 18z run of this model turns it into a powerful hurricane in the eastern Caribbean before moving it northwest. Although the thematic pattern this year has so far closely mirrored the last three years, with a mean trough over the eastern United States capable of inducing recurvature, I am not completely convinced that 94L will recurve. The GFS has in general trended farther west over the last few cycles, and the Euro has been rather adamant about a threat to the Lesser Antilles. While both models still recurve the system, there are indications within the GFS ensembles as well as the synoptic pattern portrayed in those models that recurvature may not initiate until at least 70W, possibly as far west as 80W. This is close enough that the east coast of the United States could be a viable target in the extreme long-range. However, such long-range predictions are prone to some rather large errors, and one should not make their plans based off them.

Regardless, it appears that this system could eventually become a hurricane, and quite possibly a major hurricane, as suggested by the GFS and the purely statistical guidance. It should be noted that the 850 mb vorticity fields in the global models indicate that a weaker system would tend to move more west. So if 94L remains weak throughout the next several days, we could be looking at another long-range threat to the Caribbean. As usual, there is some dry air in the central Atlantic, which could be an inhibitor for strengthening in a few days.

Probability of development in 48 hours: 20%

2012 Atlantic hurricane season Tropical Storm Gordon Tropical Storm Helene Invest 94L

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Tropical weather analysis - August 17, 2012

By: KoritheMan, 8:30 AM GMT on August 17, 2012

Gordon

Although operationally Tropical Storm Gordon is near hurricane strength as of the 11:00 PM NHC advisory, the new advisory is being written as I write, and the official ATCF estimate is down to 65 mph. Just extrapolate from the coordinates I list below:

Wind: 70 mph, with higher gusts
Location: 34.6°N 50.3°W
Movement: E at 17 mph
Pressure: 995 mb

There is no evidence of an eye on conventional satellite imagery at this time. Not even a warm spot. A TMI overpass taken near 0300 UTC indicated a rather ragged inner core structure. Gordon consists of some ragged central convection with numerous banding features to the north.



Figure 1. Latest infrared satellite image of Tropical Storm Gordon. Image credit: NOAA

Satellite images and UW-CIMSS upper air data suggest that Gordon has pretty much lost the anticyclone that was trailing it. Westerly shear is beginning to affect the tropical storm, and a little bit of dry air appears to have gotten entrained in the western semicircle. Conditions appear to have become unfavorable a little earlier than anticipated. There is still a chance Gordon recovers enough to restrengthen a bit, but it is no longer expected to become a hurricane. In about 24 hours, Gordon is forecast to cross the 26C isotherm, as well as encounter a sharp increase in westerly shear. The increase in shear will likely result in dry air being entrained into the cyclone circulation, which should hasten the extratropical transition process once the cyclone crosses the Azores. The global models remain in good agreement with this scenario. Given the seemingly deteriorating satellite appearance, I am not currently forecasting anything higher than 60 kt.

Gordon is well-embedded within mean westerly flow in advance of a deep-layer trough to the west and in the wake of a powerful extratropical low over the north Atlantic. The global models agree on a turn toward the east-northeast on Sunday morning as Gordon nears the Azores. After passing through that archipelago, there is some disagreement amongst the various models with regard to the forward speed of the cyclone, but all in all it appears that Gordon will move generally eastward subsequent to impacting the Azores.

Beyond day five, there is some indication within the models that Gordon could slow and move southward under the nose of the subtropical ridge.

Interests in the Azores should prepare for the possibility of hurricane force wind gusts, particularly in areas of higher terrain on the islands. Rainfall potential will be limited by Gordon's fast forward speed.

5-day intensity forecast

INITIAL 08/17 0600Z 55 KT 65 MPH
12 hour 08/17 1800Z 60 KT 70 MPH
24 hour 08/18 0600Z 60 KT 70 MPH
36 hour 08/18 1800Z 55 KT 65 MPH
48 hour 08/19 0600Z 50 KT 60 MPH
72 hour 08/20 0600Z 45 KT 50 MPH
96 hour 08/21 0600Z 40 KT 45 MPH...EXTRATROPICAL
120 hour 08/22 0600Z 35 KT 40 MPH...EXTRATROPICAL

5-day track forecast



Figure 2. My 5-day forecast track for Gordon.

2012 Atlantic hurricane season Tropical Storm Gordon

Updated: 8:34 AM GMT on August 17, 2012

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Tropical weather analysis - August 15, 2012

By: KoritheMan, 2:22 AM GMT on August 16, 2012

Tropical Depression Eight

The strong tropical wave we have tracked over the last week or so has finally acquired enough convective organization to be considered a tropical depression. As of the initial advisory from the National Hurricane Center, the following was posted on the tropical cyclone:

Wind: 35 mph, with higher gusts
Location: 29.9°N 55.1°W
Movement: N at 13 mph
Pressure: 1013 mb

An earlier SSMIS overpass just before 0z showed a well-organized inner structure, especially for a system at this latitude. Satellite animations clearly show a much more organized system compared to earlier. Banding features are evident, particularly west and north of the center, and the center itself is generating some relatively cold-topped convection. I've seen colder, but given the high environmental pressures and the nearby dry air, I think the depression is doing well for itself.



Figure 1. Latest infrared satellite image of Tropical Depression Eight. Image credit: NOAA

The cyclone is currently over SSTs near 28C, and underlying water temperatures are expected to remain within the 26C range for about the next three days. A sharp and dramatic increase in westerly shear is forecast, however, is forecast around roughly the same time, which should initiate a weakening trend at longer ranges. This weakening is not expected to be rapid, however, because the cyclone will be accelerating in the same direction as the shear vector. Instead, the shear will primarily help to imbue extratropical transition when combined with cool waters. The SHIPS/LGEM make the system a hurricane in about three days just before the foreseen onslaught of westerly shear. While it is certainly possible that TD Eight will become a hurricane, I am uncomfortable indicating such until I see precisely how the newly-developed tropical cyclone wards off the surrounding dry air. Water vapor imagery shows a small upper low swirling just to the northwest of the depression. However, this feature is not expected to cause an appreciable increase in vertical shear, and it is appears to be outrunning the depression.

The cyclone could become extratropical in about five days. This in good agreement with the global models, which in general show an accelerating extratropical cyclone reaching the Azores early Monday.

Large-scale observations reveal that the depression is well-embedded within high-latitude southerly flow as a large trough amplifies to its west. A secondary trough moving off the east coast of the United States will reinforce the former trough, a pattern which ultimately heralds recurvature. Model guidance is in rather remarkable agreement out to five days, giving high confidence to the track forecast. The 0z model consensus shifted fairly significantly westward. This increases the threat to the Azores, and interests there should closely monitor the progress of this system through Monday. The models do not weaken then post-tropical Gordon all that much on Monday when it is forecast to be lashing the Azores. At that point, it could still be strong enough to bring hurricane force wind gusts to certain areas of the archipelago. I agree pretty strongly with the forecast track from the National Hurricane Center.

5-day intensity forecast

INITIAL 08/16 0000Z 30 KT 35 MPH
12 hour 08/16 1200Z 35 KT 40 MPH
24 hour 08/17 0000Z 40 KT 45 MPH
36 hour 08/17 1200Z 50 KT 60 MPH
48 hour 08/18 0000Z 55 KT 65 MPH
72 hour 08/19 0000Z 60 KT 70 MPH
96 hour 08/20 0000Z 55 KT 65 MPH
120 hour 08/21 0000Z 50 KT 60 MPH...EXTRATROPICAL

5-day track forecast



Figure 2. My 5-day forecast track for Tropical Depression Eight.



Elsewhere

The global models continue to suggest the possibility of a tropical depression near the Cape Verde Islands in about 5 days from a tropical wave. Given the time of year and the model consensus, there really is no reason not to expect this.

The GFS has continued to indicate the possibility of development in the western Gulf of Mexico, most likely in the Bay of Campeche. This is forecast to occur in about a week from now from a stalled frontal boundary. Looking at the 500 mb pattern within the global models, any such system would likely move very slowly northwest toward northeast Mexico or south Texas. It takes a lot of deep southerly flow to pull a system out of the Bay of Campeche, especially this time of year.

I remain wholly unenthusiastic about this development.

2012 Atlantic hurricane season Tropical Depression Eight

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Tropical weather analysis - August 15, 2012

By: KoritheMan, 6:11 AM GMT on August 15, 2012

Remnants of TD7

The remnants of Tropical Depression Seven appear to have moved inland over Central America near the Honduras/Nicaragua border based on satellite and surface observations.



Figure 1. Latest infrared satellite image of former Tropical Depression Seven. Image credit: NOAA

The system is expected to move generally west-northwestward at about 15 mph over the next few days. Global model guidance suggests that it could enter the far southern Bay of Campeche on Thursday. Although the upper wind pattern looks favorable at that time, it is expected to move inland on Friday or Saturday, and the system will more than likely fail to significantly redevelop due to limited time over water.

Regardless of development, the system will continue to produce locally heavy rains and possible flooding over regions of Central America over the next day or two. These rains will begin to overspread portions of southern Mexico by the weekend.

Probability of development within 48 hours: Near 0%



Invest 93L

A tropical wave over the central Atlantic centered about 800 miles southeast of Bermuda ("93L") is producing disorganized showers and thunderstorms to the west of the wave axis. Based on satellite imagery, the circulation associated with this wave appears to be becoming better-defined.



Figure 2. Latest infrared satellite image of Invest 93L. Image credit: NOAA

93L is well-embedded in a weakness in the subtropical ridge along 50W. An unseasonably deep trough is in the process of amplifying across the east coast of the United States. The various models suggest that this trough will ultimately pick up 93L and recurve it well east of Bermuda on Thursday or Friday. Given current trends, I think this could occur on Thursday.

While upper-level winds are currently favorable for development, water vapor imagery suggests that dry air still surrounds the system. Although the SHIPS gradually moistens the ambient environment over the next few days, the relative humidity parameter at 0z never goes above 57%. I think SHIPS is overdoing it a notch, and I think there is potential for 93L to become a tropical depression within the next few days. This is supported by the global model forecasts, which in general bring the system to tropical cyclone status, albeit small, on Friday while the system moves toward the Azores. If 93L obtains a better structure throughout the day today (Wednesday), it is possible that genesis could occur about a day sooner.

Interests in the Azores should begin to monitor the progress of this system, as most of the models bring it through that archipelago on Monday.

Probability of development in 48 hours: 60%



Hector

Hector continues to meander as a weak tropical storm. As of the latest NHC advisory, the following was posted on the storm:

Wind: 40 mph, with higher gusts
Location: 17.2°N 115.1°W
Movement: W at 2 mph
Pressure: 999 mb

Hector remains disorganized on satellite images, although some convection has recently developed to the southwest of the low-level center. SHIPS diagnoses 20 kt of easterly shear over the cyclone at 0z. CIMSS had a similar estimate.



Figure 3. Latest infrared satellite image of Tropical Storm Hector. Image credit: NOAA

The GFS/SHIPS suggests that this relentless shear pattern is not going to abate through at least the next few days. I personally think the shear is going to abate some. But even if it does abate, Hector will also be passing over cooler waters and their associated stratocumulus cloud field as early as Thursday morning, which should continue the current weakening trend. Hector is expected to cease being a tropical cyclone in about 48 hours, and dissipate completely by 96 hours.

Water vapor imagery and objective steering analyses indicates that Hector is embedded in a col region between an upper low off the California coast and a narrow low- to mid-level ridge over central Mexico. The models respond to this pattern by forecasting Hector to turn northwestward very soon. Given that steering currents have been rather weak with this particular cyclone, I have opted to go a little slower with the forward motion, but agree that Hector should turn soon.

5-day intensity forecast

INITIAL 08/15 0600Z 35 KT 40 MPH
12 hour 08/15 1800Z 30 KT 35 MPH
24 hour 08/16 0600Z 30 KT 35 MPH
36 hour 08/16 1800Z 30 KT 35 MPH
48 hour 08/17 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
72 hour 08/18 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
96 hour 08/19 0600Z...DISSIPATED

5-day track forecast



Figure 4. My 5-day forecast track for Hector.

Gulf of Mexico development on the horizon?

Over the last several days, the GFS has been consistent in developing a weak tropical cyclone somewhere in the western Gulf of Mexico over the next 7 - 10 days. I am unsure where this development is supposed to come from; following the 850 mb vorticity tracker on that model, I am hard-pressed to believe it is the remnants of Tropical Depression Seven. It could perhaps be from the tropical wave approaching the Windward Islands, given the timeframe on that particular model. It also appears that a southward-drifting cold front stalls out and at least partially contributes to an area of disturbed weather in that area.

Since the GFS is really the only one that is enthusiastic about this development, I am not getting excited right now, but it bears watching.

The GFS/ECMWF suggest that a tropical depression could form in the eastern Atlantic from a tropical wave over the next 5 - 7 days. The GFS suggests recurvature, while the Euro suggests a more westward motion. It is impossible to favorably pick one over the other at this time.

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Depression Seven Invest 93L Tropical Storm Hector

Updated: 6:19 AM GMT on August 15, 2012

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Tropical weather analysis - August 13, 2012

By: KoritheMan, 4:26 AM GMT on August 14, 2012

Remnants of TD7

A tropical wave, the remnants of Tropical Depression Seven, continues racing across the central Caribbean Sea. This system has lost much of its thunderstorm activity today. Blending satellite, microwave, and scatterometer fixes, the wave axis appears to be located near 14.3N 78.1W, although I am not certain of this.



Figure 1. Latest infrared satellite image of former Tropical Depression Seven. Image credit: NOAA

Either way, I do not see a particularly favorable environment ahead of the system. It is still well-embedded within the strong trade winds, which is inhibiting low-level convergence and thus convective development. A continuation of this general motion is expected with only a slight decrease in forward speed is expected. The system could enter the far southern Bay of Campeche on Thursday, but the upper air environment on the GFS does not look conducive to regeneration, with a strong westerly flow prevailing south of 25N.

The disturbance may bring heavy rain to portions of Central America and the Yucatan Peninsula over the next several days, but little overall development is anticipated, brief convective bursts notwithstanding.

Probability of development in 48 hours: Near 0%



Invest 93L

A tropical wave over the central Atlantic about 1075 miles east-northeast of the Leeward Islands ("93L") is producing limited shower activity. While upper-level winds are favorable for development, water vapor imagery and the convective pattern suggests that the system is still embedded in a subsident airmass.



Figure 2. Latest infrared satellite image of Invest 93L. Image credit: NOAA

The GFS and CMC are predicting that the wave could develop as it find itself beneath a more favorable synoptic environment as it recurves well east of Bermuda on Thursday. Looking at water vapor imagery, I do see more vertical instability available in that area, and 93L could attempt to tap into this moisture, which would promote organization given the favorable upper-level wind pattern shown on the models, which should at the very least be diffluent. I see no reason to discount this at this time; experience has shown that it doesn't take much for weak and sprawling systems like 93L to resurrect themselves once they find a more favorable environment. The system could pose a long-range threat to the Azores, although whether it is tropical or extratropical at that time remains to be seen.

This wave should continue moving west-northwest to northwest at 15 to 20 mph over the next couple of days. A break is noted in the subtropical ridge near 60W between the western and eastern Atlantic ridges, so recurvature should begin close to this longitude.

Probability of development in 48 hours: 10%



Hector

Tropical Storm Hector poses no threat to land. As of the most recent NHC advisory, the following was available on the tropical storm:

Wind: 45 mph, with higher gusts
Location: 18.0°N 113.2°W
Movement: W at 6 mph
Pressure: 993 mb

After strengthening a bit today, convection has been well-removed from the center again due to easterly shear. However, a burst has recently developed about 75 miles to the southwest of the estimated low-level center.



Figure 3. Latest infrared satellite image of Tropical Storm Hector. Image credit: NOAA

The easterly shear that has prevented significant development of Hector is expected to continue for the next couple of days. Thereafter, a reduction in the shear is forecast, but the cyclone will be moving over cooler waters by that time. Model guidance is in good agreement with this. In fact, given the shear, a slow decay will likely occur before then. Hector is forecast to become a remnant low in about four days.

The tropical storm remains south of a small but well-established low- to mid-level ridge. The global models suggest this high will gradually weaken over the next couple of days as Hector comes under the influence of a mid-latitude trough that is currently amplifying off the California coast. The global models generally agree with this evolution, but they differ in regards to the depth and amplitude of the upper low and its attendant frontal zone, as well as how Hector will interact with it. The ECMWF and GFDL have a sharper and faster northward turn, while the GFS is farther to the west and much slower. While I am not willing to go quite as slow as the GFS, I tend to prefer its solution overall. My forecast track is in good agreement with that of the National Hurricane Center. Deceleration is forecast near the end of the forecast period as Hector becomes a shallow system steered by the low-level flow.

5-day intensity forecast

INITIAL 08/14 0300Z 40 KT 45 MPH
12 hour 08/14 1800Z 40 KT 45 MPH
24 hour 08/15 0300Z 35 KT 40 MPH
36 hour 08/16 1800Z 35 KT 40 MPH
48 hour 08/17 0300Z 35 KT 40 MPH
72 hour 08/18 0300Z 30 KT 35 MPH
96 hour 08/19 0300Z 30 KT 35 MPH...POST-TROP/REMNANT LOW
120 hour 08/20 0300Z 25 KT 30 MPH...POST-TROP/REMNANT LOW

5-day track forecast



Figure 4. My 5-day forecast track for Hector.



Invest 95E

A small area of low pressure over the eastern Pacific is along the southeast coast of Mexico. Although atmospheric conditions are relatively favorable, the close proximity to the coast suggests that any development will be slow. Additionally, since at this point the system is less than 75 miles offshore, it would not take much of a subtle northwestward motion to bring the low ashore and eliminate development potential entirely. In fact, a study of satellite imagery suggests the low could already be inland. On the other hand, earlier ASCAT data still suggested the system was offshore. Lacking any surface observations in the vicinity, it is difficult to tell. I will indicate it to be just offshore, though.

Regardless of development, heavy rainfall will be possible across southern Mexico over the next day or two, which could flooding in areas of orographic terrain.

Probability of development in 48 hours: Near 0%

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Depression Seven Invest 93L Tropical Storm Hector Invest 95E

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Tropical weather analysis - August 12, 2012

By: KoritheMan, 6:42 AM GMT on August 12, 2012

Invest 93L

A tropical wave producing minimal shower activity continues over the eastern Atlantic about 500 miles west-northwest of the Cape Verde Islands. Save for a small area of convection west of the wave axis, 93L is devoid of significant shower activity within the large circulation.



Figure 1. Latest infrared satellite image of Invest 93L. Image credit: RAMMB

Upper-level winds are currently only marginally favorable for development -- UW-CIMSS analyses reveals 20 knots of shear over the disturbance. Additionally, the system remains over rather cool sea surface temperatures, although they are expected to gradually warm up as 93L gains longitude over the next few days. Some of the global models suggest long-term potential for redevelopment as the system recurves well east of Bermuda, but I am wary of that at this time given the persistent subsident pattern over the central Atlantic. However, this wave still retains a rather vigorous mid-level circulation, so the potential for long-term development is definitely there. I just don't see the dry air getting out of the way.

Model guidance suggests the onset of recurvature beginning on Tuesday near 50W. Indeed, a weakness in the low- to mid-level ridge is already evident there.

Probability of development in 48 hours: 10%



Remnants of Tropical Depression Seven

Tropical Depression Seven dissipated around midday on Saturday just as it was approaching the Windward Islands. Most likely it dissipated before then, but nothing beats state of the art reconnaissance aircraft. In any event, the depression, or more accurately the remnant tropical wave, is currently lashing portions of the central and northern Leeward Islands with heavy rainfall and strong gusty winds. Satellite and surface observations indicate that the surface wind field associated with the tropical wave is extremely disorganized, with hardly even a wind shift noted in the surrounding islands. Earlier scatterometer data also suggested this, with mostly northeast winds associated with the disturbance. The observations also indicate that surface pressures are not falling at this time. However, island reports indicate that this system is producing wind gusts to tropical storm force in heavy squalls.

The system is currently battling about 40 knots of southwesterly shear according to UW-CIMSS. This shear is being induced by an upper low centered within the mid-oceanic trough to the north of the system. The GFS does not see this shear decreasing until around Monday evening, when the system will be approaching Jamaica. Afterward, the wave should encounter a break in the western Atlantic ridge, which should cause a consequent slowdown. The question after that becomes: where will the tropical wave go next? That is where the models express their respective disagreements. For the last two runs, the GFS has been moving it into the central Gulf of Mexico, and then developing it into a tropical cyclone. The Euro had a similar solution at 12z, albeit much weaker and farther south. The remainder of the models take it into Central America and/or the Bay of Campeche. The GFS is particularly good at sniffing out storms in the long-range, and one week isn't that far out, so this needs to be watched.

For now though, the former depression poses little threat to develop, as environmental conditions over the eastern and central Caribbean are not favorable for tropical cyclogenesis. The wave should continue rapidly westward at about 20 mph for the next day or two, and begin to slow down after that.

Probability of development in 48 hours: 10%



Hector

Tropical Storm Hector formed just after 0000 UTC today from what was once Hurricane Ernesto. While this tropical cyclone was technically related to Ernesto, based on a revised naming policy instituted by the National Hurricane Center, if a tropical cyclone dissipates -- even if it retains some semblance of coherence on satellite or surface data -- should it form into another tropical cyclone in a secondary basin, it will garner a new name. This was the case with Tropical Storm Arthur in 2008, which formed from the mid-level remnants of Eastern Pacific Tropical Storm Alma. Tropical Storm Hermine in 2010 formed in a similar manner.

As of the latest NHC advisory, the following was posted on Hector:

Wind: 40 mph, with higher gusts
Location: 18.5°N 108.1°W
Movement: WNW at 13 mph
Pressure: 999 mb

The center is located along the eastern edge of a rather large ball of deep convection, as indicated by satellite and microwave fixes. This is due to about 20 kt of easterly shear induced by a ridge over the New Mexico/Texas border.



Figure 2. Latest infrared satellite image of Tropical Storm Hector. Image credit: NOAA

Given that the shear is forecast to continue, conditions do not appear supportive of significant intensification, and Hector is not expected to become a hurricane. The large size of the tropical storm is also an issue. By Wednesday, Hector is expected to begin moving over cooler water, although the SHIPS guidance suggests that it will still be over 27C waters by Wednesday evening. This shouldn't be significant to disrupt Hector unless it miraculously becomes a hurricane. By Thursday, a weakening trend is forecast to become well-established.

Water vapor and synoptic data show that Hector is south of a mid-level ridge. However, a weakness is evident in this ridge near 120W, and this is apparently what is causing the models to forecast a slowing of the forward speed over the next few days. Here I am faced with another forecast of considerable uncertainty. While the models pretty much agree on a slowing of the forward speed, substantial differences arise in the timing of this slowdown, as well as the ultimate track of the cyclone. The GFS and ECMWF keep Hector well offshore Baja, while the CMC shoots it into California. The latter scenario is considered unrealistic, and is likely reflective of a stronger system being pulled more strongly to the north. The GFDL also suggests a more northward track, but again, this is likely predicated on the idea of a stronger system. Since Hector is not expected to become a hurricane, it is not currently expected that any significant northward motion will occur in the long-term. Given the usual reliability of the GFS/Euro, I will go with a consensus of those models to mold my forecast track.

While Hector is expected to remain well offshore southern Baja, high surf and rip currents will likely afflict the southern portions of that peninsula for the next few days.

5-day intensity forecast

INITIAL 08/12 0600Z 35 KT 40 MPH
12 hour 08/12 1800Z 40 KT 45 MPH
24 hour 08/13 0600Z 40 KT 45 MPH
36 hour 08/14 1800Z 45 KT 50 MPH
48 hour 08/15 0600Z 50 KT 60 MPH
72 hour 08/16 0600Z 60 KT 70 MPH
96 hour 08/17 0600Z 55 KT 65 MPH
120 hour 08/18 0600Z 45 KT 50 MPH

5-day track forecast



Figure 3. My 5-day forecast track for Hector.

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Storm Hector Invest 93L Tropical Depression Seven

Updated: 6:51 AM GMT on August 12, 2012

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Tropical weather analysis - August 10, 2012

By: KoritheMan, 11:08 PM GMT on August 10, 2012

Tropical Depression Seven

Tropical Depression Seven continues to race across the central Atlantic. As of the most recent NHC advisory, the following was available on the system:

Wind: 35 mph, with higher gusts
Location: 13.7°N 51.9°W
Movement: W at 24 mph
Pressure: 1009 mb

Satellite images suggest that the low-level center remains elongated in a southwest to northeast fashion. Oh, how I wish we still had QuikSCAT.



Figure 1. Latest infrared satellite image of Tropical Depression Seven. Image credit: NOAA

At this stage, it appears that speed shear -- not vertical -- is the primary inhibitor for organization since there are no obvious signs of strong vertical shear in the nearby environment. Broadening the horizon a bit, I see a large upper low several hundred miles north of the depression, near 26.8N 50.5W dropping rapidly southward toward the latitude of the tropical cyclone. The global models forecast this feature to gradually elongate and move westward, leaving behind a fractured piece of energy to reinforce the mid-oceanic trough. The net result should be a steady increase in southwesterly to westerly shear as the cyclone continues racing westward. This sort of unfavorable upper-level pattern is not atypical of El Nino years. Recall in 2009 that tropical storms Ana and Henri struggled against similar conditions in roughly the same area. The GFS and ECMWF continue to show the depression opening up into a wave over the eastern Caribbean, which is a viable possibility given current trends. In fact, it may even happen sooner, especially if the cyclone does not slow down.

An additional negative is the small size of the depression, which will make it quite susceptible to subtle environmental changes. Given current trends, and with no significant slowdown foreseen, I have revised my forecast significantly, and now show the system dissipating in about three days. This is in good agreement with the forecast from the National Hurricane Center, and is also harmonious with the unbeatable GFS/Euro consensus. Nevertheless, the system could still reach tropical storm status, but this prospect is becoming increasingly less likely with time.

It should be noted that the long-range pattern favors the remnants of the depression possibly getting pulled into the western Gulf of Mexico once it reaches the western Caribbean, where the GFS suggests upper-level winds could become more favorable (albeit not anticyclonic) for development. However, I assume there will be too little left of the system to take advantage of the more favorable atmospheric parameters at that time.

Synoptic data suggests that the strong subtropical ridge that is steering the depression so quickly to the west is in no hurry to budge. Indeed, all of the global models build it westward with time. Thus, a continuation of the rather fast and unclimatological westward motion is forecast, and the depression is expected to arrive in the Windward Islands as early as Saturday evening, possibly sooner. After about 72 hours, the cyclone -- or its remnants -- could encounter a break in the western extent of the ridge. The depression is expected to respond by moving more toward the west-northwest. Although a fairly significant break in this ridge is forecast, it is anticipated that the system will be too weak to fully take advantage of it.

5-day intensity forecast

INITIAL 08/10 2100Z 30 KT 35 MPH
12 hour 08/11 0600Z 30 KT 35 MPH
24 hour 08/12 2100Z 30 KT 35 MPH
36 hour 08/13 0600Z 30 KT 35 MPH
48 hour 08/13 2100Z 30 KT 35 MPH
72 hour 08/14 2100Z...DEGENERATED INTO TROPICAL WAVE OVER EASTERN CARIBBEAN SEA

5-day track forecast



Figure 2. My 5-day forecast track for Tropical Depression Seven.

No doubt as a course of least regret, the NHC has opted to issue tropical storm watches for much of the Lesser Antilles. Given the rapidity of the forward motion, it is likely that any heavy squalls will contain periods of tropical storm force winds, especially in gusts.

Watches and warnings

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* GUADELOUPE AND MARTINIQUE
* DOMINICA
* ST. LUCIA
* BARBADOS
* ST. VINCENT AND THE GRENADINES

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...IN THIS CASE IN 24 TO 48 HOURS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.


Invest 93L

A tropical wave over the eastern Atlantic ("93L") has lost organization since yesterday. While I'll admit I didn't quite expect this, it's also not that unusual either; few systems develop along or east of 20W due to cool sea surface temperatures that generally prevail in this region. This wave is embedded within a region of cool sea surface temperatures, 26.3 according to the SHIPS at 18z. The surrounding environment is also rather dry, which suggests that any short-term development potential will be slow. By Monday, 93L will be approaching warmer waters, and could start to organize a bit as it begins to approach 50W. Under a light and seemingly diffluent upper wind environment, conditions would normally be a go for strengthening. However, none of the global models are that enthusiastic with intensification. There is a large surge of dry air on water vapor imagery over much of the central and eastern Atlantic which looks like it will be hard to eradicate, so perhaps that's it. On the other hand, the purely statistical models intensify the system almost immediately after the 72 hour point, bringing to a strong tropical storm. I tend to side with the less aggressive global model camp given the abundance of dry air seen on water vapor images.

Based on global model forecasts, 93L should begin to approach a weakness in the subtropical ridge lying near 50W in a few days, which should presage recurvature. The system possibly remaining a weaker entity will not increase the odds of an eventual strike on the lower 48, it would just delay the inevitable.

Regardless of development, locally heavy rainfall and strong gusty winds are likely over much of the Cape Verde archipelago over the next 24 hours.

Probability of development in 48 hours: 20%



Gilma

Tropical Storm Gilma continues to slowly weaken over cool waters. As of the latest NHC advisory, the following information was posted:

Wind: 60 mph, with higher gusts
Location: 19.2°N 119.6°W
Movement: NNW at 5 mph
Pressure: 998 mb

The cyclone continues to do rather well for itself considering all available data suggests that it is over sub-26C SSTs. A recent AMSUB microwave overpass taken just after 2100 UTC suggests that the earlier eye feature which had been evident throughout the day has decayed, and only a small area of convection west of the center -- perhaps the fragments of an eyewall -- remain. However, recent infrared pictures suggest a more banded pattern to the convection, which could herald the onset of a more pronounced weakening trend.



Figure 3. Latest infrared satellite image of Tropical Storm Gilma. Image credit: NOAA

Environmental conditions are not favorable for Gilma to maintain its strength for much longer. Strong northeasterly shear is evident on water vapor images just to the north of the tropical storm. The SHIPS/GFS successfully recognize this and forecast Gilma to run into it within the next 12-24 hours. This shear is forecast to persist through the greater part of Sunday. That, coupled with a thermodynamic environment that is only marginal at best -- and finally -- progressively cooling sea surface temperatures -- suggests that Gilma is not long for this world. Dissipation is forecast in about 36 hours.

Gilma is moving in a poleward fashion, embedded in a weakness in the Pacific subtropical high caused by a weak shortwave trough over Nevada. A second trough is amplifying off the west coast of the United States, and this feature is expected to reinforce the broad cyclonic vorticity in this area. As a result, Gilma is expected to continue moving toward the north-northwest without significant acceleration. By Sunday, the system is forecast to begin to slow. I realize this is in contrast to the current forecast from the National Hurricane Center, but given the vigorous nature of the remnants of Ernesto, and the relative consistency in the global models to pull the circulation eastward, I am having a hard time believing some sort of interaction will not happen.

5-day intensity forecast

INITIAL 08/10 2100Z 50 KT 60 MPH
12 hour 08/11 0600Z 40 KT 45 MPH
24 hour 08/11 2100Z 30 KT 35 MPH
36 hour 08/12 0600Z 30 KT 35 MPH...POST-TROP/REMNANT LOW
48 hour 08/12 2100Z 25 KT 30 MPH...POST-TROP/REMNANT LOW
72 hour 08/13 2100Z 25 KT 30 MPH...POST-TROP/REMNANT LOW
96 HOUR 08/14 2100Z...DISSIPATED

5-day track forecast



Figure 4. My 5-day forecast track for Gilma.



Invest 94E

A large area of disturbed weather, primarily associated with the remnants of Ernesto, is centered off the southwest coast of Mexico. This large system is already showing signs of organization, and conditions appear favorable for a tropical depression to form in the area. Convection is beginning to develop closer to the center, while the large outer band that was seen encompassing the western semicircle earlier is dissipating. This is a pattern that is typical of an organizing tropical cyclone.



Figure 5. Latest infrared satellite image of Invest 94E. Image credit: NOAA

The SHIPS analyzes strong easterly shear over the system throughout the next five days, but analysis of satellite imagery suggests this is probably storm relative. The GFS builds a small anticyclone above the system, which is certainly a viable solution since sea surface temperatures are expected to remain quite warm throughout the entirety of the forecast period. I believe this system will eventually become a hurricane, although the large size of the circulation dictates that it will take time to spin up.

In the long-range, the models suggest a threat to southern Baja, with the GFS actually bringing the system inland Wednesday evening. Either way, this system will probably come close enough to the coast that watches or warnings will eventually be required, and interests across the southern and central Baja Peninsula, as well as southwestern Mexico, should carefully monitor the progress of this system over the next several days.

Regardless of development, heavy rainfall and possible flooding will continue over portions of southern Mexico over the next day or two.

Probability of development in 48 hours: 80%

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Depression Seven Invest 93L Tropical Storm Gilma Invest 94E

Updated: 11:10 PM GMT on August 10, 2012

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Tropical weather analysis - August 9, 2012

By: KoritheMan, 1:54 AM GMT on August 10, 2012

Ernesto

Ernesto made landfall along the eastern coast of Mexico just west of Coatzacoalcos near 1800 UTC this afternoon as a 50 kt tropical storm. As of the latest NHC advisory, the following information was available on Ernesto:

Wind: 45 mph, with higher gusts
Location: 18.2°N 95.8°W
Movement: W at 11 mph
Pressure: 996 mb

Considering how long the storm has been overland, the cloud pattern is well-organized. However, the decay process has clearly begun, as satellite images show two large and well-defined convective bands emanating quite far from the center: one to the west, and one to the south, while convection is diminishing closer to the center. This suggests that Ernesto has lost its inner core structure.



Figure 1. Latest infrared satellite image of Tropical Storm Ernesto. Image credit: NOAA

Given the convection, Ernesto still poses a significant flood threat over the southern half of Mexico. These rains will be especially enhanced over areas of mountainous terrain. An additional 10 to 15 inches of rain will be possible across this area through the next few days. Even after Ernesto emerges into the Pacific, a large area of low-level southwesterly flow associated with the remnants of the large circulation will continue to funnel moisture into the country. Any remaining tropical storm force winds within the circulation are very likely only occurring over water. Given the recent downturn in convection, any sustained winds of that caliber are quickly diminishing. Ernesto is expected to dissipate over the next day or so as it encounters increasingly rugged terrain. However, the global models are pretty much unanimous in regenerating the storm over the Eastern Pacific. While rare, it can happen. The last such occurrence of an Atlantic-Pacific crossover storm was Hurricane Cesar-Douglas of 1996.

I will wait one more day before calling for regeneration in the Pacific, but I have a feeling I will need to be explicit with this tomorrow. The global models suggest a long-range threat to Baja California, close enough that watches and warnings may eventually be required for the southern tip of the peninsula.

Ernesto is well-embedded within the Gulf ridge, and is expected to continue moving westward into the eastern Pacific on Saturday morning.

5-day intensity forecast

INITIAL 08/10 0000Z 40 KT 45 MPH...INLAND
12 hour 08/10 1200Z 30 KT 35 MPH...INLAND
24 hour 08/11 0000Z 30 KT 35 MPH...DISSIPATING INLAND
36 hour 08/11 1200Z...DISSIPATED

5-day track forecast



Figure 2. My 5-day forecast track for Ernesto.

Although coastal warnings are still in effect as of the 0z intermediate advisory from the National Hurricane Center, these are there for precautionary measure, and will likely be discontinued with the next full advisory package due out in about an hour and a half.



Tropical Depression Seven

The tropical wave we have been tracking for the last several days has become a tropical depression. As of the latest NHC advisory, the following information was posted on the storm:

Wind: 35 mph, with higher gusts
Location: 13.7°N 43.8°W
Movement: W at 20 mph
Pressure: 1010 mb

Typical with developing systems this time of day, convection associated with the depression has decreased.



Figure 3. Latest infrared satellite image of Tropical Depression Seven. Image credit: NOAA

The long-term survival of the tropical cyclone is in doubt. Although some intensification is possible in the near-term, water vapor imagery shows a large area of dry air surrounds the storm, which could act as a deterrent to significant strengthening. In addition, satellite images show a large convective mass about 100 miles southwest of the depression, which appears to be limiting the storm relative inflow. Although the depression is expected to become a tropical storm over the next day or two, the GFS/SHIPS shows westerly shear increasing as early as Friday afternoon, and persisting as the system enters the Caribbean. Finally, the GFS and ECMWF, which I note did quite well with Ernesto, see the cyclone degenerating into a tropical wave, which is certainly possible given the very strong ridge to the north. It should be noted that upper-level winds are forecast to become more favorable as the system nears the western Caribbean in 5 or 6 days, at which time some significant strengthening is possible. That is, assuming there is anything left of the depression at that time.

The cyclone remains situated to the south of a rather strong low- to mid-level ridge. The global models are in agreement on building this ridge westward ahead of the tropical cyclone as large-scale troughing over the western Atlantic lifts out, and the flow across that region gradually becomes more zonal. In general, this should result in a pretty straightforward westward motion for the next few days. Longer-term, the models show a weakness in the western Atlantic ridge which could induce a more poleward motion at that time, but it will depend on the respective latitude of the system at that time. A weaker system ala Ernesto would tend to move in a more uniform westerly direction to the south of the ridge, while a stronger system would likely be strong enough to move through the purported weakness. Given that both are viable possibilities at this time, I will split the difference. It appears we could have another tropical cyclone approaching the western Caribbean in four to five days. This is just a bit south of the TVCN/TVCC consensus, in better agreement with the track from the National Hurricane Center.

5-day intensity forecast

INITIAL 08/10 0000Z 30 KT 35 MPH
12 hour 08/11 1200Z 30 KT 35 MPH
24 hour 08/12 0000Z 35 KT 40 MPH
36 hour 08/13 1200Z 40 KT 45 MPH
48 hour 08/14 0000Z 45 KT 50 MPH
72 hour 08/15 0000Z 45 KT 50 MPH
96 hour 08/16 0000Z 45 KT 50 MPH
120 hour 08/17 0000Z 50 KT 60 MPH

5-day track forecast



Figure 4. My 5-day forecast track for Tropical Depression Seven.



Invest 93L

A vigorous tropical wave accompanied by a well-defined low pressure system has just emerged off the west coast of Africa. Since the system was just designated an invest about an hour ago and is still east of 20W, there is limited satellite coverage. Using available fixes, the system appears well-organized, and seems to already be on its way to becoming a tropical depression. With an estimated surface pressure of 1004 mb, there seems to be little in the way of inhibition for this system once it gets fully over water. The biggest impediment at this time appears to be a dry and stable airmass immediately ahead of the wave, accompanied of course by relatively cool sea surface temperatures. However, objective analyses of areal SSTs from both SHIPS and AOML suggest that water temperatures should remain more in the 27C range for the next day or so. This should allow the system to continue organizing. Afterward, cooler SSTs will likely halt the strengthening process. By Sunday, water temperatures are expected to slowly rebound, favoring a resumption of the earlier intensification.

Given the high latitude (15N) in which the system has emerged from the coast, as well as global model forecasts of a mid- to upper-level trough amplifying over the western Atlantic through the early part of next week, it appears that 93L is no threat to land at this time, with the possible exception of Bermuda. It is not unusual for systems originating so far east in the Atlantic to fail to make full transit. The last hurricane I recall doing so was Hurricane Dora in 1964, which went on to hit near Jacksonville, Florida as a major hurricane in early September.

Steady development is expected as the system moves west or west-northwest at 10 to 15 mph.

Probability of development in 48 hours: 40%



Gilma

After peaking as 70 knot hurricane this morning, Gilma has weakened to a tropical storm as it moves across cooler waters. As of the latest NHC advisory, the following was posted on the storm:

Wind: 70 mph, with higher gusts
Location: 17.2°N 119.1°W
Movement: NW at 6 mph
Pressure: 992 mb

Gilma still has a well-organized appearance on satellite imagery. Also, an SSMI overpass just before 0z still indicated a low- to mid-level eye feature surrounded by a ring of convection. Since the cyclone structure does not appear to have deteriorated too significantly since that time, Gilma probably hasn't weakened much.



Figure 5. Latest infrared satellite image of Tropical Storm Gilma. Image credit: NOAA

Environmental conditions are quickly becoming unfavorable for sustenance of a tropical cyclone, with dry air lying just to the west of a storm. In addition, easterly shear is forecast to abruptly increase Saturday evening, and Gilma is expected to weaken more quickly from this point on. Dissipation as a tropical cyclone is expected in about three days.

Water vapor imagery shows a weak mid- to upper-level trough continues to linger off the west coast of the United States. This trough appears to be enforcing a weakness in the Pacific subtropical high near and west of 125W. Since Gilma is still a vertically deep system at this time, it is responding by moving in a slow northwesterly fashion. As a series of shortwave impulses move through the westerlies in this area, the large-scale cyclonic flow across the region is forecast to persist. Thus, the current northwest motion is expected to continue without appreciable acceleration as synoptic steering remains somewhat weak. After 48 hours, the track becomes a little more uncertain. The global models have come into better agreement today on redeveloping Tropical Storm Ernesto in the eastern Pacific on Sunday. While the projected development may be too quick in these models, Ernesto still possess the well-defined cyclonic structure it has had ever since its landfall along the Yucatan Peninsula. So it is my thinking that just enough westerly flow will persist south of the center of the area of disturbed weather -- or tropical cyclone -- to nudge Gilma just a little bit to the east. Since Gilma is expected to steadily weaken, I am not going to commit to a full on eastward motion as some of the models seem to be doing. However, Gilma is expected to slow down significantly near the end of the forecast period as it comes under the influence of Ernesto. This track is east of what it was yesterday.

5-day intensity forecast

INITIAL 08/10 0000Z 60 KT 70 MPH
12 hour 08/10 1200Z 55 KT 65 MPH
24 hour 08/11 0000Z 45 KT 50 MPH
36 hour 08/11 1200Z 40 KT 45 MPH
48 hour 08/12 0000Z 35 KT 40 MPH
72 hour 08/13 0000Z 30 KT 35 MPH...POST-TROP/REMNANT LOW
96 hour 08/14 0000Z...DISSIPATED

5-day track forecast



Figure 6. My 5-day forecast track for Gilma.

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Depression Seven Tropical Storm Gilma Tropical Storm Ernesto Invest 93L

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Tropical weather analysis - August 9, 2012

By: KoritheMan, 7:51 AM GMT on August 09, 2012

Ernesto

Ernesto has weakened to a tropical storm, but has held up rather nicely through its adventure across the Yucatan Peninsula. As of the most recent NHC advisory, the following was posted on this storm:

Wind: 70 mph, with higher gusts
Location: 18.8°N 93.0°W
Movement: W at 16 mph
Pressure: 993 mb
Category: Tropical storm

Satellite images show that Ernesto is organizing once again. Convective banding has increased around what appears to be a developing eye. Doppler radar data from Sabancuy, Mexico confirms this, and also shows this feature is slowly becoming better defined. However, neither the satellite images nor the radar fixes suggest a formative eyewall.



Figure 1. Latest infrared satellite image of Tropical Storm Ernesto. Image credit: NOAA

Atmospheric and oceanic conditions appear conducive for intensification until the center reaches the coast this afternoon. Based on earlier SSMIS data, Ernesto does not appear to have been too disrupted by its Yucatan landfall. Dissipation is forecast to occur rather quickly after landfall. Although some of the global models still suggest the possibility of regeneration over the Eastern Pacific, the GFS shows some rather strong easterly shear impinging on whatever is left of Ernesto. Thus, this possibility appears unlikely for now.

Ernesto is south of a mid-level ridge over the Gulf of Mexico. Seeing as though Ernesto is so far south and has accelerated, it now appears unlikely that any appreciable slowing of the forward speed will occur prior to landfall. A gradual bend to the west-southwest is expected over the next 12 hours, and Ernesto should cross the coast within the hurricane warning area in about 12-18 hours. After landfall, the forward speed may slow slightly due to interaction with rugged terrain.

5-day intensity forecast

INITIAL 08/09 0600Z 55 KT 65 MPH
12 hour 08/09 1800Z 75 KT 85 MPH...NEAR COAST OF MEXICO
24 hour 08/10 0600Z 50 KT 60 MPH...INLAND
36 hour 08/11 1800Z 30 KT 35 MPH...INLAND
48 hour 08/12 0600Z 25 KT 30 MPH...DISSIPATING INLAND
72 hour 08/13 0600Z...DISSIPATED

5-day track forecast



Figure 2. My 5-day forecast track for Ernesto.

A brief period of sustained Category 1 hurricane force winds is expected near where the center comes ashore. Given the motion, it is likely that the strongest winds will be felt in the northwest quadrant of the storm. The biggest threat with Ernesto is the potential for heavy rains capable of causing flash flooding, especially in mountainous areas. Given the forecast motion of the tropical cyclone, I expect 10 to 15 inches of rain over the remainder of Ernesto's existence, with locally higher amounts.

Preparations should be completed in the warning area quickly, as conditions will begin to deteriorate later this morning.

Watches and warnings

A HURRICANE WARNING IS IN EFFECT FOR...
* VERACRUZ TO CHILITEPEC MEXICO

A HURRICANE WATCH IS IN EFFECT FOR...
* NORTH OF VERACRUZ TO BARRA DE NAUTLA

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* NORTH OF VERACRUZ TO BARRA DE NAUTLA
* EAST OF CHILITEPEC TO CAMPECHE ALONG THE GULF COAST OF THE YUCATAN
PENINSULA OF MEXICO

A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED
SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN 12 HOURS.
PREPARATIONS TO PROTECT LIFE AND PROPERTY SHOULD BE RUSHED TO
COMPLETION.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.



Invest 92L

A well-defined tropical wave is centered over the eastern Atlantic between the Cape Verde Islands and the Lesser Antilles. Although the system is generating some rather vigorous convection, the areal coverage of this convection is rather small. In addition, the low-level center appears to be located north of the strongest thunderstorm activity, which means that the system is not well-organized yet. There are hints on microwave imagery that the center could actually be a little farther than the 06z ATCF coordinates, which place it 14.6N 39.3W. This is suggested by recent satellite fixes as well, though it is always difficult to detect such motions when convection is involved.



Figure 3. Latest infrared satellite image of Invest 92L. Image credit: NOAA

The GFS forecasts an increase in westerly shear beginning in about two days, but there is little evidence of westerly flow on water vapor images at this time. I do see some moderate southerly flow from 5N 50W all the way to 15N and 55W, probably associated with an equatorial anticyclone. It is possible that if the anticyclone shifts eastward, the upper flow could transition from southerly to westerly, but I don't think this is what the GFS is seeing that will cause the shear. Instead, an upper low centered about 500 miles east of Bermuda is rapidly dropping southward. Giving it more than a cursory glance, it appears this upper low is forecast to amplify and cause westerly shear over the system. While I will not completely rule this out, 92L may be too distant to it when it makes its closest to approach, so that it doesn't get completely destroyed. In addition, if 92L is moving in a more southerly fashion like the satellite images are indicating, that is further reason to believe it will escape the shear. Dry air does still linger though, and this may retard rapid development.

The future track of the system is uncertain, but right now, the synoptic pattern favors a strong ridge building westward ahead of the system as large-scale troughing over the western Atlantic lifts out. It is possible that the aforementioned upper low could impart some sort of weakness to the ridge in a few days, but 92L will be well west of that weakness when it begins to manifest. At this point, it seems physically unlikely, barring rapid development (which appears equally unlikely) that 92L will go anywhere other than the Caribbean, but one should always be aware. Thereafter, the track becomes quite muddled. It is possible the system could eventually enter the Gulf of Mexico. Alternatively, it could stay weak like Ernesto and move generally westward. Moderate westerly shear is forecast to affect the system if it enters the Caribbean, but it will all depend on the timing, as some of the models are quicker with its arrival toward the Leeward Islands than others.

Interests in the Leeward Islands should monitor the progress of this system through the early part of next week.

Development is anticipated, and this system could become a tropical depression over the couple of days.

Probability of development in 48 hours: 70%

Gilma

Gilma has become a hurricane. As of the latest NHC advisory, the following was posted on the storm:

Wind: 75 mph, with higher gusts
Location: 16.1°N 118.4°W
Movement: W at 9 mph
Pressure: 987 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Gilma looks like a typical low-end Category 1 hurricane on satellite images, with a well-defined central dense overcast that has expanded since the previous advisory. An eye is not identifiable at this time.



Figure 4. Latest infrared satellite image of Hurricane Gilma. Image credit: NOAA

Gilma will cross the 26C isotherm in about 12 hours. Thus, time is quickly running out for strengthening. In addition, easterly shear is forecast to abruptly increase in about 72 hours, possibly in response to a broad area of disturbed weather associated with the remnants of Ernesto. It could also be associated with a building ridge over the Bay of Campeche and southern Mexico in the wake of Ernesto. Either way, Gilma will probably not last through five full days. Dissipation is shown at day four.

Although Gilma is currently moving west, a gradual turn to the northwest with a reduction in forward speed is likely. A mid- to upper-level trough is seen off the west coast of the United States. This feature is expected to weaken the subtropical ridge north of Gilma, which the global models agree will result in the northwestward motion. A turn to the west is expected by Sunday as Gilma becomes a shallow system and becomes steered by the low-level flow.

5-day intensity forecast

INITIAL 08/09 0300Z 65 KT 75 MPH
12 hour 08/09 1800Z 70 KT 80 MPH
24 hour 08/10 0300Z 65 KT 75 MPH
36 hour 08/11 1800Z 50 KT 60 MPH
48 hour 08/12 0300Z 40 KT 45 MPH
72 hour 08/13 0300Z 35 KT 40 MPH
96 hour 08/14 0300Z 30 KT 35 MPH...POST-TROP/REMNANT LOW
120 hour 08/15 0300Z 25 KT 30 MPH...POST-TROP/REMNANT LOW

5-day track forecast



Figure 5. My 5-day track forecast for Gilma.



Invest 93E

An area of low pressure centered about 500 miles south-southwest of Acapulco has become a little better organized. However, most of the associated convection is located west of the center due to moderate easterly shear. This shear is forecast to increase significantly over the next day or so, ultimately increasing to over 30 kt. Even by Eastern Pacific standards, where most storms move west, this is rather significant.



Figure 6. Latest infrared satellite image of Invest 93E.

Given the recent increase in organization, I think the NHC's probability of 30% seems a little low. Either way, 93E is not a threat to any land areas.

Probability of development in 48 hours: 40%

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Storm Ernesto Invest 92L Invest 93E Hurricane Gilma

Updated: 7:55 AM GMT on August 09, 2012

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Tropical weather analysis - August 7, 2012

By: KoritheMan, 4:48 AM GMT on August 08, 2012

Invest 92L

Shower activity remains minimal in association with a tropical wave and accompanying area of low pressure located over the eastern Atlantic about 600 miles west-southwest of the Cape Verde Islands.

However, earlier microwave data suggested that the low-level center is fairly well-defined, albeit not closed. This is also supported by recent satellite pictures.



Figure 1. Latest infrared satellite image of Invest 92L. Image credit: NOAA

I am not sure why the global models are so down on this system; water vapor imagery does suggest dry air lies to the west of the storm, but it is not being entrained into the core at this time. In addition, a large moisture field surrounds 92L, making any such intrusions unlikely in the near term. High cloud motions do still suggest some easterly shear, though, which could certainly slow rapid strengthening. One other reason the global models may not be quite so enthusiastic about development potential with this is due to sea surface temperatures of only 26/27C along the forecast track for the next several days. Remember, we are still somewhat behind the climatological due date for the arrival of the Cape Verde season, so the eastern Atlantic is just beginning to warm.

Based on global model forecast pressure fields, 92L is expected to move westward over the next several days. Should it begin to develop, it would likely take a more west-northwest track as it nears a weakness in the subtropical ridge near 55W. This would likely occur over the weekend. It is entirely possible 92L will fail to develop significantly for the next five days, and eventually enter the Caribbean. I am in between the aforementioned tracks for now.

Probability of development in 48 hours: 30%



Gilma

Tropical Storm Gilma formed today. As of the most recent NHC advisory, the following was posted on the tropical cyclone:

Wind: 60 mph, with higher gusts
Location: 15.4°N 115.0°W
Movement: WNW at 13 mph
Pressure: 994 mb
Category: Tropical storm

Gilma is a well-organized storm; a well-defined curved band is evident to the west of the center, while the center itself is composed of a steadily maturing CDO. In addition, a robust inflow jet appears to be setting up over the eastern quadrant of the tropical cyclone.



Figure 2. Latest infrared satellite image of Tropical Storm Gilma. Image credit: NOAA

An earlier SSMIS overpass at the 37 GhZ channel showed what appeared to be a small warm spot north of the strongest convection, but this was not evident on the 85 GhZ channel. The former is more sensitive to the lower-levels, but we would expect the upper-levels to begin forming an eye first due to the intense release of latent heat energy at that level of the troposphere. I am not quite sure what to make of this, but given Gilma's seemingly well-defined inner core structure, it seems likely that it will become a hurricane soon. After about 24 hours, the cyclone should be crossing the 26C isotherm, at which time a weakening trend is expected. One interesting thing to note is the GFS shows easterly shear increasing over the storm beginning in about four days. This is probably due to the influence of regenerating Atlantic Hurricane Ernesto. While regeneration of Ernesto on the Pacific side is not impossible, it would be a very rare event. The last Atlantic/Pacific crossover storm was Hurricane Cesar-Douglas of 1996.

Given the rugged terrain of the Sierra Madre Oriental mountain range over central Mexico, as well as climatology, I find it doubtful that Ernesto will actually regenerate once its remnants enter the eastern Pacific. Presumably the easterly shear just mentioned emanates from Ernesto's outflow, but again, since regeneration seems unlikely for now, I will not base my intensity forecast off the presumed vertical shear.

Given the cool waters and increasing shear at longer ranges, Gilma is expected to weaken to a tropical depression by the end of the forecast period. However, if the shear increases like the GFS says, Gilma could easily dissipate prior to the end of the forecast period.

It is notable that both the Euro AND GFS regeneate Ernesto. We saw how well they did with the storm when it was in the Caribbean, defying all of my forecasts, as well as others who hypothesized that a United States strike was the most likely course. I have learned a thing or two from Ernesto, so I may have to reevaluate the possibility of regeneration tomorrow.

Gilma is on the south side of a low- to mid-level ridge. Water vapor imagery shows a rather substantial ridge is in the process of deamplifying across the southwestern United States as a large upper trough and attendant frontal zone approach coastal California. Over the next day or so, this trough is expected by all of the global models to cause an erosion of the ridge that currently provides the steering for Gilma. This should more or less cause a continued west-northwest motion, albeit at a slower forward speed. After about 72 hours, the track becomes uncertain because the models disagree on the handling of Ernesto's entrance into the Pacific. Some of them, such as the GFS and ECMWF, forecast a fully regenerated Ernesto to slowly pull the low-level center eastward as it rides the low-level westerly flow found on the south side of the center. The remainder of the global models still slow Gilma, but do not show near as much interaction between the two systems. Because of the faint possibility of regeneration as purported by the godlike GFS/Euro duo, I will compromise between the solutions from both camps. In the end, this makes my forecast track a fair bit to the right of the National Hurricane Center's, as I show a slow northward drift near the end of the forecast period. Confidence in this track is not particularly high.

5-day intensity forecast

INITIAL 08/08 0300Z 50 KT 60 MPH
12 hour 08/08 1800Z 60 KT 70 MPH
24 hour 08/09 0300Z 70 KT 80 MPH
36 hour 08/09 1800Z 75 KT 85 MPH
48 hour 08/10 0300Z 55 KT 65 MPH
72 hour 08/11 0300Z 45 KT 50 MPH
96 hour 08/12 0300Z 35 KT 40 MPH
120 hour 08/13 0300Z 30 KT 35 MPH

5-day track forecast



Figure 3. My 5-day forecast track for Gilma.



Invest 93E

Another area of disturbed weather, this one centered approximately 450 miles south of Acapulco, remains disorganized. Satellite images show a well-defined -- probably closed -- circulation exists, but there is virtually no convection associated with this circulation.



Figure 4. Latest infrared satellite image of Invest 93E. Image credit: NOAA

93E still has a ways to go. There appears to be some easterly shear over the system, as evidenced by the large outflow channel in the western semicircle. The SHIPS and GFS don't really show this shear decreasing too much. Also, 93E appears to be practically stationary at the moment, which would most definitely favor penetration of the fragile low-level circulation by said shear. There is also a little bit of dry air to the east that make entrain into the circulation -- indeed, that is probably why no convection exists now.

Having said all that, this low appears to possess a fairly well-defined lower-tropospheric structure, which would favor intensification if the environment becomes more favorable. It should be noted that due to the possibility of Ernesto regenerating in this part of the basin, the global models will likely have a difficult time resolving -- and thus successfully forecasting -- the future evolution of 93E.

Probability of development in 48 hours: 40%

2012 Atlantic hurricane season 2012 East Pacific hurricane season Invest 92L Tropical Storm Gilma Invest 93E

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Tropical weather analysis - August 7, 2012

By: KoritheMan, 1:41 AM GMT on August 08, 2012

Ernesto

After six days, Ernesto has finally become a hurricane. As of the 8:00 PM EDT intermediate advisory, the National Hurricane Center said the following about the storm:

Wind: 85 mph, with higher gusts
Location: 18.7°N 87.1°W
Movement: W at 18 mph
Pressure: 980 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Satellite and radar data from Belize indicate that Ernesto is still intensifying; a warm spot is evident in conventional satellite images right where the center is estimated to be.



Figure 1. Latest infrared satellite image of Hurricane Ernesto. Image credit: NOAA

Ernesto should be inland along the eastern coast of the Yucatan Peninsula just north of the Belize/Yucatan border within the next hour or two. It's not impossible that a little more strengthening could occur during the small period of time before landfall, and if I lived in Belize or the Yucatan Peninsula, I would be preparing for a Category 2 hurricane. Indeed, Ernesto is probably going through a rapid intensification phase of sorts, which will ultimately be thwarted by land interaction.

I have been unable to find significant wind reports in the landfall area given the sparse observations that are available there. Regardless, sustained winds between 80 - 100 mph will be possible for a brief period on the coast at and around the landfall point, particularly north of the eye. Although Ernesto is naturally expected to weaken subsequent to landfall, the terrain of the southern Yucatan is not particularly mountainous, and it is expected that Ernesto will emerge into the Bay of Campeche on Wednesday morning at or near hurricane strength. Once it emerges in the Bay of Campeche, conditions are expected to remain conducive for strengthening. Assuming the inner core is not too disrupted by passage overland, another period of rapid intensification is possible prior to Ernesto's final landfall, especially given the relatively slow motion of the hurricane that is being forecast at that time. However, Ernesto is already south of the 12 hour forecast point from the 5:00 PM NHC advisory, and if this continues, the hurricane will have less time over water than previously anticipated. One caveat to the intensity forecast when Ernesto enters the Gulf of Mexico is the possibility of some northerly shear, as alluded to be the GFS. While some of this is no doubt storm relative given the size of the hurricane, I doubt all of it is. This could be a fly in the ointment for Ernesto to make one last hurrah.

Ernesto is forecast to be a well-inland remnant low by Friday evening.

Ernesto is being steered into land by a narrow mid-level ridge over the Gulf of Mexico. The global models forecast this feature to persist, although a shortwave trough is forecast to drop southward out of Canada and weaken the ridge over the next few days. Should this happen, Ernesto would slow. However, with the southward deviation from the 12 hour NHC forecast point, Ernesto may be too far south to slow down significantly. Based on the above, my new forecast track is south of the one from the National Hurricane Center, and I imagine they will follow suit with the advent of the next advisory.

5-day intensity forecast

INITIAL 08/08 0000Z 75 KT 85 MPH...NEAR THE COAST
12 hour 08/08 1200Z 65 KT 75 MPH...INLAND
24 hour 08/09 0000Z 60 KT 70 MPH...OVER WATER
36 hour 08/09 1200Z 70 KT 80 MPH
48 hour 08/10 0000Z 75 KT 85 MPH
72 hour 08/11 0000Z 25 KT 30 MPH...DISSIPATING INLAND
96 hour 08/12 0000Z...DISSIPATED

5-day track forecast



Figure 2. My 5-day forecast track for Ernesto.

Watches and warnings


A HURRICANE WARNING IS IN EFFECT FOR...
* CHETUMAL TO TULUM ON THE EAST COAST OF THE YUCATAN PENINSULA
* COZUMEL
* COAST OF BELIZE FROM BELIZE CITY NORTHWARD TO THE BORDER OF MEXICO

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* NORTH OF TULUM TO CABO CATOCHE ON THE EAST COAST OF THE YUCATAN
PENINSULA
* SOUTH OF BELIZE CITY SOUTHWARD TO THE BORDER OF GUATEMALA
* CELESTUN SOUTHWARD AND WESTWARD TO CHILITEPEC ALONG THE GULF COAST
OF MEXICO.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED
STATES...INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE
MONITOR PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE
FORECAST OFFICE. FOR STORM INFORMATION SPECIFIC TO YOUR AREA OUTSIDE
THE UNITED STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR NATIONAL
METEOROLOGICAL SERVICE.

2012 Atlantic hurricane season Hurricane Ernesto

Updated: 1:43 AM GMT on August 08, 2012

Permalink

Tropical weather analysis - August 6, 2012

By: KoritheMan, 6:38 AM GMT on August 06, 2012

Ernesto

Ernesto continues cruising through the Caribbean Sea as a tropical storm. As of the latest NHC advisory, the following was posted in regards to the storm's status:

Wind: 50 mph, with higher gusts
Location: 15.0°N 79.9°W
Movement: W at 15 mph
Pressure: 1003 mb
Category: Tropical storm

Ernesto is still not exceptionally well organized, but it is definitely improving. Although an earlier SSMIS overpass just after 0000 UTC suggested that the lower- and middle-tropospheric vortexes were displaced in a west-to-east fashion, as alluded to above, Ernesto appears to be organizing a little bit, and it is presumed that these centers are starting to realign.



Figure 1. Latest infrared satellite image of Tropical Storm Ernesto. Image credit: NOAA

Whatever shear was there is diminishing, as upper-level outflow is now evident in the western semicircle, a first in quite awhile for Ernesto. The SHIPS and GFS forecast a slow reduction in the shear, which should presumably lead to intensification, but Ernesto could surprise. It is expected that Ernesto will attain hurricane status prior to moving across the Yucatan Peninsula late Tuesday or early Wednesday, but assuming the cyclone follows the current forecast track, it will likely miss the warmest waters of the western Caribbean. Thus, rapid intensification is not being explicitly shown at this time (although it remains a possibility). Not that I have any skill in predicting such an episode, anyway. Although temporary weakening is expected to begin as Ernesto moves across the Yucatan Peninsula, reintensification is possible once the cyclone emerges over Bay of Campeche -- if it does so. At that point, Ernesto could become become a hurricane again before moving into Mexico. After its second landfall, Ernesto should quickly dissipate over the rugged terrain of central Mexico, although the threat for heavy rainfall and flooding will persist even after dissipation.

Ernesto remains south of a rather strong subtropical ridge extending to about the longitude of the tropical cyclone. Dynamical models and synoptic data suggest this ridge is beginning to weaken somewhat with the approach of a shortwave trough over the eastern United States. While this trough is not particularly strong, it should be sufficient to induce some poleward component of motion to the cyclone. In a couple of days, another shortwave currently dropping out of southern Canada is forecast to be vastly more potent. This trough and its attendant frontal zone are poised to slide eastward through the Ohio Valley and the mid-Atlantic region. This was the trough that was originally projected to turn Ernesto northward in the Gulf of Mexico. At this point, Ernesto will be too far south to feel an appreciable northward tug from this trough. I would not be surprised if subsequent forecasts have to be shifted southward. A good analogue for Ernesto at this point is Hurricane Debby of 1988, which moved across the Yucatan Peninsula and went on to hit eastern Mexico as a minimal hurricane.

An alternate scenario is that Ernesto runs into Central America, though this seems highly unlikely since the storm appears to be organizing as well as slowly gaining latitude. Should Ernesto move slower or farther south than forecast, a significant flood threat could manifest over portions of Central America through the latter part of the week.

5-day intensity forecast

INITIAL 08/06 0600Z 45 KT 50 MPH
12 hour 08/06 1800Z 50 KT 60 MPH
24 hour 08/07 0600Z 55 KT 65 MPH
36 hour 08/07 1800Z 65 KT 75 MPH
48 hour 08/08 0600Z 70 KT 80 MPH...INLAND
72 hour 08/09 0600Z 65 KT 75 MPH...OVER WATER
96 hour 08/10 0600Z 75 KT 85 MPH
120 hour 08/11 0600Z 35 KT 40 MPH...INLAND

5-day track forecast



Figure 2. My 5-day forecast track for Ernesto.

Watches and warnings

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* JAMAICA

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* GRAND CAYMAN
* THE COAST OF HONDURAS FROM THE HONDURAS/NICARAGUA BORDER WESTWARD
TO PUNTA CASTILLA

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN
12 HOURS.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...IN THIS CASE WITHIN THE NEXT 24 TO
36 HOURS.

INTERESTS IN BELIZE AND THE YUCATAN PENINSULA OF MEXICO SHOULD
MONITOR THE PROGRESS OF ERNESTO. A TROPICAL STORM OR HURRICANE WATCH
MAY BE NEEDED FOR THESE AREAS MONDAY MORNING.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.



Florence

Tropical Storm Florence has continued to weaken over the eastern Atlantic as of the latest NHC advisory:

Wind: 40 mph, with higher gusts
Location: 16.2°N 37.9°W
Movement: W at 13 mph
Pressure: 1008 mb
Category: Tropical storm

Florence looks deathly ill, with no deep convection to be found anywhere within the cyclone vortex. However, satellite imagery suggests that the circulation and low-level wind field remain well-defined.



Figure 3. Latest infrared satellite image of Tropical Storm Florence. Image credit: NOAA

Coincident with the loss of convection, Dvorak numbers have quickly plummeted, ranging anywhere from 1.0 to 1.5, the very bottom of the scale. Given the degraded satellite presentation and lack of inner core convection, I prefer the lower end of these estimates. I highly doubt there are any remaining tropical storm force winds anywhere within the circulation given the cool waters and absence of convection, but using the National Hurricane Center operational estimates for my blog, I am forced to follow restrictions.

If convection does not return soon, Florence could degenerate into a remnant area of low pressure later today. Water vapor and upper shear data from UW-CIMSS show a swath of southwesterly vertical shear along and west of 40W extending to around 70W. The SHIPS model correctly recognizes this shear, as does the GFS. The tropical cyclone is forecast to encounter these winds within about 36 hours. With 25 - 30 kt of zonal shear forecast to infringe on the cyclone, Florence will not survive, even though the forecast track takes it into gradually warmer water. In addition to the factors mentioned above, dry air is seen to the west of the storm on water vapor images.

Most of the global models quickly lose the circulation as of 0z, and I strongly agree with this. Given current trends, dissipation is now forecast to occur much sooner. Although vertical shear is shown by the GFS to be decreasing at day five and beyond as Florence approaches 70W, there will likely be too little left of Florence for any regeneration to occur.

Florence is south of a well-established subtropical ridge. A break in the ridge is evident west of 40W. Although Florence is now a shallow system devoid of deep convection, the global models suggest there will still be enough of a weakness for the storm to move more toward the west-northwest with time. However, I remain on the south side of the guidance envelope. Not that it matters much.

5-day intensity forecast

INITIAL 08/06 0600Z 35 KT 40 MPH
12 hour 08/06 1800Z 25 KT 30 MPH
24 hour 08/07 0600Z 25 KT 30 MPH
36 hour 08/08 1800Z 20 KT 25 MPH...POST-TROP/REMNANT LOW
48 hour 08/09 0600Z 20 KT 25 MPH...POST-TROP/REMNANT LOW
72 hour 08/10 0600Z...DISSIPATED

5-day track forecast



Figure 4. My 5-day forecast track for Florence.



Invest 92E

An area of disturbed weather centered about 500 miles southwest of Acapulco is producing only minimal shower activity. However, satellite images and earlier microwave data suggest that the circulation is fairly well-defined. The system is currently experiencing easterly shear, which is not uncommon for weak Eastern Pacific tropical entities. A ragged curved band feature has developed in the thunderstorms on the western side.



Figure 5. Latest infrared satellite image of Invest 92E. Image credit: NOAA

Atmospheric conditions appear favorable for development, and this low has the potential to become a tropical depression over the next day or two. However, the SHIPS model suggests that cooler waters awaits the system in about two days. Given that the global models are forecasting a poleward bending cyclone, this forecast may not be quite so unreasonable. This system does not currently pose a threat to land.

Probability of development in 48 hours: 50%



Atlantic getting active

Now would be a good time to remind everyone to ensure they have a hurricane preparedness kit. Yes, the last few years have been fairly kind to us. But eventually, we will see another year where a major hurricane -- or worse, a series of major hurricanes -- batters the United States mainland. Central America and Mexico have taken the brunt for us the last few years, but this cannot be expected to last. A lot of our luck can be attributed to the persistent drought conditions that have generally been prevalent across the southern United States during the last couple years.

Anyway, the GFS, complimented by the NOGAPS -- and to a loose extent the CMC -- forecasts the development of Tropical Storm Gordon between the west coast of Africa and the Cape Verde Islands. While this would be unusually far east for a tropical cyclone even by Cape Verde standards, the GFS tends to do well in sniffing out storms in the long-range. Additionally, it has been consistent enough in this solution over the last few days for me to believe it. The formation is forecast to occur in about five days.

Broadening my horizons a bit, the GFS has been consistent in developing at least one or two more storms behind Gordon during the upcoming two week forecast period. While things will probably not pan out exactly as the GFS is saying, the MJO should be at least neutral in our area of the world throughout much of this month, with the possibility of a significant upward surge in September. This means that despite the oncoming El Nino, the 2012 tropical season is not quite over, and I anticipate that these next two months will feature anywhere between 4 and 6 storms.

2012 Atlantic hurricane season 2012 East Pacific hurricane season Invest 92E Tropical Storm Ernesto Tropical Storm Florence Tropical Storm Gordon

Updated: 6:43 AM GMT on August 06, 2012

Permalink

Tropical weather analysis - August 5, 2012

By: KoritheMan, 5:31 AM GMT on August 05, 2012

Ernesto

Tropical Storm Ernesto continues racing across the central Caribbean. As of the latest NHC advisory, the following was posted on the storm:

Wind: 60 mph, with higher gusts
Location: 15.4°N 72.7°W
Movement: WNW at 22 mph
Pressure: 1007 mb
Category: Tropical storm

Ernesto has been a humbling storm, to say the least. Defying all of my previous forecasts, the storm has failed to significantly intensify up to this point. In fact, reports from a reconnaissance aircraft suggest that Ernesto is less organized than earlier, and the National Hurricane Center admits the current intensity estimate may be generous. In hindsight, there were a couple things I perhaps didn't fully consider, but this is not the time. Officially, the center is purported to be on the northwestern edge of the convection, which consists of a circular ball of thunderstorms with little banding. However, I have my doubts as to whether Ernesto has a closed circulation at all based on recent aircraft fixes.



Figure 1. Latest infrared satellite image of Tropical Storm Ernesto. Image credit: NOAA

I can see pretty easily based on recent aircraft reports and the satellite signature that Ernesto lacks an inner core. It is possible Ernesto is pulling in drier air from South America to its south, as evidenced by recent TPW imagery from CIMSS; the fast forward motion isn't helping, either. However, when Ernesto approaches 80W, conditions should become more conducive to strengthening as the tropical cyclone finds itself beneath the hottest waters in the basin and under a light upper-level shear pattern. Assuming Ernesto retains some structural coherence, rapid intensification is still a possibility at that time. However, since we have little apparent skill in predicting the onset of these events, coupled with Ernesto's poor structure, I will not be indicating such at this time. Gradual strengthening is still forecast, but this is a low confidence forecast.

Ernesto could still become a major hurricane once it hits the untapped heat reservoir of the western Caribbean. However, I do not consider this possibility to be as likely as it was yesterday. The intensity forecast is reduced from yesterday to account for the likelihood of a landfall along the Yucatan Peninsula.

Ernesto has made the long anticipated turn to the west-northwest as it comes under a slight erosion in the subtropical ridge. The global models continue to suggest that a weakness will develop in the ridge over the next few days. Previously, I had anticipated that this weakness would be great enough to potentially turn Ernesto northward. But given the lack of strengthening of the cyclone thus far, which was not expected, this possibility is becoming increasingly less likely. Additionally, the HWRF has joined the model consensus as of 18z when it was previously forecasting a landfall along the northern Gulf Coast, which adds confidence to the forecast track.

0z upper air data over the lower 48 suggests that the trough currently moving across the northern plains is rather strong above 500 mb. As this trough moves eastward, it should weaken the ridge a bit further. By Thursday, the models suggest the arrival of another trough, this one far more potent, but Ernesto may be too far south to feel the influence of the secondary trough. The only way I can see Ernesto moving northward is from a more a poleward center reformation under the deeper convection, or it becomes a significant hurricane in the western Caribbean (Category 3 or 4). Even then, I feel that any such northward turn would occur much farther west than I was indicating yesterday. At this point I've pretty much taken areas from Louisiana eastward out of the woods given recent model trends and the relative storm organization. It is possible that the secondary trough could cause some rather slow motion prior to Ernesto's final landfall.

5-day intensity forecast

INITIAL 08/05 0300Z 50 KT 60 MPH
12 hour 08/05 1200Z 50 KT 60 MPH
24 hour 08/06 0000Z 55 KT 65 MPH
36 hour 08/06 1200Z 60 KT 70 MPH
48 hour 08/07 0000Z 65 KT 75 MPH
72 hour 08/08 0000Z 75 KT 85 MPH
96 hour 08/09 0000Z 70 KT 80 MPH...INLAND
120 hour 08/10 0000Z 75 KT 85 MPH...OVER WATER

5-day track forecast



Figure 2. My 5-day forecast track for Ernesto.

Watches and warnings

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* JAMAICA

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

INTERESTS IN THE CAYMAN ISLANDS AND THE YUCATAN PENINSULA OF MEXICO
SHOULD MONITOR THE PROGRESS OF ERNESTO. A TROPICAL STORM WATCH MAY
BE REQUIRED FOR PORTIONS OF THE CAYMAN ISLANDS ON SUNDAY.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.



Florence

Tropical Storm Florence continues to intensify as of the latest NHC advisory:

Wind: 60 mph, with higher gusts
Location: 16.1°N 33.0°W
Movement: WNW at 15 mph
Pressure: 1000 mb
Category: Tropical storm

Florence has changed little in organization since the advent of the most recent NHC advisory, with fairly deep convection near the low-level center.



Figure 3. Latest infrared satellite image of Tropical Storm Florence. Image credit: NOAA

Earlier microwave data indicated that the inner core structure of the tropical cyclone was well-defined. Although Florence is experiencing little vertical shear, water vapor imagery suggests that it is approaching a rather stable airmass. In addition, the SHIPS analysis of areal sea surface temperatures is at the lower end of 26C, rather cool waters for a hurricane. While a little more intensification is possible over the next 12-24 hours, in all likelihood Florence is near its peak intensity. Watch it surprise me and become a hurricane. Although the cyclone is forecast to approach warmer water temperatures on Tuesday, that will be coincident with a significant increase in westerly shear, which is expected to slowly chew away at Florence. Because Florence is a small storm, this shear could kill it off before the end of the forecast period, but I am not explicitly showing it at this time.

The track forecast is relatively straightforward. Florence is on the south side of the Bermuda-Azores ridge, which is propelling it toward the west-northwest. This general motion is expected to continue throughout the forecast period as the cyclone maintains enough vertical integrity to feel a rather large weakness in the subtropical ridge forecast to lie along 50-55W at the end of the forecast period. With slight differences in the strength of the system, the global models are in good agreement with this.

5-day intensity forecast

INITIAL 08/05 0300Z 50 KT 60 MPH
12 hour 08/05 1200Z 55 KT 65 MPH
24 hour 08/06 0000Z 55 KT 65 MPH
36 hour 08/06 1200Z 50 KT 60 MPH
48 hour 08/07 0000Z 45 KT 50 MPH
72 hour 08/08 0000Z 35 KT 40 MPH
96 hour 08/09 0000Z 25 KT 30 MPH...POST-TROP/REMNANT LOW
120 hour 08/10 0000Z...DISSIPATED

5-day track forecast



Figure 4. My 5-day track forecast for Florence.

2012 Atlantic hurricane season Tropical Storm Ernesto Tropical Storm Florence

Updated: 5:34 AM GMT on August 05, 2012

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Tropical weather analysis - August 3, 2012

By: KoritheMan, 3:46 AM GMT on August 04, 2012

Ernesto

Tropical Storm Ernesto appears to be intensifying. The latest NHC advisory had this to say about the storm:

Wind: 50 mph, with higher gusts
Location: 13.9°N 65.6°W
Movement: W at 18 mph
Pressure: 1002 mb
Category: Tropical storm

Ernesto looks the best it ever did on satellite, and it is likely recon will find stronger winds during their next scheduled penetration. Unfortunately, the storm is out of range of both the San Juan and Curacao radars, making a decent look at the center difficult from that vantage point. As I write this, Dvorak numbers are starting to come up, and it is likely that satellite estimates will correspond to the improving satellite signature soon enough.



Figure 1. Latest infrared satellite image of Tropical Storm Ernesto. Image credit: NOAA

Although not yet perfect, the upper flow pattern over Ernesto is becoming more diffluent. The GFS/SHIPS forecasts the environment to gradually become anticyclonic over the next few days. This is particularly true when the system enters the western Caribbean on Monday. At that point, an upper low is forecast to be over the western Gulf of Mexico, which could provide some sort of upper air ventilation to the tropical cyclone at that time. This low is already apparent on water vapor imagery developing off the southwest Florida coast. Given the very high oceanic heat content currently lying dormant in the western Caribbean Sea, such a pattern could herald rapid intensification at that stage. One negative is the relatively quick forward motion of the cyclone -- it is moving at 18 mph, which makes surface convergence (and thus thunderstorms) difficult. However, Ernesto is forecast to gradually slow, and all indications are that it will be a dangerous hurricane in the western Caribbean and Gulf of Mexico. I admit that my intensity forecast is a conservative one, but I still don't know how Ernesto is going to interact with land. If it moves over the Yucatan Peninsula like the hurricane center predicts, that could temporarily halt strengthening. On the other hand, if it avoids the peninsula, it could become stronger than anticipated. My forecast is a compromise of these possibilities, and one is reminded that 5-day intensity forecasts are subject to large errors, up or down.

I am still having difficulty in pinpointing exactly where Ernesto will go. Now that it's safely tucked in the Caribbean, we know it's not going to recurve. But the million dollar question is: how strong will the central plains ridge be? I note that it has been a very persistent feature this summer, what with the extreme heat waves over that portion of the country. While I doubt this pattern will break away completely, as they often don't, the global models all agree on some sort of trough amplifying over the central and southern United States by Tuesday. Although the trough is forecast to slowly lift out by Thursday, a secondary trough, this one a bit more potent, is forecast to begin amplifying over the western United States at around roughly the same time. This could theoretically reinforce any northward motion, but this is contingent on how strong Ernesto becomes over the Caribbean. As a general rule, stronger systems tend to move in a more poleward fashion.

The first trough is already in the process of amplifying over the Great Basin. Upper air data and water vapor images show that this feature is quite strong, but I would rather be conservative before I deviate too far to the right of my previous forecasts. For one thing, there's the GFS and ECMWF solution of a Mexico landfall that remains a possibility. When these two models are in harmonious agreement, their consensus is usually hard to beat. However, these models do not seem to be handling the synoptic environment ahead of Ernesto particularly well, and the assumption of a more westward motion at longer ranges seem to hinge almost entirely on a weaker cyclone vortex missing the trough. However, it's also possible the trough won't be strong enough to fully pick up Ernesto, especially given the strength of the central United States ridge this summer. Keeping this in the back of my mind, I will not yet fully commit to a definitive northward motion when the system approaches the Gulf of Mexico. But given the uncertainties, interests along the Gulf Coast from Mexico to Alabama should monitor the progress of Ernesto, as it has the potential to be a major hurricane.

It should be noted that 91L could play an important role in the future track of Ernesto, especially if intensifies into a tropical cyclone. This option would tend to reinforce the weakness in the ridge at longer ranges, allowing Ernesto to move more northward. For now, I am leaving this possibility out of the forecast due to uncertainties in how 91L is going to evolve.

5-day intensity forecast

INITIAL 08/04 0000Z 45 KT 50 MPH
12 hour 08/04 1200Z 50 KT 60 MPH
24 hour 08/05 0000Z 55 KT 65 MPH
36 hour 08/05 1200Z 55 KT 65 MPH
48 hour 08/06 0000Z 60 KT 70 MPH
72 hour 08/07 0000Z 65 KT 75 MPH
96 hour 08/08 0000Z 75 KT 85 MPH
120 hour 08/09 0000Z 85 KT 100 MPH

5-day forecast track



Figure 2. My 5-day forecast track for Ernesto.



Tropical Depression Six

A well-defined tropical wave in the eastern Atlantic located about 230 miles southwest of the Cape Verde Islands ("90L") has become a tropical depression. As of the initial NHC advisory, the following information was posted on the storm:

Wind: 35 mph, with higher gusts
Location: 13.8°N 27.8°W
Movement: WNW at 16 mph
Pressure: 1009 mb
Category: Tropical depression

The low-level center appears to be the on eastern edge of the convection, which is exhibiting a curved band pattern on satellite imagery.



Figure 3. Latest infrared satellite image of Tropical Depression Six. Image credit: RAMMB

Upper-level winds in the path of the system are fairly light, which could favor some additional intensification over the next day or so. After that, the cyclone is forecast to enter a drier airmass and move across slightly cooler sea surface temperatures, the combination of which should halt further intensification. None of the global models intensify this system much, but neither do they with Ernesto. However, in this case, they appear to have a more realistic presentation of the surrounding environment so that their forecasts are more believable. Beyond 72 hours, some westerly shear could begin to affect the system. However, this shear isn't too strong in the GFS fields relative to the vortex location, so it is uncertain how much of this shear will actually penetrate the core. My forecast will call for slight strengthening over the next 24 hours, followed by slowly weakening storm. However, the cyclone could just as easily dissipate.

5-day intensity forecast

INITIAL 08/04 0300Z 30 KT 35 MPH
12 hour 08/04 1200Z 35 KT 40 MPH
24 hour 08/05 0000Z 40 KT 45 MPH
36 hour 08/05 1200Z 40 KT 45 MPH
48 hour 08/06 0000Z 40 KT 45 MPH
72 hour 08/07 0000Z 35 KT 40 MPH
96 hour 08/08 0000Z 30 KT 35 MPH
120 hour 08/09 0000Z 25 KT 30 MPH

The depression is south of a mid-level ridge. An upstream shortwave trough over France carries lingering southwesterly flow all the way to north Africa, which is creating a slight weakness in this ridge. Therefore, a more climatological due westward motion is not anticipated in the near-term. By Monday, the tropical cyclone is forecast to be moving west. However, concurrent with this, the storm will be approaching a break in the subtropical ridge associated with a large upper-level low pressure system; water vapor imagery indicates a general weakness west of 35W. Assuming the cyclone maintains some semblance of convection at that time, it could follow this weakness out to sea, or it could move more westward with the low-level flow as a weaker entity. I favor the latter option.

5-day track forecast



Figure 4. My 5-day forecast track for Tropical Depression Six.



Invest 91L

A large area of showers extending across the Bahamas southward toward Cuba is associated with a surface trough. The associated shower activity is poorly-organized, surface pressure are not falling, there are no signs of a circulation, and upper-level winds are not currently favorable for development. Based on a couple of surface observations, satellite interpretation, and doppler radar data out of Miami, any center is probably located to the south of Nassau, but there is certainly margin for error. However, since the system is disorganized, a center reformation could occur underneath the deepest convection to the south, closer to the north coast of Cuba. If this happens, 91L will have more time over water, and would likely be a stronger entity in the Gulf of Mexico.



Figure 5. Latest infrared satellite image of Invest 91L. Image credit: NOAA

The upper low west of Naples is forecast to move westward away from 91L over the next few days, which would place the system in a more diffluent southeasterly shearing regime when it enters the Gulf of Mexico. I am not particularly enthusiastic about a strong system, but the environment over the Gulf appears favorable enough for the formation of a tropical depression or tropical storm should 91L hold together. I am not yet going to call for genesis, however.

Based on 0z upper air data from the Bahamas and Florida, the system is not in a hurry to move. I expect a very slow northwestward motion for the next 24 hours, with the system picking up speed after that. Depending on where the center is, the system should be inland by tomorrow night. However, if no significant motion occurs, this may be somewhat delayed. Once in the Gulf of Mexico, the models have differing solutions regarding landfall, and areas anywhere from Louisiana to the Florida panhandle should monitor the progress of 91L. Interestingly, the Euro does not bring the system into the Gulf, instead calling for a more northward motion toward Georgia. The latter scenario seems unlikely since the weak trough over the eastern US appears to be lifting out.

Regardless of development, locally heavy rainfall will affect portions of the Bahamas and south Florida over the next day or so.

Probability of development in 48 hours: 30%

2012 Atlantic hurricane season Tropical Storm Ernesto Invest 91L Tropical Depression Six

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Tropical weather analysis - August 3, 2012

By: KoritheMan, 6:00 AM GMT on August 03, 2012

Ernesto

Tropical Storm Ernesto is nearing the Windward Islands, and is located very near Barbados. As of the latest NHC advisory, the following information was posted on the cyclone:

Wind: 50 mph, with higher gusts
Location: 13.4°N 58.3°W
Movement: W at 22 mph
Pressure: 1005 mb
Category: Tropical storm

While by no means fabulous, the satellite presentation is slowly improving, with new convective cells developing closer to the low-level center.



Figure 1. Latest infrared satellite image of Tropical Storm Ernesto. Image credit: NOAA

Westerly to southwesterly wind shear associated with an anticyclone over Venezuela is still affecting Ernesto. The SHIPS models shows this shear decreasing over the next 12-24 hours, which if it verifies, should allow Ernesto to strengthen. Current large-scale trends appear to support this. Additionally, water vapor and CIMSS Total Precipitable Water (TPW) imagery shows little evidence of any nearby dry air. Taking this into consideration, there seems to be little reason to assume the cyclone will not continue intensifying. Ernesto has escaped the influence of the upper low near 27.5N 45.0W that had been slinging westerly shear, and from this point onward the upper flow looks relatively favorable up to 75W. I note however, that there appears to be a bit of southeasterly shear over the waters of the eastern Caribbean. These are not trade winds, but are likely associated with the South American anticyclone. Should this feature fail to weaken or collocate with Ernesto, there could be a short-term struggle for intensification. However, since this could just as easily be part of the outflow related to Ernesto, I am not going to count this as a deterrent to strengthening.

The upper-level tropospheric wind pattern over the Caribbean appears conducive for at least steady strengthening over the next couple of days. When Ernesto approaches the longitude of Jamaica in about three days, a very favorable upper-level environment awaits the tropical cyclone. Thus, at longer ranges, there is definitely the possibility of rapid intensification. But until I can get a little more confidence in how the shear pattern is going to evolve, I will remain conservative. Ernesto has the potential to be a major hurricane in the Gulf of Mexico beyond day five.

5-day intensity forecast

INITIAL 08/03 0600Z 45 KT 50 MPH
12 hour 08/03 1800Z 50 KT 60 MPH
24 hour 08/04 0600Z 50 KT 60 MPH
36 hour 08/04 1800Z 55 KT 65 MPH
48 hour 08/05 0600Z 60 KT 70 MPH
72 hour 08/06 0600Z 70 KT 80 MPH
96 hour 08/07 0600Z 75 KT 85 MPH
120 hour 08/08 0600Z 80 KT 90 MPH

The track forecast is still a bit of a challenge, primarily due to how much strength Ernesto will gain while moving across the Caribbean. The global models suggest that the strong subtropical ridge to the north of the system will remain intact throughout the next five days, steering the system generally toward the west-northwest. By Tuesday, an upstream trough is forecast by all of the models to begin developing over the southern plains. That favors a slowing of the forward speed, which is supported by the models. Ordinarily this would be the harbinger of recurvature, but the GFS and ECMWF still insist on taking the system westward toward Mexico and/or the western Gulf at that point. However, examination of the lower- to middle-tropospheric fields within those models shows that the depicted ridge in much weaker at 500 mb than it is at 850 mb. Hence, these more westward forecasts are likely predicated on a weaker system moving in tandem with the lower-tropospheric flow. Since Ernesto is expected to be a fairly strong system, I consider this scenario to be unlikely (but still possible if Ernesto struggles).

As a course of least regret, I will not deviate too far east of the model consensus -- yet. However, I anticipate that they will come more east over the next couple of days. A significant slowing of the forward speed is possible by day five and beyond as the large-scale steering weakens with the approach of the aforementioned trough. Given the inherent uncertainties ever present in intensity forecasts, which the track is contingent upon, confidence in the forecast track beyond day five is not particularly high.

Interests in the Windward Islands and western Caribbean should closely monitor the progress of Ernesto. The Barbados radar already shows rainbands beginning to affect portions of the Lesser Antilles within the tropical storm warning area. Preparations to protect life and property should be completed quickly before conditions begin to deteriorate.

5-day track forecast



Figure 2. My 5-day forecast track for Ernesto.

Watches and warnings

THE GOVERNMENT OF TRINIDAD AND TOBAGO HAS ISSUED A TROPICAL STORM
WATCH FOR GRENADA AND ITS DEPENDENCIES.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* BARBADOS...ST. VINCENT AND THE GRENADINES...AND DOMINICA
* ST. LUCIA
* MARTINIQUE AND GUADELOUPE

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* GRENADA AND ITS DEPENDENCIES

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN
THE NEXT 12 TO 24 HOURS.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...IN THIS CASE WITHIN THE NEXT
12 TO 24 HOURS.

INTERESTS ELSEWHERE IN THE LESSER ANTILLES SHOULD MONITOR THE
PROGRESS OF THIS SYSTEM.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.

2012 Atlantic hurricane season Tropical Storm Ernesto

Updated: 6:08 AM GMT on August 03, 2012

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Tropical weather analysis - August 1, 2012

By: KoritheMan, 2:06 AM GMT on August 02, 2012

Tropical Depression Five

The tropical wave formerly known as Invest 99L has become a tropical depression. As of the first NHC advisory, the following information was posted on the cyclone:

Wind: 35 mph, with higher gusts
Location: 12.4°N 49.8°W
Movement: WNW at 18 mph
Pressure: 1008 mb
Category: Tropical depression

The depression is rather ragged at the moment, with the center exposed along the northern edge of the convection. Satellite images show that the tropical cyclone is being pummeled by southwesterly shear in association with a large upper low centered several hundred miles to the north. Dry air doesn't currently appear to be an issue, although there is some just to the west of the depression. I should note that although the current cloud pattern is rather ragged looking, it is not uncommon for weak tropical cyclones to look, to put it bluntly, pitiful, at this time of day.



Figure 1. Latest infrared satellite image of Tropical Depression Five. Image credit: NOAA

The intensity forecast is quite problematic. Satellite and water vapor imagery shows that the depression is not currently embedded within a particularly favorable environment, with dry air and southwesterly shear noted in front of it. High cloud motion vectors indicate that this shear is probably not sufficient to decapitate the cyclone, but the shear in combination with the surrounding dry air will likely slow intensification in the short-term. However, the shear is forecast to slowly diminish, and I see faint indications of this on water vapor imagery as well. In particular, conditions should become much more conducive after 24 hours as the ambient upper flow becomes more diffluent. However, there may still be some dry air for the system to contend with in the eastern Caribbean, so rapid intensification seems unlikely for the next few days despite the forecast for a slowly improving upper-tropospheric flow pattern. Some of the models still insist the system will not survive the Caribbean, which is possible if the depression remains disorganized or the shear does not relax as anticipated. However, assuming the current cloud pattern is just a sprawl, slow intensification seems like a more reasonable bet. The SHIPS shows relative humidity values increasing by 72 hours, which is when a more steady strengthening trend may become established. Although the 5-day intensity forecast reflected below does not show a strong hurricane, given the very favorable upper-level environment forecast by the GFS, intensities at longer-ranges may be underestimated. Tropical Depression Five certainly has the potential to become a dangerous hurricane in the western Caribbean Sea next week.

The CMC shows stronger shear, and the ECMWF fails to develop the cyclone throughout its Caribbean cruise. The presumed catalyst for the forecast vertical shear in those models is probably the mid-oceanic trough, which typically lies along and/or north of the Greater Antilles. Given the broad anticyclonic flow over the tropical wave in that region, most of the associated westerly shear is being pushed upstream. Thus, I will go with the GFS for now. However, I will remain conservative until I see how the depression fares against its environment.

5-day intensity forecast

INITIAL 08/02 0000Z 30 KT 35 MPH
12 hour 08/02 1200Z 30 KT 35 MPH
24 hour 08/03 0000Z 35 KT 40 MPH
36 hour 08/03 1200Z 40 KT 45 MPH
48 hour 08/04 0000Z 50 KT 60 MPH
72 hour 08/05 0000Z 55 KT 65 MPH
96 hour 08/06 0000Z 65 KT 75 MPH
120 hour 08/07 0000Z 70 KT 80 MPH

And now it's time to discuss the track. Synoptic data indicates that the depression is located to the south of a low- to mid-level ridge. This should continue the west-northwest motion throughout the forecast period, as the GFS and ECMWF, the ones I'm in greatest agreement with, do not really show the ridge budging all that much. On the other hand, the CMC and NOGAPS still continue to show a poleward track around the periphery of the Atlantic ridge on a trajectory that would take the storm out to sea, or at least toward the southeast United States coast. This seems quite suspect in light of current trends, and the fact that the system is not expected to rapidly intensify. Considerable uncertainty exists after day 5, and at that point it's anyone's guess as to where the cyclone might go. Model guidance suggests that a trough could move from the western United States to the central plains, causing a more northwestward motion by that time. On the other hand, the ridge depicted in these models, particularly within the GFS and ECMWF, is quite strong in the lower troposphere. Thus, if the system miraculously stays weak, a more southern path toward Texas or Mexico is possible.

5-day track forecast



Figure 2. My 5-day forecast track for Tropical Depression Five.

Interests in the southern Windward Islands should closely monitor the progress of this depression. Tropical storm force winds and heavy rainfall are expected to begin overspreading these islands on Friday. Tropical storm watches have already been posted for portions of the Lesser Antilles:

Watches and warnings

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* BARBADOS...ST. VINCENT AND THE GRENADINES...AND DOMINICA
* ST. LUCIA
* MARTINIQUE AND GUADELOUPE

INTERESTS ELSEWHERE IN THE LESSER ANTILLES SHOULD MONITOR THE
PROGRESS OF THIS SYSTEM.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.

2012 Atlantic hurricane season Tropical Depression Five

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.

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