KoritheMan's WunderBlog

Tropical weather analysis - September 30, 2012

By: KoritheMan, 7:37 AM GMT on September 30, 2012

Nadine

Nadine is one tenacious bugger. As of the latest NHC advisory, the storm actually strengthened a little:

Wind: 85 mph, with higher gusts
Location: 35.6°N 37.5°W
Movement: NNW at 10 mph
Pressure: 984 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Satellite images suggest that the eye has become better defined over the last few hours, although cloud tops in the eyewall have warmed a bit.



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

There really isn't any real evidence of vertical shear over Nadine, or in the nearby storm environment. Nor do I see many downstream features that would be capable of causing a sudden spike in wind shear over the hurricane. Indeed, it looks as if the upper tropospheric flow, barring subtle upper level changes due to alternating passages of shortwaves -- troughs and ridges -- I don't really expect an appreciable increase in shear until about 72 hours. Beyond that time, an increase in southerly shear is forecast as Nadine interacts with a rather strong baroclinic zone over the north Atlantic.

It should be noted that Nadine's current operational intensity of 75 kt is the same as its initial peak two weeks ago. This is somewhat remarkable considering that the cyclone has been over sub-26C SSTs for about 36 hours. I initially did not expect this given that there appeared to be more ridging than troughing over the subtropics, but a similar synoptic situation that allowed Chris to become a hurricane over cool waters all the way back in June appears to have set up again for Nadine. AMSU temperature cross section diagrams from yesterday (Saturday) revealed that the upper troposphere is rather cold. Hence, it appears that the relative temperature gradient between the lukewarm sea surface temperatures and the rather cold upper-level temperatures are providing the mechanisms for intensification with Nadine, in an environment not generally considered conducive for tropical cyclone formation and intensification. It is a little uncertain how long this upper support can continue, but I suspect for at least the next 36 hours.

Gradual weakening is expected beyond 36 hours, although it should be noted that with the expectation of lower vertical shear through 72 hours, any subtle environmental change -- namely one akin to the one being experienced now, with cold temperatures aloft relative to warm water temperatures -- could allow the hurricane to remain stronger than indicated below. Also, I have kept the 96 and 120 hour intensities up a little, as I see a strong likelihood of the north Atlantic low giving Nadine a burst of baroclinic forcing.

Regardless, Nadine is forecast to become extratropical at the end of the forecast period as it interacts with a large mid- to upper-level trough over the north Atlantic. The global models are in good agreement with this, and for the sake of my own sanity, I sincerely hope they are correct.

The steering for Nadine remains a little complex, at least in the short-term. Water vapor imagery shows a mid- to upper-level trough passing to the north of the hurricane. This would coincide well with the uniform zonal (west to east) flow north of about 35N. Downstream and closer to home, another trough with its parent upper low over the Ohio Valley, appears to be slowly amplifying over the western Atlantic waters. This appears to be the trough that should ultimately cause Nadine to lose tropical characteristics as well as accelerate her out of the subtropics. However, this developing system is not moving much at the moment, and will likely gain little latitude over the next day or two. After that time, model guidance is unanimous in bringing the system into the north Atlantic, which should increase the southerly flow over Nadine and cause a gradual acceleration.

The models continue to show a tight cyclonic loop over the next 24-48 hours as surrounding steering currents collapse. This is supported by current observational trends, as the zonal flow over the north Atlantic suggests that the shortwave to the north will bypass Nadine. I will show a southward motion as well, but not as much as most of the guidance.

5-day intensity forecast

INITIAL 09/30 0900Z 75 KT 85 MPH
12 hour 09/30 1800Z 75 KT 85 MPH
24 hour 10/01 0600Z 75 KT 85 MPH
36 hour 10/01 1800Z 75 KT 85 MPH
48 hour 10/02 0600Z 70 KT 80 MPH
72 hour 10/03 0600Z 65 KT 75 MPH
96 hour 10/04 0600Z 55 KT 65 MPH
120 hour 10/05 0600Z 55 KT 65 MPH...POST-TROPICAL/EXTRATROPICAL

5-day track forecast



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

With a total of nearly 18 days as a tropical cyclone, Nadine has become the longest-lasting Atlantic hurricane since Hurricane Bertha in 2008.



Jelawat

I will not have a post on Jelawat tonight, as it is poised to make landfall in southern Japan soon, accelerating in mid-latitude southwesterly flow. Strong winds may impact the Tokyo metropolitan area, which may produce some sporadic power outages. Rainfall will be quite limited, especially in the southern and western quadrants of the storm, which appear to be entraining dry air. This is consistent with extratropical transition, which is likely to occur in about a day or so.



Elsewhere

None of the computer models are showing anything remotely suggestive of a tropical threat in the Atlantic for the next seven days. The rainy low pressure system over the northern Gulf Coast is expected to remain inland, and development is no longer feasible.

2012 Atlantic hurricane season Hurricane Nadine

Updated: 7:41 AM GMT on September 30, 2012

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Tropical weather analysis - September 28, 2012

By: KoritheMan, 4:26 AM GMT on September 29, 2012

Nadine

Tenacious Nadine became a hurricane again around midday today. As of the latest NHC advisory, the following information was available on the hurricane:

Wind: 75 mph, with higher gusts
Location: 31.0°N 35.5°W
Movement: NNW at 10 mph
Pressure: 988 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

There has been little change to the structure of Nadine this evening. The hurricane continues to generate a circular area of cold topped convection. Although an eye is not readily apparent on conventional satellite images, recent microwave data suggests the internal structure remains well-organized. However, the eyewall is open to the south, and the lower and middle-level circulations exhibit a northeastward displacement of about 25 miles. This is probably due to about 20 kt of southwesterly vertical wind shear as depicted by analysis from UW-CIMSS and the SHIPS model.



Figure 1. Latest infrared satellite image of Hurricane Nadine. Image credit: RAMMB Colorado State University (CSU).

The shear is anticipated to continue for another 48 hours or so. After that time, the both the SHIPS and GFS forecast a relative decrease in the shear. Given that the GFS has been indicating this for the last several days, it is a little difficult to ignore. While restrengthening is not being explicitly indicated at this time, if enough baroclinic forcing happens to develop, Nadine will probably strengthen a little from the 48-72 hour point. However, the models are not consistent in when or if this will occur. Beyond day three, northerly shear is forecast to abruptly increase over the storm, which should put an end to any possible intensification.

Nadine has made a turn north-northwestward, as water vapor imagery and UW-CIMSS steering data suggest that the nearby subtropical ridge continues to weaken with the approach of a vigorous upper low/baroclinic zone over the central Atlantic. This synoptic pattern is forecast to continue the north-northwestward motion for the next day or two. Beyond 48 hours, Nadine is expected to move north and northeast as it gradually becomes enraptured in mid-latitude westerly flow associated with a deep-layer trough that the global models are unanimous in bringing across the north Atlantic shipping lanes. It appears that, finally, that Nadine may have had her fun. With the models having come into better agreement over the past 24 hours, confidence in the long-range forecast track is a little higher, but not completely. And while a relatively slow motion is still shown at the end of the forecast period, I am considerably faster than the last several days. But I'm still prepared for surprises, and I'm not going to go too fast.

5-day intensity forecast

INITIAL 09/29 0300Z 65 KT 75 MPH
12 hour 09/29 1200Z 65 KT 75 MPH
24 hour 09/30 0000Z 65 KT 75 MPH
36 hour 09/30 1200Z 65 KT 75 MPH
48 hour 10/01 0000Z 65 KT 75 MPH
72 hour 10/02 0000Z 65 KT 75 MPH
96 hour 10/03 0000Z 55 KT 65 MPH
120 hour 10/04 0000Z 45 KT 50 MPH

5-day track forecast



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



Norman

Tropical Storm Norman quickly developed today off the southwest coast of Mexico, but has already weakened to a tropical depression. As of the latest NHC advisory, the following information was posted on the cyclone:

Wind: 35 mph, with higher gusts
Location: 25.2°N 109.0°W
Movement: NNW at 10 mph
Pressure: 1003 mb

Norman is a poorly-organized tropical cyclone, to say the least. Based on satellite images, surface observations from mainland Mexico and southern Baja, and finally, doppler radar data from Guasave, the center appears to be located to the north of an elongated band of relatively deep convection.



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

The highest sustained wind speed I could find overland was a 23 mph sustained wind at Los Mochis, located about 60 miles from the coast of Mexico. There is no indication that Norman is producing any sustained winds to tropical storm force, although 40 to 45 mph gusts will still be possible within the rather vigorous band to the east of the center. The previously cited wind report from Los Mochis was actually outside of this band, so people living near or underneath it should exercise caution.

Norman is located just off the coast, and it will not take much for it to make landfall. However, some of the more reliable guidance, such as the GFS, has come westward, and now show Norman moving parallel to, but just offshore, the coast before turning westward. Since the storm has recently turned north-northwest, a motion just off the coast is indeed within the realm of possibility. That being said, given that not all of the models agree with the GFS scenario, I will go ahead and pull Norman inland, but will more or less keep it between the coastal waters and the immediate coast, obviously out of respect for the GFS.

I should note that Norman's possible landfall is of little consequence, as all of the weather is in bands to the south and east of the center. These rains will be capable of causing flash flooding and mudslides, particularly in areas of higher terrain. This will be especially if true the farther Norman moves inland.

After landfall, or even if not, the cyclone is forecast to quickly wind down. I am showing dissipation in 36 hours, but it could certainly occur sooner.

It should be noted that some of Norman's mid-level energy is expected to get caught up in deep-layer southwesterly flow ahead of a cold front and help to initiate a surface low over south Texas over the weekend, which will likely be very heavy rains to the northern Gulf Coast.

5-day intensity forecast

INITIAL 09/29 0300Z 30 KT 35 MPH...APPROACHING THE SOUTHWEST COAST OF MEXICO
12 hour 09/29 1200Z 25 KT 30 MPH...INLAND POST-TROPICAL/REMNANT LOW
24 hour 09/30 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
36 hour 09/30 1200Z...DISSIPATED

5-day track forecast



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



Jelawat

Typhoon Jelawat continues to steadily weaken as it plows through Okinawa. As of the 0300Z JTWC advisory, the following information was available on the typhoon:

Wind: 115 mph, with higher gusts
Location: 26.1N 127.4E
Movement: NE at 15 mph
Category: 3 (Saffir-Simpson Hurricane Scale)

Recent satellite fixes indicate that Jelawat is slightly west of the 0z forecast point from the JTWC. Other than that, there has been no significant deviation from the projected track.

Satellite images still show a formidable typhoon, but the eye is extremely ragged.



Figure 5. Latest infrared satellite image of Typhoon Jelawat. Image credit: NOAA

There is not much to say about the storm. The long-anticipated southwesterly shear has arrived, as a 0100Z microwave pass indicated the southern eyewall had fragmented. In addition, some drier air has periodically filtered into the circulation. Unlike the episodes of the last several days, there is no indication that Jelawat is going through another eyewall replacement cycle. In fact, considering that it is forecast to quickly weaken subsequent to this point, it is likely that there will be no additional cycles. All of that being said, Jelawat is still expected to approach the southern coast of Japan as a strong tropical storm. It could pass directly over the heavily populated city of Tokyo, and residents there should be taking it seriously. Rainfall potential will be limited, since the cyclone will be moving quite rapidly as it moves across the country.

Jelawat is expected to lose tropical characteristics in about 72 hours as it moves over cold water and encounters even stronger upper-level shear.

The track forecast remains straightforward. The typhoon is well-embedded within a southwesterly steering current between a strong ridge to the east and a deep-layer trough to the west. A continued acceleration to the northeast is expected over the next few days. Tonight's track merely updates the previous few.

Okinawa reported a maximum wind gust of 112 mph, along with a central pressure of 955 mb as the eye passed over. This was along the southern end of the archipelago, on Naha.

5-day intensity forecast

INITIAL 09/29 0300Z 100 KT 115 MPH
12 hour 09/29 1200Z 90 KT 105 MPH
24 hour 09/30 0000Z 70 KT 80 MPH
36 hour 09/30 1200Z 50 KT 60 MPH...INLAND OVER SOUTHEASTERN JAPAN
48 hour 10/01 0000Z 30 KT 35 MPH...OVER WATER
72 hour 10/02 0000Z 25 KT 30 MPH...POST-TROPICAL/EXTRATROPICAL
96 hour 10/03 0000Z...ABSORBED BY FRONT

5-day track forecast



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



Gulf of Mexico disturbance taking shape

The much-advertised Gulf of Mexico disturbance is beginning to manifest over southern Texas, as evidenced by a recently-formed cluster of convection. This is occurring at the southern end of a rather potent shortwave trough currently moving across central Texas. The global models have generally trended toward a more inland/less time over water solution. However, there could still be a narrow corridor of lighter upper-level winds that could assist in development if the low moves over water longer than predicted. As previously mentioned, this low is being fed in part by Eastern Pacific Tropical Depression Norman. This is a rare synoptic setup.

Regardless of development, heavy rainfall, gusty winds along the coast, as well as possible coastal flooding will occur over portions of the northern Gulf Coast through about Tuesday. These rains could be capable of causing flooding. Some severe weather will also be possible with this activity, although large-scale cloud cover may inhibit instability somewhat.

This system currently looks like it will move across Louisiana based on the model runs.

2012 Atlantic hurricane season 2012 East Pacific hurricane season 2012 Western Pacific typhoon season Hurricane Nadine Typhoon Jelawat Tropical Depression Norman

Updated: 4:27 AM GMT on September 29, 2012

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Tropical weather analysis - September 28, 2012

By: KoritheMan, 7:29 AM GMT on September 28, 2012

Nadine

Nadine continues as a tropical storm. The following information was available on the storm as of the latest National Hurricane Center forecast package:

Wind: 65 mph, with higher gusts
Location: 28.8°N 33.6°W
Movement: W at 7 mph
Pressure: 993 mb

Nadine looks more tropical than it has in millenia. Convection is actually near the center for once, and a large curved band wraps around the western semicircle. Upper-level outflow is becoming established as well, something Nadine has lacked for awhile. An eye feature briefly tried to appear around 4z as well. A well-timed microwave pass at 0327 UTC suggested this feature also, but the associated convection was more reminiscent of banding rather than a formative eyewall.



Figure 1. Latest infrared satellite image of Tropical Storm Nadine. Image credit: RAMMB Colorado State University (CSU).

Environmental conditions are forecast to remain marginally conducive for the next 24 hours or so, with relatively warm sea surface temperatures and weak vertical shear. However, satellite data and water vapor imagery indicate strong upper-level southwesterly flow a couple hundred miles southwest of the storm. This shear is probably going to interfere with development before long, but then again, Nadine has shielded herself well against the shear thus far, which was forecast to pick up by this time. It should be noted that some of the guidance now calls for Nadine to regain hurricane strength. This is a distinct possibility, although acknowledging the aforementioned vertical shear pattern, I am not ready to call for that at this time. In addition to the shear, Nadine will be moving over slowly decreasing sea surface temperatures subsequent to this point. Hopefully this will be her final hurrah, but I wouldn't count on it.

Nadine is approaching the southwestern side of a deep-layer ridge over the central Atlantic, which is being pushed slowly eastward by an approaching shortwave trough. Recent satellite fixes and earlier microwave fixes suggest that the tropical cyclone may have begun to turn toward the west-northwest; it's certainly not moving west anymore. Nadine is expected to turn northwestward over the course of the day today as it rounds the axis of the subtropical ridge. A turn toward the north is anticipated in about 36 hours. The models agree up to the 72 hour point, and then begin to diverge. I would say I see signs of Nadine's departure in the global model fields, but I've said this before only to get burned. Hence, I fully intend to keep my mouth shut this time.

To err on the side of caution, and because the models have been anything but consistent with this storm, I will continue to forecast a quasi-stationary storm beyond the 72 hour point. This remains a low-confidence forecast, as per usual.

5-day intensity forecast

INITIAL 09/28 0600Z 55 KT 65 MPH
12 hour 09/28 1800Z 60 KT 70 MPH
24 hour 09/29 0600Z 60 KT 70 MPH
36 hour 09/29 1800Z 60 KT 70 MPH
48 hour 09/30 0600Z 55 KT 65 MPH
72 hour 10/01 0600Z 50 KT 60 MPH
96 hour 10/02 0600Z 50 KT 60 MPH
120 hour 10/03 0600Z 45 KT 50 MPH

5-day track forecast



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



Invest 94E

A broad area of low pressure centered off the Mexican coast about 125 miles southeast of the southern tip of the Baja Peninsula ("94E") continues to produce a large area of showers and thunderstorms.



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

Looks can be deceiving. Unlike yesterday, I was fortunate enough to be witness to a 0430 UTC AMSU microwave pass, which depicted the inner structure of the disturbance rather well. It suggested that the low-level center remains broad, and the surface wind field is probably not closed. Based on the microwave data as well as shortwave infrared satellite fixes, the center appears to be on the southwest side of a large ball of cold-topper convection.

While environmental conditions are generally not unfavorable for development, global model guidance suggests the low will move inland in no more than 24 hours. If I am correct in thinking that the system is moving even faster now (which is admittedly very hard to tell using nighttime satellite imagery), it could be as early as 18 hours. Notwithstanding, as long as the circulation remains over water, the low could become a tropical cyclone prior to landfall along the southwest coast of Mexico.

The low is rounding the western extent of the subtropical ridge, and is expected to move generally northward at about 15 to 20 mph until landfall occurs.

Regardless of whether or not this system becomes a tropical cyclone, heavy rainfall and winds to tropical storm force -- primarily in gusts -- can be anticipated in heavy squalls associated with the disturbance. These rains could cause flash flooding and mudslides in areas of mountainous terrain along southwestern Mexico. A recent ASCAT pass indicated that the low is already producing winds to near tropical storm force in the deep convection.

Probability of development in 48 hours: 50%



Jelawat

Jelawat is slowly weakening while heading for Okinawa. As of the latest JTWC advisory, the following information was available on the typhoon:

Wind: 145 mph, with higher gusts
Location: 22.4N 124.1E
Movement: NNE at 7 mph
Category: 4 (Saffir-Simpson Hurricane Scale)

The ragged eye has degraded significantly, and is now completely cloud filled.



Figure 4. Latest infrared satellite image of Typhoon Jelawat. Image credit: NOAA

Microwave data throughout the last 12 hours have suggested the formation of a concentric eyewall, which suggests Jelawat is experiencing its third eyewall replacement cycle in three days. This is not uncommon in strong typhoons/hurricanes. This one is fortuitously timed, as the collapse of the inner eyewall likely heralds a weaker storm for Okinawa. It will probably take the cyclone another 12-18 hours to reach the climax of this cycle. Seeing as though it is already moving over progressively cooler water as well as encountering strong upper tropospheric vertical shear, restrengthening is not anticipated subsequent to this particular cycle. However, the outer wind field may expand as a result. I still anticipate winds of 100 to 130 mph on Okinawa as the typhoon passes by. I also expect the storm to be at or near the threshold of minimal typhoon intensity as it moves across Tokyo late Sunday or early Monday local time.

The system is forecast to lose tropical characteristics at the end of the period, although it could occur sooner given the highly baroclinic environment that is typically found north of Japan.

I don't really have an issue with the track. It remains straightforward, and the models remain in good agreement. I am merely updating the previous few.

5-day intensity forecast

INITIAL 09/28 0600Z 125 KT 145 MPH
12 hour 09/28 1800Z 115 KT 135 MPH
24 hour 09/29 0600Z 105 KT 120 MPH
36 hour 09/29 1800Z 90 KT 105 MPH
48 hour 09/30 0600Z 80 KT 90 MPH
72 hour 10/01 0600Z 55 KT 65 MPH...INLAND
96 hour 10/02 0600Z 45 KT 50 MPH...OVER WATER
120 hour 10/03 0600Z 30 KT 35 MPH...POST TROPICAL/EXTRATROPICAL

5-day track forecast



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



Gulf of Mexico development possible

Not only does the GFS continue to suggest the formation of a non-tropical area of low pressure in the northwestern Gulf of Mexico, but so do the rest of the global models, as well as the regional models like the NAM. This low is forecast to form in about 48 hours from the vigorous shortwave trough moving across west Texas. Given the strong model agreement on this evolving scenario, as well as the fact that it falls within the window of short-range weather forecasting, I see little reason to doubt it.

Yesterday I noted that upper-level winds looked unfavorable for development. Upon analysis tonight, however, I have noticed that the GFS keeps the upper flow just a little bit lighter near where the surface low is purported to develop. The placement of the low will be the critical player in determining whether or nor tropical or subtropical cyclogenesis occurs. The farther offshore the low forms, the likelihood of development lessens, since it will move inland quicker (and farther west, I might add). Conversely, a system that originates farther south in the Gulf will have more time over water, and naturally will have greater potential for development.

This system could move inland anywhere from Louisiana to the western Florida panhandle. The most likely areas still seem to be southeast Louisiana or southern Mississippi to me, although there is still time to evaluate things.

Regardless of development, it appears that widespread heavy rainfall and gusty winds along the coastal waters will affect a large section of the northern Gulf Coast through the weekend into early next week. These rains could be rather prodigious at times.

2012 Atlantic hurricane season 2012 East Pacific hurricane season 2012 Western Pacific typhoon season Tropical Storm Nadine Invest 94E Typhoon Jelawat

Updated: 7:35 AM GMT on September 28, 2012

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Tropical weather analysis - September 27, 2012

By: KoritheMan, 5:10 AM GMT on September 27, 2012

Nadine

Tropical Storm Nadine continues to slowly move across the eastern Atlantic. As of the latest NHC advisory, the following information was available on the storm:

Wind: 50 mph, with higher gusts
Location: 29.5°N 31.4°W
Movement: SW at 6 mph
Pressure: 993 mb

Nadine has become a little better organized, and the winds have come up today. Convection is limited to a curved band that wraps around the eastern semicircle.



Figure 1. Latest infrared satellite image of Tropical Storm Nadine. Image credit: RAMMB Colorado State University (CSU).

Having said that, conditions are still not ideal for strengthening. The cyclone continues to be surrounded by a very dry airmass, as evidenced by an ambient stratocumulus cloud deck. How long will Nadine linger around? Will it live to see the light of Christmas Day, and then abruptly die as a nice Christmas gift for all to see? If enough children write Santa, it just might happen.

In about 48 hours, an increase in southwesterly shear is forecast as Nadine approaches an upper-level trough. Beyond that time, cooler waters, shear, and dry air will hopefully take their toll on this tropical cyclone, as I am tired of forecasting it. There is some indication that the shear could decrease a little at day five as suggested by the GFS, but there is too much uncertainty in long-range vertical shear forecasts to be particularly confident of this.

Following the expected motif, the future track of Nadine has become uncertain once again. After a little bit of mutuality yesterday, the models have become significantly divergent after 96 hours. The ECMWF suggests that Nadine will move southwestward under a building ridge over the central Atlantic. The CMC shows little motion, while the HWRF shows Nadine getting pushed back to the south. Finally, the GFS and GFDL indicate a slow eastward motion as the cyclone remains embedded in broad westerly mid-latitude flow. Since meteorologically -- my personal longings for the demise of this tropical cyclone notwithstanding -- there is little reason to favor one solution over the other, all I can do is show a quasi-stationary storm at the end of the period. This remains a low confidence forecast.

5-day intensity forecast

INITAL 09/27 0300Z 45 KT 50 MPH
12 hour 09/27 1200Z 45 KT 50 MPH
24 hour 09/28 0000Z 50 KT 60 MPH
36 hour 09/28 1200Z 50 KT 60 MPH
48 hour 09/29 0000Z 50 KT 60 MPH
72 hour 09/30 0000Z 45 KT 50 MPH
96 hour 10/01 0000Z 40 KT 45 MPH
120 hour 10/02 0000Z 40 KT 45 MPH

5-day track forecast



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



Miriam

Miriam continues to weaken as of the most recent NHC advisory:

Wind: 45 mph, with higher gusts
Location: 21.5°N 115.7°W
Movement: NNW at 7 mph
Pressure: 999 mb

What was once a powerful hurricane seems to be quickly decaying. The low-level center is completely exposed, but ill-defined shower activity continues well north of the center, most likely associated with the mid-level remnants of the storm, rather than the actual low-level circulation.



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

There are probably still some tropical storm force winds occurring within the circulation though, as per Dvorak constraints. It generally takes a while for the surface winds to match the satellite signature of a tropical cyclone in its decaying stage. Oftentimes the circulation itself can help to maintain tropical storm force winds long after the absence of deep convection. This is why the National Hurricane Center continuously follows what seem to be naked swirls.

With no respite from the vertical shear anticipated for the next few days, the only future ahead of Miriam appears to be a short one. Dissipation is expected in about 36 hours, although it could obviously occur a little sooner, particularly if the core of strongest shear moves directly over the cyclone. Although the National Hurricane Center official forecast shows Miriam surviving as a distinct entity through 96 hours, I have serious doubts about this given the magnitude of shear forecast by the GFS/SHIPS. A forecast track out to 72 hours seems reasonable, so I'll go with that.

Water vapor imagery shows the upper trough slowly moving through the four corners region. In the wake of this trough, a building mid-level ridge is noted upstream from Tropical Storm Miriam. While another trough is seen amplifying offshore British Columbia, Miriam is expected to be too shallow to feel any significant poleward influence from this trough. This in good agreement with the global models, which show Miriam moving westward in the low-level flow south of a building ridge. The track forecast is relatively straightforward, unlike with Nadine.

If the remnants survive beyond three days, the models show them turning southwest. While this would take them into warmer water, strong vertical shear is expected to preclude regeneration.

Remnant high-level moisture is still expected to overspread portions of the southwestern United States through the weekend.

5-day intensity forecast

INITIAL 09/27 0300Z 40 KT 45 MPH
12 hour 09/27 1200Z 35 KT 40 MPH
24 hour 09/28 0000Z 30 KT 35 MPH
36 hour 09/28 1200Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
48 hour 09/29 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/30 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
96 hour 10/01 0000Z...DISSIPATED

5-day track forecast



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



Invest 94E

An area of disturbed weather and associated surface low is located about 350 miles south of Manzanillo. Unfortunately, a 0230 UTC SSMI overpass missed the center of the disturbance, but satellite images suggest that the low is becoming better organized.



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

Based on shortwave infrared imagery, the center appears to be just north of a ball of very cold convection. However, there is little evidence of curved banding at this time. The northerly shear that had been afflicting the system appears to be gradually relaxing as the large anticyclone over the Gulf of Mexico moves eastward.

I expect this low to move northwestward between the southerly flow associated with Miriam and the trough over the central United States. This system poses a threat to Baja over the next 4-5 days, and interests there should monitor it.

The SHIPS suggests the system will run into cool waters in about 48 hours, but on my projected path, this seems a little dubious. Upper-level winds are forecast to remain light throughout the next several days, and may even attempt to become anticyclonic at times.

This system could become a tropical depression during the next couple of days.

Probability of development in 48 hours: 60%



Jelawat

Jelawat remains a super typhoon, but continues to weaken as of the latest JTWC advisory:

Wind: 150 mph, with higher gusts
Location: 19.7N 124.7E
Movement: NW at 9 mph
Category: 4 (Saffir-Simpson Hurricane Scale)

There have been no significant change to the cloud pattern this evening. Jelawat remains a large and extremely well-organized typhoon, with a large eye amidst a vigorous central dense overcast that is well-encompassing. In addition, a large curved band exists north of the center.



Figure 6. Latest infrared satellite image of Super Typhoon Jelawat. Image credit: NOAA

While upper-level winds should soon increase over the typhoon, they clearly haven't just yet. In fact, the storm has become even more organized compared to six hours ago. While I doubt this will translate to a sudden spike in the winds, it should at least prevent any significant weakening for the next 24 hours. Beyond that point, increasing southwesterly shear -- already evident on water vapor imagery just north of Taiwan should begin affecting the system. In combination with steadily decreasing sea surface temperatures and underlying oceanic heat content, the net should result be weakening, with more rapid weakening anticipated beyond 48 hours. However, Jelawat is expected to remain a formidable typhoon throughout the duration of its passage over Okinawa. A weaker storm is expected to impact southern Japan.

The track forecast remains straightforward. Water vapor imagery and UW-CIMSS data suggest that Jelawat is moving through a well-established weakness in the subtropical ridge caused by a combination of a shortwave trough developing between China and South Korea and accelerating Tropical Storm Ewiniar. Consequently, I expect the typhoon to turn northward over the next 24 hours, followed by recurvature toward the northeast as it moves closer to the trough axis and comes under increasing southwesterly flow. After some disagreement, the computer models have come into better agreement on the future track of the typhoon, with a path through Okinawa and into southern Japan early next week. Interests in those areas should carefully monitor the progress of this typhoon. Winds could be tremendously high on Okinawa.

Rainfall and storm surge potential with this system will be limited due to the fast forward motion of the system, which is only forecast to get faster throughout the next five days. My forecast track has not changed much, and represents an update of the previous one.

5-day intensity forecast

INITIAL 09/27 0300Z 130 KT 150 MPH
12 hour 09/27 1200Z 130 KT 150 MPH
24 hour 09/28 0000Z 125 KT 145 MPH
36 hour 09/28 1200Z 115 KT 135 MPH
48 hour 09/29 0000Z 100 KT 115 MPH
72 hour 09/30 0000Z 70 KT 80 MPH
96 hour 10/01 0000Z 55 KT 65 MPH...ON THE COAST OF SOUTHERN JAPAN
120 hour 10/02 0000Z 40 KT 45 MPH...OVER WATER

5-day track forecast



Figure 7. My 5-day forecast track for Jelawat.



Gulf of Mexico development possible

Over the last several days, the GFS has been consistent in developing some sort of lower tropospheric vorticity maximum/area of low pressure in the western Gulf of Mexico in about 3 to 5 days. The cold front now moving into the Rockies appears to be what generates this. Any system here would likely be more baroclinic than tropical given that the subtropical jet has made its seasonal migration southward.

Synoptic steering favors any system getting drawn northeast and hitting areas between Louisiana and the western Florida panhandle. Given where the GFS has been insisting that the low will develop, Louisiana or Mississippi seems like the best bet at the moment. I want to reiterate that I do not anticipate a strong system. In fact, should this system actually become extant at any point over the next five days, I would not expect much in the way of tropical development either.

2012 Atlantic hurricane season 2012 East Pacific hurricane season 2012 Western Pacific typhoon season Tropical Storm Nadine Tropical Storm Miriam Super Typhoon Jelawat Invest 94E

Updated: 7:51 AM GMT on September 27, 2012

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Tropical weather analysis - September 26, 2012

By: KoritheMan, 5:08 AM GMT on September 26, 2012

Nadine

Tropical Storm Nadine continues to be persistent. As of the latest NHC advisory, the following information was available on the storm:

Wind: 45 mph, with higher gusts
Location: 31.2°N 30.4°W
Movement: SSW at 5 mph
Pressure: 997 mb

An eye-like feature was evident at the 37 GhZ channel in an SSMIS overpass taken at 2140Z, surrounded by a ring of convection. While Nadine certainly retains a well-defined circulation, I question how much of this is actually an eye, or dry air. My hunch is that an eye did legitimately try and develop earlier, but has since been overtaken by the strong subsident flow that prevails in this part of the Atlantic.



Figure 1. Latest infrared satellite image of Tropical Storm Nadine. Image credit: RAMMB Colorado State University (CSU).

Since there appears to be little vertical shear over Nadine, the primary reason why the storm has not strengthened is no doubt the very dry airmass in which it is embedded. The SHIPS analyzes the 700-500 mb ambient relative humidity values at a mere 33% as of 0z. This is not a favorable pattern for a tropical cyclone. While the warmer sea surface temperatures that Nadine is expected to move into could mitigate this factor, the overall effect should be negligible at best.

Nadine's future doesn't look too bright. Southwesterly shear is forecast to increase sharply in about 48 hours as the cyclone approaches an upper level low/trough zone over the central Atlantic. If Nadine is going to strengthen, it has only a little over a day to do so. I am not particularly optimistic. I am a little below the intensity consensus. An alternate scenario is that Nadine doesn't strengthen at all.

Recent satellite fixes indicate that the tropical storm is gradually making the long anticipated southward turn as an upper trough over the western Atlantic shunts the subtropical ridge eastward and northerly mid-level flow increases over the storm. A turn toward the west is forecast over the next couple of days, followed by a turn to the northwest on day three as Nadine comes under increasing southerly flow associated with the western Atlantic trough. The models, while still a little divergent, seem to be in better agreement than they were yesterday, although there is still a fairly sizable spread amongst the 18z GFS ensemble members.

My forecast track is close to the National Hurricane Center.

5-day intensity forecast

INITIAL 09/26 0300Z 40 KT 45 MPH
12 hour 09/26 1200Z 40 KT 45 MPH
24 hour 09/27 0000Z 45 KT 50 MPH
36 hour 09/27 1200Z 50 KT 60 MPH
48 hour 09/28 0000Z 50 KT 60 MPH
72 hour 09/29 0000Z 50 KT 60 MPH
96 hour 09/30 0000Z 45 KT 50 MPH
120 hour 10/01 0000Z 40 KT 45 MPH

5-day track forecast



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



Miriam

Hurricane Miriam continues to steadily weaken. As of the most recent NHC advisory, the following information was available on the storm:

Wind: 80 mph, with higher gusts
Location: 19.1°N 115.2°W
Movement: NW at 5 mph
Pressure: 983 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Miriam is beginning to shear apart. While upper-level outflow remains fairly well-defined, the cloud pattern suggests shear. This shear is coming from the southwest and appears to be confined beneath the outflow layer judging by the convective pattern. Also, recent microwave fixes suggest that the low- and mid-level circulations have started to decouple.



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

Water vapor imagery shows a sharp upper trough axis over the central Pacific. The southwesterly flow on the east side of this feature is forecast to maintain some semblance of vertical shear over the hurricane. In about 72 hours, a vigorous upper low currently centered near 33N 144W is forecast by the global models to begin approaching Miriam from the west. The net result will be an abrupt and rather gruesome increase in westerly shear. That along with cool ocean temperatures is expected to significantly disrupt the hurricane; so much so that I am now expecting dissipation in 72 hours, which is in sharp contrast to yesterday. It could certainly occur sooner.

Miriam is moving through a substantial break in the Pacific subtropical high. This is being generated by a large trough over the western United States. As the central Pacific upper low pivots eastward over the next few days, this weakness is expected to become reinforced. The global models remain in excellent agreement on the track, which has greater certainty than normal. Given the increasingly shallow depth of the cyclone circulation, a significant poleward bend now seems unlikely. Instead, Miriam is expected to move pretty much uniformly northwestward in the lower tropospheric flow.

While Miriam is forecast to dissipate before reaching Baja, as the vortex shears apart in the vertical, the 500 mb circulation should accelerate northeastward ahead of the decadent low-level center under the influence of the upper low/trough and bring rain to portions of the southwestern United States this weekend into early next week.

5-day intensity forecast

INITIAL 09/26 0300Z 70 KT 80 MPH
12 hour 09/26 1200Z 60 KT 70 MPH
24 hour 09/27 0000Z 50 KT 60 MPH
36 hour 09/27 1200Z 40 KT 45 MPH
48 hour 09/28 0000Z 30 KT 35 MPH
72 hour 09/29 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/30 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
120 hour 10/01...DISSIPATED

5-day track forecast



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



Jelawat

Jelawat has weakened a bit this evening, but remains a rather formidable typhoon. As of the latest JTWC advisory, the following information was posted on the storm:

Wind: 155 mph, with higher gusts
Location: 17.3N 126.6E
Movement: NW at 3 mph
Category: 4 (Saffir-Simpson Hurricane Scale)

The typhoon has jogged a little to the right of the 0300Z JTWC forecast track. Smoothing out the fits and wobbles yields a northwesterly motion as advertised in the advisory. Also, the storm appears to have accelerated a bit.



Figure 5. Latest infrared satellite image of Super Typhoon Jelawat. Image credit: NOAA

A slew of microwave fixes suggest that Jelawat may be undergoing another concentric eyewall cycle, as a large outer band was becoming prominent in the data, while the inner eyewall -- especially to the north -- was decaying. This is somewhat supported by conventional satellite fixes as well, which clearly imply a weakening of the inner eyewall, once again most recognizable to the north.

I see a flattening of the outflow on the northern side by looking at water vapor imagery. This is likely due to an upper cold low that has formed to the northwest of Tropical Storm Ewiniar. As this low moves southward, it will likely begin affecting Jelawat with southwest to westerly shear. Steadily decreasing sea surface temperatures should also help to initiate weakening. However, Jelawat is expected to remain a powerful typhoon as it bears down on Okinawa this weekend.

The synoptic situation over the western Pacific seems a little different tonight. Rather than a trough I now see a ridge over China. This is confirmed by analyses from UW-CIMSS and water vapor imagery, which show a mostly zonal flow over the country. There is very clearly some ridging going on upstream of the typhoon, which wasn't expected yesterday. Because of this, I have shifted my track a bit to the left for the next 48 hours, but the storm is still expected to remain well offshore from Taiwan. It should be noted that if the ridging does not give way as predicted, the eye could come a little closer to -- but still safely offshore -- the coast of Taiwan. Locally heavy rains and gusty winds in showery bands are still possible there, although they will be on the weaker side. Okinawa will face much harsher conditions, with wind gusts anywhere from 100 to 140 mph as Jelawat accelerates.

Subsequent to passing Okinawa, the storm is forecast to become fully embedded within the mid-latitude southwesterly flow ahead of a developing trough. This pattern favors a landfall in southern Japan, possibly near Tokyo, Monday morning local time. Interests there should begin monitoring the progress of Jelawat at this time.

5-day intensity forecast

INITIAL 09/26 0300Z 135 KT 155 MPH
12 hour 09/26 1200Z 130 KT 150 MPH
24 hour 09/27 0000Z 125 KT 145 MPH
36 hour 09/27 1200Z 115 KT 135 MPH
48 hour 09/28 0000Z 100 KT 115 MPH
72 hour 09/29 0000Z 90 KT 105 MPH
96 hour 09/30 0000Z 75 KT 85 MPH
120 hour 10/01 0000Z 60 KT 70 MPH...APPROACHING SOUTHERN JAPAN

5-day track forecast



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

2012 Atlantic hurricane season 2012 East Pacific hurricane season 2012 Western Pacific typhoon season Tropical Storm Nadine Hurricane Miriam Super Typhoon Jelawat

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Tropical weather analysis - September 24, 2012

By: KoritheMan, 4:12 AM GMT on September 25, 2012

Nadine

There has been little change to tenacious Nadine as of the latest NHC advisory:

Wind: 45 mph, with higher gusts
Location: 32.2°N 29.0°W
Movement: W at 7 mph
Pressure: 996 mb

The storm continues to suffer from dry air, as the convection is limited to a small band in the west quadrant.



Figure 1. Latest infrared satellite image of Tropical Storm Nadine. Image credit: RAMMB Colorado State University (CSU).

There also appears to be some southwesterly shear over the cyclone. In about 24-36 hours, the GFS/SHIPS forecast an environment a bit more conducive to strengthening, with weaker shear and slightly warmer sea surface temperatures. However, the latest output of the SHIPS model suggests that if Nadine follows the current forecast track, water temperatures may never go above 26C. Looking at objective sea surface temperature data in this region, the SHIPS model output for SSTs appear to be generally correct. Lukewarm waters and a dry environment expected to persist even as the shear decreases is not what I consider to be a favorable environment. Nevertheless, the expectation of a decrease in shear may allow Nadine to stop ingesting so much dry air, and strengthen at least a little. That said, I seriously doubt the prospects of Nadine ever becoming a hurricane. In about 72 hours, southwesterly shear is forecast to increase again, possibly of even greater magnitude than the current shear vector. This is expected to promote weakening of Nadine at later ranges.

Nadine is riding the southeastern periphery of the subtropical ridge, and should soon begin executing a prolonged but gentle clockwise loop around its southern periphery. In about three days, the cyclone is expected to begin turning northwestward at increasing forward speeds due to a broad trough extending southward from a large upper low forecast to be moving across Atlantic Canada at that time. The global models are becoming divergant once again on the future trajectory of Nadine. The GFDL and HWRF, for example forecast a prolonged southward movement of Nadine. The best recourse seems to be an endeavor to maintain some continuity with previous model prognostications.

Out of respect for these models (the GFDL in particular has performed very well in the past), as well as the fact that I don't see a particularly deep trough on the models, my forecast track is a little south of and slower than the NHC track. But Nadine has surprised us before, and I am sure there are still many left.

5-day intensity forecast

INITIAL 09/25 0300Z 40 KT 45 MPH
12 hour 09/25 1200Z 40 KT 45 MPH
24 hour 09/26 0000Z 40 KT 45 MPH
36 hour 09/26 1200Z 45 KT 50 MPH
48 hour 09/27 0000Z 50 KT 60 MPH
72 hour 09/28 0000Z 55 KT 65 MPH
96 hour 09/29 0000Z 55 KT 65 MPH
120 hour 09/30 0000Z 50 KT 60 MPH

5-day track forecast



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



Miriam

Miriam became the fourth major hurricane of the season earlier today, but has since weakened. As of the latest NHC advisory, the following was available on the hurricane:

Wind: 105 mph, with higher gusts
Location: 18.5°N 113.9°W
Movement: NW at 8 mph
Pressure: 968 mb
Category: 3 (Saffir-Simpson Hurricane Scale)

Satellite images show a significant degradation of what was once a beautiful hurricane. An eye is no longer evident, and earlier microwave data suggested that the western eyewall was almost completely open. The data also indcated, particularly after 12z, that the hurricane was undergoing an eyewall replacement cycle. Satellite images also supported this.



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

If there ever was such a cycle, it seems to have plateaued. Water vapor images show strong southwesterly shear north of about 20N. Miriam should run into this shear in about a day or so, which along with cooler waters is expected to promote further weakening. Sea surface temperatures will begin to warm again as Miriam approaches the central coast of Baja. While the shear is expected to be too strong to allow for any sort of restrengthening, Miriam has a large enough circulation that I assume it will be able to survive as a weak tropical cyclone prior to landfall. This is especially true if it hits southern Baja. Heavy rainfall associated with the mid-level remnants will likely overspread portions of the southwest United States this weekend into early next week.

Water vapor images and UW-CIMSS steering analyses indicate that Miriam continues to move along a weakness in the Pacific subtropical high. An upper low moving into the four corners region is dropping southeastward. The models forecast it to move eastward over the next 12-24 hours, which should keep enough southerly flow on Miriam to pull it towards Baja. The weakness will be reinforced by another upper low forecast to approach the hurricane in about 72 hours. Presumably, this low is the same one currently near 33N 144W.

While there are still disagreement on whether or not Miriam will recurve, they are between our less reliable models. Our best performers, the GFS and ECMWF, remain in good agreement on a track towards Baja. Interests there should monitor the progress of Miriam.

5-day intensity forecast

INITIAL 09/25 0300Z 90 KT 105 MPH
12 hour 09/25 1200Z 85 KT 100 MPH
24 hour 09/26 0000Z 80 KT 90 MPH
36 hour 09/26 1200Z 70 KT 80 MPH
48 hour 09/27 0000Z 65 KT 75 MPH
72 hour 09/28 0000Z 50 KT 60 MPH
96 hour 09/29 0000Z 40 KT 45 MPH...APPROACHING CENTRAL BAJA
120 hour 09/30 0000Z 30 KT 35 MPH...INLAND OVER CENTRAL BAJA

5-day track forecast



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



Jelawat

Dangerous Super Typhoon Jelawat achieved the Category 5 equivalent on the Saffir-Simpson Hurricane Scale today. As of the latest JTWC advisory, the following information was available on the typhoon:

Wind: 160 mph, with higher gusts
Location: 15.7N 127.8E
Movement: N at 4 mph
Category: 5 (Saffir-Simpson Hurricane Scale)

Jelawat remains large and well-organized, with a large circular eye surrounded by very deep convection.



Figure 5. Latest infrared satellite image of Super Typhoon Jelawat. Image credit: NOAA

Upper-level winds are light, and now that last night's eyewall replacement cycle is done, no significant weakening is anticipated in the near-term. In about 36 hours, the typhoon is forecast to begin weakening as it moves over less oceanic heat content and encounters gradually increasing shear. However, Jelawat is expected to remain a dangerous typhoon throughout the forecast period, especially when it lashes the entire chain of Okinawa, where winds could easily gust in excess of 140 mph in the southern end of the archipelago.

Water vapor imagery shows that the main shortwave trough has bypassed the typhoon, and with ridging building in to the east of Japan as heights gradually rise. There is another deep trough amplifying over central China. This synoptic evolution favors a gradual turn to the north and northeast over the next five days, bringing the eye over Okinawa late Friday local time. Thereafter, the system is forecast to head toward southern Japan as a weakening (but still significant) typhoon. My forecast track is in excellent agreement with that from the JTWC.

Interests in Taiwan should continue monitoring the progress of this system, as, although the system is likely to stay well offshore, it could deliver locally heavy rains and gusty winds, especially in areas of higher terrain.

5-day intensity forecast

INITIAL 09/25 0300Z 140 KT 160 MPH
12 hour 09/25 1200Z 140 KT 160 MPH
24 hour 09/26 0000Z 125 KT 145 MPH
36 hour 09/26 1200Z 115 KT 135 MPH
48 hour 09/27 0000Z 105 KT 120 MPH
72 hour 09/28 0000Z 100 KT 115 MPH
96 hour 09/29 0000Z 90 KT 105 MPH
120 hour 09/30 0000Z 80 KT 90 MPH

5-day track forecast



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

2012 Atlantic hurricane season 2012 East Pacific hurricane season 2012 Western Pacific typhoon season Tropical Storm Nadine Hurricane Miriam Super Typhoon Jelawat

Updated: 4:12 AM GMT on September 25, 2012

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Tropical weather analysis - September 24, 2012

By: KoritheMan, 5:41 AM GMT on September 24, 2012

Due to issues with the GOES-13 satellite, I am physically incapable of doing a blog on Nadine. So I will be sticking with Miriam and Jelawat for now.

Miriam

Miriam has rapidly strengthened to a hurricane this evening, the ninth of the season. As of the latest NHC advisory, the following information was available on the hurricane:

Wind: 90 mph, with higher gusts
Location: 16.8°N 111.3°W
Movement: NW at 12 mph
Pressure: 979 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Miriam is a well-organized hurricane. A small eye was evident in a recent microwave pass, and this feature has become apparent in conventional satellite images as well.



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

Miriam has no more than about 36 hours to intensify, most likely closer to 24. After that time, the hurricane is forecast to encounter cooler waters, a more stable airmass, and an increase in southwesterly shear beyond 48 hours. These factors should initiate a weakening trend, possibly fairly rapid near the end of the period. While the intensity consensus does not currently anticipate Miriam becoming a major hurricane, I think it's possible given current trends. Alas, it is difficult to tell how long these rapid intensification episodes will last. Given the nature of intense hurricanes, it is possible that Miriam will undergo an eyewall replacement cycle subsequent to assuming its peak intensity. That would likely result in more short-term weakening than indicated below, but it would also result in a larger circulation that is more fit to survive cold waters later in the period.

One fly in the ointment pertaining to the intensity of Miriam beyond day three is the point at which it recurves. Since the GFS and ECMWF are now in agreement that Miriam will get picked up by the west coast trough, I suppose the only real question is to the specific longitude that she will recurve. A cyclone that curves farther east, south of central Baja, would feel warmer sea surface temperatures and be less to prone to weakening. On the hand, a storm that recurves left -- or north -- will find itself in a harsher thermodynamic environment, with much cooler water temperatures.

The steering for Miriam is relatively straightforward, with the biggest uncertainty being precisely where the hurricane will recurve. Miriam is expected to begin recurving in about 48 hours as the mid-level ridge to the north weakens with the amplification of a large upper trough along the west coast of the United States. There are considerable differences in forward speed though, likely due to the influence of a secondary vortex noted in the various model forecast fields. This vortex has not been evident in either the GFS or the Euro, so it is most likely of a spurious nature. The GFS and Euro aren't that far apart in terms of speed, which leads me to believe the disagreements are outliers being falsely influenced by said vortex.

Interests in Baja California and southwestern Mexico should monitor the progress of the hurricane. Regardless of whether or not Miriam makes landfall, heavy rainfall associated with the mid-level vortmax will likely overspread portions of the southwestern United States 5 - 7 days from now.

5-day intensity forecast

INITIAL 09/24 0300Z 80 KT 90 MPH
12 hour 09/24 1200Z 90 KT 105 MPH
24 hour 09/25 0000Z 100 KT 115 MPH
36 hour 09/25 1200Z 95 KT 110 MPH
48 hour 09/26 0000Z 85 KT 100 MPH
72 hour 09/27 0000Z 65 KT 75 MPH
96 hour 09/28 0000Z 45 KT 50 MPH
120 hour 09/29 0000Z 30 KT 35 MPH

5-day track forecast



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



Jelawat

Jelawat became a super typhoon yesterday (Sunday). I mentioned this was possible, since western Pacific storms often accomplish this feat fairly easily. As of the latest JTWC advisory, the following was posted on the storm:

Wind: 150 mph, with higher gusts
Location: 13.6N 128.5E
Movement: N at 5 mph
Category: 4 (Saffir-Simpson Hurricane Scale)

There have been some structural changes with Jelawat this evening. The eye has shrunk a bit, at times becoming completely cloud filled. In addition, satellite images show considerable erosion of the northern eyewall. Microwave data throughout the day on Sunday, continuing up through 0z Monday, have shown a proclivity toward a concentric eyewall structure. This is not uncommon for western Pacific typhoons, which again, can become monstrous quite effortlessly.



Figure 3. Latest infrared satellite image of Super Typhoon Jelawat. Image credit: NOAA

Once Jelawat reaches the culminaton of its inner core fiasco, it should be able to reintensify a little. By day three, the typhoon is forecast to encounter gradually decreasing sea surface temperatures and underlying oceanic heat content. Increasing vertical shear will also be a problem at those forecast ranges. This pattern is expected to bring about a weakening of the system at that time, although it is expected to remain large and powerful as it bears down on Okinawa early this weekend. Interests there should carefully monitor the progress of Jelawat. However, structures on that island are generally built to withstand the force of powerful typhoons like Jelawat, so I do not anticipate a catastrophe. However, some very strong winds could occur on that island if Jelawat follows the intensity forecast, possibly upwards of over 140 mph in wind gusts. Beyond day five, Jelawat is expected to accelerate northeastward in the mid-latitude westerly flow and move toward mainland Japan.

My track forecast has come a little westward today, as there appears to be more ridging than I originally thought. A ridge is building in over mainland China in the wake of a shortwave trough, which is now lingering near the southwestern coast of Japan. This synoptic evolution favors a gradual turn to the north and northeast, but will allow for a more northweserly course than I was prognosticating yesterday. There does not appear to be enough forcing with the trough to produce a particularly sharp recurvature subsequent to the storm passing Okinawa. If I lived in Japan I would be preparing for a formidable typhoon hitting the southern coast early next week. I would also watch this if I lived in Taiwan, just in case Jelawat turn does turn as predicted.

5-day intensity forecast

INITIAL 09/24 0300Z 130 KT 150 MPH
12 hour 09/24 1200Z 120 KT 140 MPH
24 hour 09/25 0000Z 125 KT 145 MPH
36 hour 09/25 1200Z 130 KT 150 MPH
48 hour 09/26 0000Z 130 KT 150 MPH
72 hour 09/27 0000Z 125 KT 145 MPH
96 hour 09/28 0000Z 115 KT 135 MPH
120 hour 09/29 0000Z 105 KT 120 MPH

I should note that my intensity forecast carries greater than normal uncertainty due to not well understood inner core dynamics (namely eyewall replacement cycles) as well as the somewhat erratic behavior of storms in this region.

5-day track forecast



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

2012 East Pacific hurricane season 2012 Western Pacific typhoon season Hurricane Miriam Super Typhoon Jelawat

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Tropical weather analysis - September 23, 2012

By: KoritheMan, 7:04 AM GMT on September 23, 2012

Remnants of Nadine

Satellite images and surface observations suggest that the area of low pressure associated with former Hurricane Nadine is becoming better organized. Earlier microwave data suggested the presence of a well-defined, albeit exposed circulation. Since those passes, convection has increased in both coverage and extent, and a curved band is prominent north of the center. All it would take is detachment from the front and the system could probably be considered tropical. The global models suggest this will happen soon, and water vapor imagery supports it as well, as the parent upper low moves off toward Europe.

The low is expected to moves slowly westward over the next day or two.



Figure 1. Latest infrared satellite image of ex-Nadine. Image credit: RAMMB Colorado State University (CSU).

Probability of development in 48 hours: 70%



Miriam

Tropical Storm Miriam is intensifying as of the latest NHC advisory:

Wind: 45 mph, with higher gusts
Location: 14.9°N 108.5°W
Movement: WNW at 8 mph
Pressure: 1002 mb



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

Conditions appear conducive for additional strengthening. The shear should remain low for the next 48-72 hours. In fact, the only apparent hurdle I see that would prevent Miriam from becoming a major hurricane is a tongue of cooler waters that lie west of 115W. Given the extremely favorable environmental conditions that lie ahead, my intensity forecast is a little higher than the intensity consensus. It is possible that Miriam could get a little stronger than anticipated. Beyond 48 hours, a weakening trend is expected to initiate as the cyclone encounters cooler waters, drier air, and increasing southwesterly shear.

Miriam is situated on the southwest periphery of a well-established low- to mid-level ridge over the southwestern United States. This should continue the current west-northwest motion for about the next 72 hours. After that time, there is a significant divergence in the model guidance, which since seems to hinge primarily on how strong Miriam is, as well as the amplitude of the upper trough now amplifying off the coast of the western United States. The GFS continues to call for an eventual landfall on the coast of western or central Baja, and has shifted southward in the 0z run. The ECMWF shows a westward motion beyond 72 hours as the trough lifts out. Since it is impossible to pinpoint the strength of the trough and Miriam's depth at this point in time, the best course of action seems to be an average of the extremes. That leads to a northwestward bend beyond day three under the assumption that Miriam feels the trough, but not to the extent that it recurves. This is a low confidence forecast.

I am similar to that from the National Hurricane Center for lack of a reason to disagree with it.

5-day intensity forecast

INITIAL 09/23 0600Z 40 KT 45 MPH
12 hour 09/23 1800Z 50 KT 60 MPH
24 hour 09/24 0600Z 60 KT 70 MPH
36 hour 09/24 1800Z 70 KT 80 MPH
48 hour 09/25 0600Z 80 KT 90 MPH
72 hour 09/26 0600Z 70 KT 80 MPH
96 hour 09/27 0600Z 60 KT 70 MPH
120 hour 09/28 0600Z 45 KT 50 MPH

5-day track forecast



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



Jelawat

I don't normally blog for western Pacific systems, but Typhoon Jelawat is simply too marvelous to pass up. As of the latest advisory from the Joint Typhoon Warning Center (JTWC), the following information was available on the typhoon:

Wind: 115 mph, with higher gusts
Location: 12.2N 128.9E
Movement: Stationary



Figure 4. Latest infrared satellite image of Typhoon Jelawat. Image credit: NOAA

The satellite signature is nothing short of impressive. Very deep convection wraps around a well-defined eye, and numerous rainbands are emanating from the cyclone.

Environmental conditions ahead of the typhoon appear favorable for strengthening. Since Jelawat has been rapidly intensifying, with Dvorak numbers rising as fast as constraints allow, it has the potential to become a super typhoon, and possibly a Category 5 equivalent on the Saffir-Simpson Hurricane Scale. The Western Pacific is home to a vast reservoir of deep warm water, and it is not uncommon to see storms with winds over 140 mph on a regular basis here. Beyond 72 hours, an increase in southwesterly shear is likely as the system moves closer to an upper-level trough.

An upper trough near the north coast of Japan is pivoting eastward in the mid-latitude westerly flow over mainland China. While the main trough is moving generally eastward, there is apparently enough northwesterly flow along the back side of the parent upper low to help push the southern portion of the frontal trough southward, closer to the vicinity of the tropical cyclone. This is allowing for a small break in the ridge to the north of the storm, which is also depicted on UW-CIMSS steering imagery. The net result should be a bend to the north later today. I don't disagree too strongly with the current JTWC track for the first couple of days, although I'm not going to show the strong northwestward bend that they are showing, since there appears to be a ton of shortwave energy over China coming out of the westerlies. This could theoretically help to reinforce the large-scale troughiness in this region.

Next week, this system is expected to threaten Okinawa and possibly mainland Japan.

5-day intensity forecast

INITIAL 09/23 0900Z 100 KT 115 MPH
12 hour 09/23 1800Z 105 KT 120 MPH
24 hour 09/24 0600Z 110 KT 130 MPH
36 hour 09/24 1800Z 115 KT 135 MPH
48 hour 09/25 0600Z 125 KT 145 MPH
72 hour 09/26 0600Z 125 KT 145 MPH
96 hour 09/27 0600Z 105 KT 120 MPH
120 hour 09/28 0600Z 90 KT 105 MPH

Unfortunately, I could not find a map with which to draw track forecasts on. However, my track is basically for a general northward motion for the next three to four days, followed by recurvature toward the northeast.

2012 Atlantic hurricane season 2012 East Pacific hurricane season Remnants of Nadine Tropical Storm Miriam Typhoon Jelawat 2012 Western Pacific typhoon season

Updated: 8:46 AM GMT on September 23, 2012

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Tropical weather analysis - September 22, 2012

By: KoritheMan, 5:35 AM GMT on September 22, 2012

Remnants of Nadine

After many days of diligence, Nadine finally made the transition to a post-tropical cyclone tonight. As of the final NHC advisory, the following information was posted on the post-tropical low:

Wind: 60 mph, with higher gusts
Location: 31.9°N 26.6°W
Movement: SSE at 13 mph
Pressure: 984 mb

A poorly-defined band of convection still persists in the southwest quadrant, but it is well-removed from the center and not well organized.



Figure 1. Latest infrared satellite image of Post-Tropical Cyclone Nadine. Image credit: RAMMB Colorado State University (CSU).

It should be noted that while Nadine is no longer tropical, the GFS and SHIPS continue to forecast a more favorable upper wind environment over the next couple of days; the former even suggests an anticyclone. Since Nadine is heading toward warmer water, regeneration back into a tropical cyclone is a distinct possibility as the low gradually turns east and north over the next day or two.

Probability of development in 48 hours: 30%



Invest 94L

A non-tropical area of low pressure centered about 350 miles east-northeast of Bermuda has become less organized. Upper-level winds are forecast to be only marginally conducive for development as the low recurves. Beyond 36 hours, a sharp increase in westerly shear is anticipated, which should halt any possible accolades from the low.



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

Probability of development in 48 hours: 10%



Tropical Depression Thirteen-E

A new tropical depression formed in the Eastern Pacific today from what was previously known as "Invest 93E". The first NHC advisory had the following to say about the system:

Wind: 35 mph, with higher gusts
Location: 13.7°N 107.5°W
Movement: W at 12 mph
Pressure: 1005 mb

As is typical of developing systems, the outer banding seen earlier has decreased, while convection has developed closer to the low-level center. This typically heralds intensification, and I expect the system will be a tropical storm soon.



Figure 4. Latest infrared satellite image of Tropical Depression Thirteen-E. Image credit: NOAA

The environment ahead of the depression looks quite favorable; favorable enough that rapid intensification could occur. The 0z SHIPS rapid intensification parameter shows a 57% chance of a 25 kt increase in wind speed in association with the tropical cyclone, which is five times the sample mean. In about four days, the cyclone is forecast to encounter cooler waters and begin to weaken. Lacking any obvious sign of negativity, I will forecast a 35 kt increase in wind speed over the next 48 hours. However, I am fully prepared to wake up to a storm even stronger than that, in which case my intensity forecast will be revised significantly upward in my next entry.

Water vapor imagery and UW-CIMSS steering data shows that the depression remains south of a well-established subtropical ridge. The global models respond to this thematic pattern by forecasting a general west-northwest motion throughout the next five days. A more poleward bend is possible beyond day three as the system encounters a bigger weakness in the ridge. My forecast is close to the National Hurricane Center official track, albeit slower near the end to account for the aforementioned weakness.

The models have come significantly westward today, and are now in better agreement that this cyclone will not threaten Baja.

5-day intensity forecast

INITIAL 09/22 0300Z 30 KT 35 MPH
12 hour 09/22 1200Z 35 KT 40 MPH
24 hour 09/23 0000Z 45 KT 50 MPH
36 hour 09/23 1200Z 55 KT 65 MPH
48 hour 09/24 0000Z 65 KT 75 MPH
72 hour 09/25 0000Z 75 KT 85 MPH
96 hour 09/26 0000Z 70 KT 80 MPH
120 hour 09/27 0000Z 60 KT 70 MPH

5-day track forecast



Figure 5. My 5-day forecast track for Tropical Depression Thirteen-E.

2012 Atlantic hurricane season 2012 East Pacific hurricane season Remnants of Nadine Invest 94L Tropical Depression Thrteen-E

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Tropical weather analysis - September 20, 2012

By: KoritheMan, 2:55 AM GMT on September 21, 2012

Nadine

Tropical Storm Nadine continues to meander in the far eastern Atlantic south of the Azores. As of the latest NHC advisory, the following information was available on the storm:

Wind: 65 mph, with higher gusts
Location: 35.4°N 28.0°W
Movement: ESE at 9 mph
Pressure: 981 mb

Central convection has decreased again, and the only convection is confined to a vigorous band well northwest of the center.



Figure 1. Latest infrared satellite image of Tropical Storm Nadine. Image credit: RAMMB Colorado State University (CSU).

While Nadine's winds have come up, this probably isn't due to tropical processes, but due to baroclinic forcing and the increasing pressure gradient northwest of the center due to a building mid-level ridge over the central Atlantic.

The intensity forecast is a bit of a conundrum this evening. On one hand, the satellite signature of Nadine is less tropical than 24 hours ago, and water vapor imagery suggests that the north Atlantic upper low and its attendant baroclinic zone continue to edge closer to the cyclone, which could cause extratropical transition given the current structure. On the other hand, the GFS/SHIPS have continuously forecast a brief decrease in the vertical shear over Nadine over the next day or two as the front bypasses Nadine. Since the relaxation of the shear will coincide with passage over warmer waters, strengthening, or at least a steady state storm, seems possible. For the sake of continuity, I will continue to call for extratropical transition, but I will compromise between the two extremes I presented above. This continues to be a low-confidence forecast, and Nadine has surprised us before.

Nadine is wedged between a developing anticyclone over the central-north Atlantic and an upper low dropping southeastward toward the tropical cyclone. The net result should be a continued southeast to east-southeast movement for the next 24-48 hours. After that time, there is again divergence in the model guidance. The ECMWF and GFS both agree on an eastward solution, but the former is significantly slower by a margin of several days. In addition, the HWRF moves the storm southwestward at 18z, as did the 12z NOGAPS. The GFS also cannot make up its mind on how fast Nadine is going to recurve, with some runs being substantially slower than others. Given the uncertainty, the best course of action still seems to be for a quasi-stationary motion beyond 48 hours.

5-day intensity forecast

INITIAL 09/21 0300Z 55 KT 65 MPH
12 hour 09/21 1200Z 55 KT 65 MPH
24 hour 09/22 0000Z 55 KT 65 MPH
36 hour 09/22 1200Z 55 KT 65 MPH...EXTRATROPICAL
48 hour 09/23 0000Z 55 KT 65 MPH...EXTRATROPICAL
72 hour 09/24 0000Z 55 KT 65 MPH...EXTRATROPICAL
96 hour 09/25 0000Z 55 KT 65 MPH...EXTRATROPICAL
120 hour 09/26 0000Z 55 KT 65 MPH...EXTRATROPICAL

5-day track forecast



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

I imagine the tropical storm warning for the Azores will probably be taken down tomorrow.



Invest 94L

A non-tropical gale area centered several hundred miles east of Bermuda has not changed much in organization this evening. Showers continue to develop, but are not in close proximity to the low-level center. In addition, earlier scatterometer data suggested that the radius of maximum winds was rather large, on the order of at least 150 miles. This suggests that 94L is not tropical at this time. Nonetheless, the shower activity appears to be slowly consolidating, and this system still has a legitimate shot at subtropical development before conditions become less favorable on Saturday.



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

94L could impact portions of Atlantic Canada in about three or four days.

Probability of development in 48 hours: 60%



Invest 93E

A broad area of low pressure centered several hundred miles southwest of Acapulco is showing signs of organization this evening, with the shower activity developing a little closer to the low-level center. An SSMIS overpass just after 0z suggested that the low-level center is still a little broad, and remains somewhat south of the convection.



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

Upper-level winds appear conducive for additional development, and this system could become a tropical depression during the next day or two as it moves west to west-northwestward. In the longer range, some of the global models -- including the GFS -- suggest a threat to portions of Baja California. It is simply too early to assess the specifics of this threat. It will depend on the evolution of a large upper low and attendant trough currently off the coast of Washington/Oregon.

The SHIPS suggests the system will encounter sub-26C SSTs in about four days, but the SHIPS parameter is based on the GFS output. On a more poleward track, the system will encounter cooler SSTs prior to the end of the forecast period, but if it takes a more westward path, it may find itself over warmer waters through day five.

Interests across Baja California should begin to monitor the progress of this disturbance over the next few days.

Probability of development in 48 hours: 60%

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Storm Nadine Invest 94L Invest 93E

Updated: 3:02 AM GMT on September 21, 2012

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Tropical weather analysis - September 19, 2012

By: KoritheMan, 4:13 AM GMT on September 20, 2012

Nadine

Nadine continues meandering near the Azores. As of the latest NHC advisory, the following was posted on the system:

Wind: 50 mph, with higher gusts
Location: 37.1°N 31.4°W
Movement: ENE at 3 mph
Pressure: 990 mb

A band of deep convection has developed in the northwest quadrant a little closer to the low-level center. A glance at buoy and ship reports in this area suggest that Nadine will encounter SSTs of 24-25C as it moves southeastward, which is a little warmer than what it has been experiencing. This could be why convection appears to have developed closer to the low-level center this evening.



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

Water vapor imagery suggests that an upper low is orbiting around the south side of the low-level center. Depending on how close the low stays relative to the cyclone center, this may not allow for much in the way of truly tropical organization from Nadine despite the system moving into warmer waters. If the upper low moves directly above the center, however, Nadine will find itself in an environment of minimal shear. In combination with the relatively warm underlying waters, that could allow for a steep temperature gradient, enabling Nadine's low-level warm core to gradually warm the upper troposphere, eliminate the upper low, and finally maintain some persistent convection. A similar evolution occurred with Tropical Storm Beryl earlier in the year as it was approaching the southeast United States coast.

There is simply too much uncertainty in the forecast evolution of Nadine and the upper low to say definitively what the technical status of the tropical cyclone will be. It's possible that Nadine will lose convection and degenerate into a remnant low, get enough baroclinic forcing to become an extratropical cyclone, or become absorbed by the large frontal system over the north Atlantic. While all three scenarios remain possible, I still favor the middle option. One other option, as alluded to above, is for Nadine to maintain tropical characteristics throughout the next several days. This could occur with slow movement over warm water, especially if the current convective signature persists or expands to other parts of the circulation. It is virtually impossible to tell which one of them will verify at this point.

The upper trough north of Nadine has reached its point of closest to the tropical cyclone. With the flow becoming more zonal in that direction, Nadine is expected to begin moving southeastward between a strengthening mid-level ridge over the central Atlantic and the northerly flow on the back side of the north Atlantic upper low. The models still disagree substantially on the future trajectory of Nadine, but it is mostly after 48 hours, as they have come into better agreement that Nadine will move southeastward at a faster forward speed before 72 hours. My forecast will go along with this, but given the continued inherent uncertainty beyond that time, I am slower than the National Hurricane Center. This continues to be a low-confidence forecast.

Beyond 48 hours, the upper low is forecast to be just north of Nadine. This will be the make it or break it pitch. Will Nadine throw a curveball, avoid the low and move southwestward around the subtropical ridge, or will it throw a fastball and move quickly northeastward with the low? Unfortunately, there is no reason to favor either scenario over the other at this time.

Interests in the Azores should continue following Nadine's progress.

5-day intensity forecast

INITIAL 09/20 0300Z 45 KT 50 MPH
12 hour 09/20 1200Z 45 KT 50 MPH
24 hour 09/21 0000Z 45 KT 50 MPH
36 hour 09/21 1200Z 45 KT 50 MPH...EXTRATROPICAL
48 hour 09/22 0000Z 45 KT 50 MPH...EXTRATROPICAL
72 hour 09/23 0000Z 45 KT 50 MPH...EXTRATROPICAL
96 hour 09/24 0000Z 45 KT 50 MPH...EXTRATROPICAL
120 hour 09/25 0000Z 45 KT 50 MPH...EXTRATROPICAL

5-day track forecast



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

Watches and warnings

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE AZORES ISLANDS OF FLORES...CORVO...FAIAL...PICO...SAO JORGE...
GRACIOSA...TERCEIRA...SAO MIGUEL...AND SANTA MARIA.

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



Invest 94L

A non-tropical area of low pressure over the central Atlantic about 700 miles east-southeast of Bermuda ("94L") is showing signs of organization. Low cloud lines on satellite imagery indicates that the surface wind field associated with this low is not as broad as yesterday. In addition, shower activity is wrapping closer to the center.



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

Water vapor imagery and UW-CIMSS 200 mb vorticity data suggests that an upper low is entangled with 94L, pretty much over the low-level center. The GFS suggests that this upper low will more or less move in tandem with 94L for the next three days before becoming dismantled thereafter. By that time, however, the system is forecast to enter a weakness in the subtropical ridge and recurve into cooler waters. At this point, any organization would probably be subtropical, although I cannot rule out this system briefly becoming a tropical storm. A subtropical or tropical storm could form on Thursday, but the system would probably not get much stronger than 50 kt.

Interests in Atlantic Canada should monitor the progress of this system over the next few days.

Probability of development in 48 hours: 50%



Invest 93E

A broad area of low pressure centered about 350 miles south-southeast of Acapulco ("93E" is producing disorganized showers and thunderstorms. Based on analysis of satellite imagery this evening, the center is devoid of deep convection. Recent microwave data suggested that the center was not at all well-defined, which is confirmed by low cloud movements on satellite photos as well.



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

Upper-level winds are forecast to be of at least a diffluent nature, which should allow for steady development, particularly if the system can get convection. This system does not pose a threat to any landmasses.

Probability of development in 48 hours: 40%

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Storm Nadine Invest 94L Invest 93E

Updated: 4:16 AM GMT on September 20, 2012

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Tropical weather analysis - September 19, 2012

By: KoritheMan, 6:26 AM GMT on September 19, 2012

Nadine

Nadine has weakened a little more this evening. As of the latest NHC advisory, the following information was available on the storm:

Wind: 50 mph, with higher gusts
Location: 36.2°N 32.1°W
Movement: NNE at 7 mph
Pressure: 993 mb

Nadine continues to lose organization. While no recent microwave passes are available, the satellite signature is starting to become reminiscent of a frontal low, although cyclone phase diagrams from FSU still showed at least a weak warm core in association with the system at 12 and 18z, respectively. While the satellite signature has continued to deterioriate since then, it takes time for extratropical transition, and Nadine probably still retains at least a weak mid- to upper-level warm core.



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

Significant vertical shear and cool sea surface temperatures should continue throughout the next five days. While these factors would normally argue for significant weakening, Nadine is obviously not very tropical, and is deriving at least some of its energy from temperature differentials. This should preclude significant weakening. Indeed, the majority of the guidance continues to suggest little change in strength, but Nadine has continued to weaken over the last 24 hours. However, with a major storm system now approaching the storm from the west, it seems likely that the cyclone will garner a little bit of baroclinic energy, which should act to keep the winds up, or at least not allow them to experience much additional dwindling. The future evolution -- structurally -- of Nadine, remains highly speculative. I see three possibilities: number one is for the cyclone to continue to lose central convection and degenerate into a remnant low pressure area; this seems a bit unlikely to me given the frontal appearance of the convection on satellite imagery. The second is for Nadine to transition to an extratropical cyclone over the next day or two, which is supported by the current structure. Finally, another potential event is for the cyclone to become absorbed by the north Atlantic low pressure system. This has been inconsistently suggested by some of the global models over the last few days. I tend to favor the middle option.

The steering currents around Nadine are beginning to weaken as the large scale flow to the north of the tropical cyclone becomes more zonal. A large upper low centered between the southern tip of Greenland and Newfoundland continues moving southeastward well to the north of Nadine. This evolution will drive the aforementioned frontal boundary eastward, but the amplitude of the front and the zonal flow over that portion of the north Atlantic suggests that this front will be insufficient to fully pick up Nadine. The storm is forecast to decelerate over the next day or two until the upper low drops southward as suggested by the global models. Beyond 24 hours, the approach of the low is forecast to turn Nadine southeastward. The global models have come into much better agreement on the future evolution of Nadine over the last several cycles, although the ECMWF remains an outlier at 12z in calling for a clockwise loop around the subtropical ridge. However, I recall the GFS solidly outperforming that particular model in the case of Isaac, when the latter stubbornly insisted on a hit along the Alabama coast even as the system was approaching Louisiana.

Despite the eastward shift, there continues to be some rather large disagreement pertaining to the details of that turn, as well as its duration. The GFS, after showing a much faster northeastward motion at 18z, has come in significantly slower at 0z, and keeps Nadine meanding between the Azores and the Canary Islands.

Interests in the Azores, particularly the northwestern section, should monitor the progress of Nadine over the next couple of days. Tropical storm force winds, especially in areas of higher elevation, are possible. A tropical storm warning has been issued for the archipelago (see below).

Watches and warnings


A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE AZORES ISLANDS OF FLORES...CORVO...FAIAL...PICO...SAO JORGE...
GRACIOSA...AND TERCEIRA

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

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

5-day intensity forecast

INITIAL 09/19 0600Z 45 KT 50 MPH
12 hour 09/19 1800Z 45 KT 50 MPH
24 hour 09/20 0600Z 45 KT 50 MPH
36 hour 09/20 1800Z 45 KT 50 MPH
48 hour 09/21 0600Z 45 KT 50 MPH...EXTRATROPICAL
72 hour 09/22 0600Z 45 KT 50 MPH...EXTRATROPICAL
96 hour 09/23 0600Z 45 KT 50 MPH...EXTRATROPICAL
120 hour 09/24 0600Z 45 KT 50 MPH...EXTRATROPICAL

5-day track forecast



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



Central Atlantic low

A non-tropical area of low pressure has developed about 900 miles east of Bermuda. Satellite imagery suggests a well-defined surface circulation, and upper-level winds appear conducive for some development -- tropical or subtropical -- of this disturbance over the next couple of days. While a westward motion is anticipated in the interim, the models suggest recurvature ahead of a shortwave trough moving from North America by day three.

Probability of development in 48 hours: 20%



Lane

Lane has weakened to a tropical storm as of the latest NHC advisory, which is as follows:

Wind: 40 mph, with higher gusts
Location: 20.8°N 130.2°W
Movement: WNW at 9 mph
Pressure: 1006 mb

There is no deep convection associated with the circulation of Lane, which remains quite vigorous. There is only a small shower located about 150 miles northeast of the center. This suggests that Lane is quickly becoming a remnant low as it succumbs to southwesterly shear and cold waters. In fact, the cyclone may actually be a remnant low as of this writing.



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

Regardless of the technical status of the system, the low should dissipate in a few days and move westward in the low-level flow. A more definitive turn to the west should begin shortly as Lane has lost convection.

The track and intensity forecast philosophies remain unchanged. My track is similar to the National Hurricane Center 11:00 PM one, albeit faster and farther south based on the current motion relative to the 1200Z forecast point.

5-day intensity forecast

INITIAL 09/19 0600Z 35 KT 40 MPH
12 hour 09/19 1800Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
24 hour 09/20 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
36 hour 09/20 1800Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
48 hour 09/21 0600Z 15 KT 15 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/22 0600Z...DISSIPATED

5-day track forecast



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

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Storm Nadine Tropical Storm Lane

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Tropical weather analysis - September 17, 2012

By: KoritheMan, 4:59 AM GMT on September 18, 2012

Nadine

Nadine continues to slowly wind down. As of the latest NHC advisory, the following information was posted on the storm:

Wind: 60 mph, with higher gusts
Location: 33.9°N 34.2°W
Movement: NE at 9 mph
Pressure: 989 mb

After a burst of deep convection occured earlier briefly causing the system to look more tropical-like, the cloud pattern has significantly deteriorated, and Nadine is beginning to look less tropical, with little convection near the center, and a large shallow band of convection to the west. In addition, the cyclone is quickly approaching cool waters, and in fact may actually be sitting above water temperatures that are below the magical 26C threshold.



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

While the shear has decreased a little over Nadine today, there is no reason to suggest any reintensification given the highly baroclinic environment the system is moving into. In fact, given this environment, Nadine is expected to become extratropical in about three days, although it could occur sooner.

Nadine has slowed down as the mid-latitude westerly flow weakens. The global models suggest the trough should begin lifting out over the next day or so, allowing Nadine to avoid recurvature. Beyond day five, the models are now pretty consistent that Nadine will not become absorbed by the north Atlantic low pressure system to its west, and will instead be forced southward amidst a building mid-level ridge as a viable extratropical entity. There are still some substantial differences, however, in where and when Nadine makes the southward turn. The GFS and ECMWF are pretty close to each other, showing a gentle clockwise loop around the Atlantic subtropical high. When these two models are in relative agreement, it is usually best to follow. This would also make sense given the synoptic pattern, which does not suggest a trough strong enough to pick up the storm. I still think Nadine could be with us for awhile.

The prolonged southward motion around the mid-Atlantic ridge would bring the system into an environment characterized by warm water and low shear, which will probably allow the system to reorganize into a tropical cyclone. The evolution from tropical to extratropical back to tropical rarely occurs, but it is not unprecedented. The 1899 San Ciriaco Hurricane which struck Puerto Rico as a major hurricane lasted over a month, but spent some of that time as an extratropical cyclone, only to regenerate later.

5-day intensity forecast

INITIAL 09/18 0600Z 50 KT 60 MPH
12 hour 09/18 1800Z 50 KT 60 MPH
24 hour 09/19 0600Z 50 KT 60 MPH
36 hour 09/19 1800Z 50 KT 60 MPH
48 hour 09/20 0600Z 50 KT 60 MPH
72 hour 09/21 0600Z 50 KT 60 MPH...EXTRATROPICAL
96 hour 09/22 0600Z 50 KT 60 MPH...EXTRATROPICAL
120 hour 09/23 0600Z 50 KT 60 MPH...EXTRATROPICAL MOVING INTO WARMER WATERS

5-day track forecast



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



Lane

Lane became the eighth hurricane of the 2012 Pacific season, but is now a downward decline. As of the latest NHC advisory, the following was posted on the hurricane:

Wind: 80 mph, with higher gusts
Location: 18.4°N 127.5°W
Movement: NNW at 10 mph
Pressure: 989 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Satellite data shows a less organized hurricane. The deepest convection is confined to the west side of the low-level center, and earlier microwave data suggested a deterioration of the eastern eyewall. This doesn't seem to be due to any sort of vertical shearing force over the hurricane, but is probably due to cooler waters.



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

In addition to the cool waters, water vapor imagery shows strong southwesterly upper flow just ahead of the hurricane. Lane is expected to respond to this shear pattern by rapidly weakening. The hurricane is forecast to be a shell of its former self -- a remnant low -- in about 48 hours, although it is possible that it could occur a little sooner.

Data from the University of Wisconsin CIMSS group shows an upper trough approaching Lane from the west. This trough is expected to continue moving the hurricane northwestward to north-northwestward for about the next 36-48 hours. Subsequent to that time, Lane is expected to turn westward and west-southwestward in the low-level flow. The models remain tightly clustered because the steering pattern is so simplistic. My forecast track is pretty similar to the one from the National Hurricane Center.

5-day intensity forecast

INITIAL 09/18 0600Z 70 KT 80 MPH
12 hour 09/18 1800Z 65 KT 75 MPH
24 hour 09/19 0600Z 50 KT 60 MPH
36 hour 09/19 1800Z 35 KT 40 MPH
48 hour 09/20 0600Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/21 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/22 0600Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
120 hour 09/23 0600Z...DISSIPATED

5-day track forecast



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

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Storm Nadine Hurricane Lane

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Tropical weather analysis - September 16, 2012

By: KoritheMan, 3:38 AM GMT on September 17, 2012

Nadine

Nadine continues to move across the cenrtal Atlantic Ocean, and has weakened to a tropical storm. As of the latest NHC advisory, the following coordinates were available on the storm:

Wind: 70 mph, with higher gusts
Location: 31.4°N 38.1°W
Movement: ENE at 18 mph
Pressure: 987 mb

The convective pattern has continued to deteriorate this evening, with the shower activity taking on a more banded appearance. The center actually appears to be partially exposed to the south of the convection, suggesting that Nadine has finally succumbed to the strong upper tropospheric shear. A homogeneous comparison of various microwave fixes over the last 12-24 hours further attests to the erosion of the inner core convection.



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

Since the shear is not forecast to abate over the next few days, Nadine may weaken a little more, especially given the disorganized appearance on satellite images. I have revised my intensity forecast downward from the last several days. In about 48 hours, Nadine is expected to cross the 26C isotherm, which should induce faster weakening beyond that time. It should be noted that the GFS has become more consistent that Nadine will ultimately interact with a powerful upper level low pressure system forecast to dive out of the north Atlantic in about three days. If true, this could significantly alter the structure of the tropical cyclone, and may actually cause extratropical transition at or just beyond the end of the forecast period, especially given the cold water in the vicinity of the Azores. Conversely, the ECMWF continues to insist that Nadine will not interact much with the low, which would of course keep Nadine tropical for longer. At this point it is physically impossible to pick sides.

The intensity forecast shows weakening through 72 hours. After that, Nadine may get a shot of baroclinic energy from the aforementioned upper low. While none of the guidance explicitly shows this, I feel it is the best course of action given the uncertainty in the future evolution of the cyclone.

Nadine continues to move eastward in the fast mid-latitude westerly flow associated with an upper tropospheric cold low rotating north of the Azores. Water vapor imagery shows a trough approaching the cyclone. This models suggest this trough should be sufficient to turn Nadine northward in the next 24-36 hours as the upper low lifts out. Thereafter, the trough is expected to bypass the storm, leaving Nadine in a region of weak steering. Thereafter, the northerly flow between another upper low and the subtropical ridge should force Nadine southward. While the model guidance generally agrees with this, there are still significant differences in forward speed, as well as how sharp of a southward turn Nadine makes. The best course of action for now still appears to be the order for a slow motion at the end of the forecast period.

5-day intensity forecast

INITIAL 09/17 0300Z 60 KT 70 MPH
12 hour 09/17 1200Z 60 KT 70 MPH
24 hour 09/18 0000Z 55 KT 65 MPH
36 hour 09/18 1200Z 55 KT 65 MPH
48 hour 09/19 0000Z 50 KT 60 MPH
72 hour 09/20 0000Z 45 KT 50 MPH
96 hour 09/21 0000Z 45 KT 50 MPH
120 hour 09/22 0000Z 45 KT 50 MPH

5-day track forecast



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



Invest 92L

A weak tropical wave continues its approach to the Lesser Antilles. Convection has virtually vanished near the low-level center (I use that term quite loosely here).



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

I am beginning to want to give up on this wave, at least in the short-term. It may still have a shot in the western Caribbean at longer ranges, but that too will be contingent on how much of a circulation remains with the wave by the time it approaches that particular longitude. The GFS still insists on the easterly shear abating, and an anticyclone building over the system. This has not happened yet, and the global models tend to perform poorly in a shearing environment, oftentimes weakening the shear too fast. Indeed, the GFS seems to be latching onto the idea of a weaker, more broad anticyclone as the system traverses the Caribbean. There is some subsident flow to contend with across the eastern and central Caribbean, which will also inhibit development. Even as the system reaches the western Caribbean in about five days, the GFS does not portray a shear pattern that is all that favorable for intensification, with strong southwesterly shear lying in very close proximity to the disturbance.

Given these parameters, I do not expect significant development of 92L in the next few days. At this point a track through the length of the Caribbean into -- you guessed it ladies and gentlemen -- Mexico -- seems a little more likely barring some unlikely strengthening. A stronger system would still have the potential to move toward peninsular Florida.

Probability of development in 48 hours: 10%



Invest 93L

Although officially a low pressure area for now, the area of disturbed weather in the western Gulf of Mexico is highly disorganized. Based on satellite images, a circulation center is still evident near 26.2N 95.0W, and surface and buoy/oil rig observations suggest it remains well-defined.



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

However, surface pressures are not falling, and if there is a low, it remains very weak and completely devoid of convection. Water vapor imagery shows a well-defined upper trough is approaching the system from Texas. This trough/attendant frontal zone is producing strong southwesterly vertical shear over the disturbance, analyzed at 25 kt by UW-CIMSS. While 93L is moving northeast parallel to the upper flow, the shear vector is blowing faster than 93L is moving, which will counteract the general rule that systems can still intensify in shearing environments. Dry air covering the western Gulf should also preclude significant development before the system moves over the central or southern Louisiana coast sometime late Monday evening. The GFS and ECMWF are in good agreement on the timing, and I trend toward the faster side of the guidance given the depth of the aforementioned trough/low.

Regardless of development, episodic heavy rains are likely to accompany this low as it moves quickly northeastward. These rains could produce localized flooding in short bursts.

It should be noted that the global models show the circulation becoming better defined just prior to crossing the coast.

Probability of development in 48 hours: 10%



Lane

Lane is just under hurricane strength. As of the latest NHC advisory, the following was posted on the tropical storm:

Wind: 70 mph, with higher gusts
Location: 15.0°N 125.8°W
Movement: NW at 9 mph
Pressure: 995 mb

Satellite and microwave data suggests that Lane continues to become better organized, with a small but well-developed central dense overcast. Tropical cyclones with tight inner cores like Lane tend to intensify quicker than the climatological mean, but they also weaken just as fast once the environment becomes hostile. A warm spot has not yet become apparent on conventional satellite pictures, but a small eye-like feature surrounded by a developing eyewall was present in an earlier microwave taken shortly after 0z.



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

Lane has only a very narrow window of opportunity for further strengthening, 24 hours at the very most. After that time, the cyclone is forecast to move across cooler waters and encounter increasing westerly shear to the central Pacific trough. Weakening should begin by that time, with degeneration into a remnant low expected in about three days. This is consistent with forecasts from the global models and the National Hurricane Center.

Water vapor imagery shows a large amplitude trough approaching Lane from the west. Consequently, the storm is moving northwestward, and this motion is expected to continue for the next 48-72 hours. After that time, Lane is forecast to weaken and turn west then west-southwest in the low-level flow. My track remains very similar to that from the National Hurricane Center.

5-day intensity forecast

INITIAL 09/17 0300Z 60 KT 70 MPH
12 hour 09/17 1200Z 65 KT 75 MPH
24 hour 09/18 0000Z 65 KT 75 MPH
36 hour 09/18 1200Z 55 KT 65 MPH
48 hour 09/19 0000Z 45 KT 50 MPH
72 hour 09/20 0000Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/21 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
120 hour 09/22 0000Z...DISSIPATED

5-day track forecast



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

2012 Atlantic hurricane season 2012 East Pacific hurricane season Hurricane Nadine Invest 92L Invest 93L Tropical Storm Lane

Updated: 3:38 AM GMT on September 17, 2012

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Tropical weather analysis - September 16, 2012

By: KoritheMan, 6:17 AM GMT on September 16, 2012

Nadine

Nadine continues to move over the central Atlantic. As of the latest NHC advisory, the following was posted on the hurricane:

Wind: 80 mph, with higher gusts
Location: 30.6°N 46.6°W
Movement: E at 17 mph
Pressure: 983 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Despite 30 kt of southwesterly shear as analyzed by UW-CIMSS and the SHIPS model, Nadine continues to hang on, and an eye briefly appeared in satellite pictures around 0200Z.



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

It is unclear how much longer Nadine can maintain its theatrics in such an environment. In addition to the shear, water vapor imagery indicates that the hurricane is surrounded by a large region of dry air, but so far, Nadine has neglected to pay much attention to it. Nadine will be crossing the 26C isotherm in about 48 hours. That should bring about weakening during that time.

Based on recent satellite fixes, Nadine remains pretty much on the 11:00 PM EDT National Hurricane Center forecast track. Water vapor images reveal that the hurricane is coming under increasing mean westerly flow to the south of a deep-layered low pressure system in the vicinity of the Azores. Near the end of the forecast period, the forecast becomes highly uncertain due to run to run differences amongst the model guidance pertaining to how well Nadine responds to a vigorous upper low forecast to dive from the north Atlantic in about four days. If Nadine misses the low, it would likely turn southward under a building ridge over the north Atlantic. Due to the inherent uncertainties in how Nadine will interact with this low, I'll just go with a virtual stall at days four and five. I will revise this as needed. Nadine may be with us for awhile longer.

5-day intensity forecast

INITIAL 09/16 0600Z 70 KT 80 MPH
12 hour 09/16 1800Z 70 KT 80 MPH
24 hour 09/17 0600Z 70 KT 80 MPH
36 hour 09/17 1800Z 70 KT 80 MPH
48 hour 09/18 0600Z 65 KT 75 MPH
72 hour 09/19 0600Z 60 KT 70 MPH
96 hour 09/20 0600Z 50 KT 60 MPH
120 hour 09/21 0600Z 45 KT 50 MPH

5-day track forecast



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



Invest 92L

A broad area of low pressure several hundred miles east of the Windward Islands is associated with a tropical wave. The National Hurricane Center dubbed this wave "Invest 92L" on Saturday. This system is sprawling and not well-organized at the moment. There is evidence of strong easterly shear impinging on the cloud pattern due to a broad upper-level anticyclone to the northeast.



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

Earlier estimates had the center around 13N, but convection has weakened considerably in this area. I see little evidence of cyclonic rotation in this area by looking at satellite imagery. In the meantime, a new burst of convection has developed well to the northwest of this diffuse area. Large scale vorticity data from UW-CIMSS suggests that 92L may trying to form a surface circulation under the mid-level circulation/convective burst. However, the axis of the mid-level circulation passed directly over buoy 41101 located about 250 miles east of Martinique. This buoy did not show significant pressure falls, nor any indication of west winds. This suggests that 92L is broad and disorganized, and I would not be surprised to see the eastern area refire later this morning and into the afternoon.

Water vapor imagery suggests that 92L is embedded in a region of dry air. This subsident airmass and the broad nature of the circulation should inhibit rapid development over the next few days. The current easterly shear is forecast to relax over the next couple of days, and 92L will have the opportunity to strengthen in a few days. It should be noted that if the mid-level circulation is indeed taking over, upper-level winds will become more favorable more quickly.

Regardless of development, this wave is expected to produce locally heavy rains and periods of gusty winds across portions of the Lesser Antilles over the next few days.

The long-range track of this system is uncertain, but an unseasonably deep trough forecast to amplify over the southern United States this weekend favors an eventual path toward peninsular Florida. This assumes the system maintains some vertical integrity. If not, the tropical wave could just as easily continue further west in the low-level flow.

Probability of development in 48 hours: 20%



Kristy

Persistent Kristy remains a tropical cyclone as of the latest NHC advisory:

Wind: 40 mph, with higher gusts
Location: 23.7°N 116.2°W
Movement: NW at 9 mph
Pressure: 1004 mb

Finally, however, there are signs that the cool waters underneath the cyclone are beginning to have adverse effects. Convection has decreased significantly in coverage and extent, and now consists of a few loose bands around the circulation center. However, the cyclone maintains a well-defined surface circulation.



Figure 4. Latest infrared satellite image of Tropical Storm Kristy. Image credit: NOAA

There's really not much I can say for a system in the process of dying. The northwesterly shear that was affecting the cyclone over the last few days has relaxed, but water vapor images and the GFS/SHIPS suggest the possibility of easterly shear impacting the system very soon. In combination with increasingly falling sea surface temperatures, this should lead to a quick demise of the tropical cyclone. Kristy has been a fighter for sure, and has earned my respect even though she was weak.

There is no change to the track philosophy. Kristy should continue moving northwestward due to a mid-level ridge over Mexico and the southwestern United States. Near the end of the period, the cyclone -- then remnant low actually -- is forecast to slow down and turn north.

5-day intensity forecast

INITIAL 09/16 0600Z 35 KT 40 MPH
12 hour 09/16 1800Z 30 KT 35 MPH
24 hour 09/17 0600Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
36 hour 09/17 1800Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
48 hour 09/18 0600Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/19 0600Z...DISSIPATED

5-day track forecast



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



Lane

Tropical Storm Lane formed on Saturday from what was previously known as Invest 91E. As of the latest NHC advisory, the following information was available on the storm:

Wind: 40 mph, with higher gusts
Location: 13.3°N 124.1°W
Movement: W at 7 mph
Pressure: 1003 mb

A large burst of convection has developed, possibly an incipient central dense overcast. However, a recent microwave pass taken at 0443 UTC suggested that the center was along the north side of the convective mass; not completely exposed, but not totally underneath the convective canopy, either.



Figure 6. Latest infrared satellite image of Tropical Storm Lane. Image credit: NOAA

Since the time of the microwave pass, the low-level center has moved closer to the convection, and since the shear appears to be decreasing, it will not take long for the center to move fully underneath the convection.

Lane has about two days under warm waters and light shear. These conditions favor strengthening, and the current cyclone structure would certainly suggest that. After that time, Lane is forecast to begin moving across cooler waters, and the cyclone is also forecast to encounter increasing westerly shear. This should lead to weakening, possibly faster than indicated below. While not much of the guidance calls for Lane to become a hurricane, this is a viable possibility if the tropical storm strengthens quickly today. I would certainly be prepared for a hurricane. Lane is forecast to dissipate in about four days.

Satellite and microwave fixes from the last several hours suggest that Lane may have turned west-northwestward. This would be consistent with the evolving synoptic pattern, as a deep-layer trough amplifies over the central Pacific. The guidance pretty much agrees on a gradual turn to the northwest, followed by an eventual turn toward the west as Lane weakens and becomes a shallow vortex susceptible to the lower tropospheric flow. My forecast track is fairly similar to that of the National Hurricane Center, albeit a little farther to the right at days two and three.

5-day intensity forecast

INITIAL 09/16 0600Z 35 KT 40 MPH
12 hour 09/16 1800Z 40 KT 45 MPH
24 hour 09/17 0600Z 50 KT 60 MPH
36 hour 09/17 1800Z 55 KT 65 MPH
48 hour 09/18 0600Z 55 KT 65 MPH
72 hour 09/19 0600Z 40 KT 45 MPH
96 hour 09/20 0600Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
120 hour 09/21 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW

5-day track forecast



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

2012 Atlantic hurricane season 2012 East Pacific hurricane season Hurricane Nadine Invest 92L Tropical Storm Kristy Tropical Storm Lane

Updated: 6:18 AM GMT on September 16, 2012

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Tropical weather analysis - September 15, 2012

By: KoritheMan, 6:53 AM GMT on September 15, 2012

Nadine

Nadine became the eighth hurricane of the 2012 season late Friday evening. Behind 2005 and 1893, Nadine marks the third earliest formation of the eighth hurricane in a given season. As of the latest NHC advisory, the following information was available on the hurricane:

Wind: 75 mph, with higher gusts
Location: 30.0°N 52.8°W
Movement: NNE at 14 mph
Pressure: 985 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Nadine continues to exhibit an impressive convective pattern on satellite imagery, although the central dense overcast does not completely wrap around the center, most notably in the eastern semicircle.



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

CIMSS wind shear analyses and the 0z run of the SHIPS suggest that Nadine is experiencing 25 to 30 kt of southwesterly vertical shear due to a well-defined upper tropospheric cold low a couple hundred miles north of Bermuda. The global models suggest this low should move northeastward and quickly weaken, which is supported by water vapor imagery. While this will likely cause a reduction in the shear, another upper low developing between the Azores and the Canary Islands is forecast to move southward and increase the shear over the hurricane in about a day. The reason the shear may not be affecting Nadine as much as one would expect can probably be attributed to the northeastward movement of the hurricane. Storms moving in the same direction as the shear vector rarely experience much weakening. In fact, since the upper flow is forecast to remain between southwest and west throughout the forecast period, I find it unlikely to assume that Nadine will weaken much. Later in the period, cooler waters should help hasten the weakening process.

Nadine is moving through a weakness in the subtropical ridge. Water vapor imagery shows an upper trough amplifying to the west of the hurricane, over Atlantic Canada. As the trough and the aforementioned upper low exert their influence, Nadine is expected to respond by turning eastward. Later in the period, a gradual east-northeast turn toward the Azores is represented. My track continues to agree fairly well with the National Hurricane Center, albeit it remains slower.

Beyond day five, there is some indication that Nadine will not move very much.

5-day intensity forecast

INITIAL 09/15 0600Z 65 KT 75 MPH
12 hour 09/15 1800Z 70 KT 80 MPH
24 hour 09/16 0600Z 70 KT 80 MPH
36 hour 09/16 1800Z 70 KT 80 MPH
48 hour 09/17 0600Z 70 KT 80 MPH
72 hour 09/18 0600Z 65 KT 75 MPH
96 hour 09/19 0600Z 55 KT 65 MPH
120 hour 09/20 0600Z 45 KT 50 MPH

5-day track forecast



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



Kristy

Kristy continues as a tropical storm. As of the latest NHC advisory, the following was posted on the cyclone:

Wind: 45 mph, with higher gusts
Location: 21.2°N 113.8°W
Movement: NW at 9 mph
Pressure: 1001 mb

Despite being over sub-26C SSTs, Kristy appears to have become a little better organized during the last several hours, with convection wrapping around more of the center, most notably in a band to the north.



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

While vertical shear remains weak, Kristy will continue to move across increasingly cooler waters, which is expected to result in a continued decay of the cyclone circulation. Kristy is forecast to become a remnant low in about 36 hours, but it could occur sooner.

Kristy continues moving northwest, and remains pretty much on track. I see no reason to disagree with the National Hurricane Center forecast track in light of the evolving synoptic pattern over this portion of the Pacific.

5-day intensity forecast

INITIAL 09/15 0600Z 40 KT 45 MPH
12 hour 09/15 1800Z 35 KT 40 MPH
24 hour 09/16 0600Z 30 KT 35 MPH
36 hour 09/16 1800Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
48 hour 09/17 0600Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/18 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/19 0600Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
120 hour 09/20 0600Z...DISSIPATED

5-day track forecast



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



Invest 91E

A well-organized area of disturbed weather in the far open Pacific located about 1000 miles southwest of the southern tip of Baja California could develop into a tropical depression at any time.

An SSMIS microwave pass taken just after 0z suggested that the low-level center was exposed to the north of the thunderstorms. Since the time of the pass, the low-level center has more or less moved under the northern portion of the convection. The current northeasterly shear affecting the system is forecast to relax, and this should allow the system to organize into a tropical depression sometime today. There is the potential for the system to become a tropical storm, but it only has about three days to intensify, after which time it will begin moving across cooler waters and into an environment of increasing westerly shear.

The global model guidance suggests that the amplified trough located between the Hawaiian Islands and the coast of California will tug the system northward in a couple of days, followed by a turn back toward the west under a building ridge.

Probability of development in 48 hours: 90%

2012 Atlantic hurricane season 2012 East Pacific hurricane season Hurricane Nadine Tropical Storm Kristy Invest 91E

Updated: 10:07 AM GMT on September 15, 2012

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Tropical weather analysis - September 14, 2012

By: KoritheMan, 7:42 AM GMT on September 14, 2012

Nadine

Tropical Storm Nadine continues to move across the central Atlantic Ocean. As of the latest NHC advisory, the following information was available on the tropical cyclone:

Wind: 70 mph, with higher gusts
Location: 25.0°N 53.7°W
Movement: NNW at 15 mph
Pressure: 989 mb

Satellite fixes suggest little change to Nadine. A 0000 UTC AMSUB overpass indicated that the center was a little to the southwest of the central dense overcast, symptomatic of southwesterly shear, as diagnosed by UW-CIMSS and the SHIPS model.



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

Interestingly, the outflow pattern is still quite marked on satellite images, which suggests that the majority of the shear is confined below the outflow level.

The GFS shows the shear decreasing in about 24-36 hours. The SHIPS takes Nadine to hurricane strength in 36 hours, but curiously still shows shear. Either way, the tropical cyclone is moving away from the upper low northeast of Puerto Rico. As this happens, the upper tropospheric flow over Nadine should veer from southwesterly to southeasterly, which is a pattern more conducive to intensification. The HWRF makes Nadine a major hurricane in about three days, but the remainder of the intensity models are well below that value. Additionally, the HWRF seems to overdo the intensity of tropical cyclones quite often.

Since most of the guidance makes Nadine a hurricane in what appears to be a favorable environment on the GFS, I will do the same. Beyond day three, sea surface temperatures should decrease, which is expected to induce a slow weakening of Nadine at longer ranges.

Water vapor imagery shows that Nadine is about to round the western periphery of the subtropical ridge over the central Atlantic, although for now a more north-northwest continues. In about 6-12 hours, Nadine is expected to turn northward, then gradually turn toward the east-northeast as the cyclone becomes embedded in high latitude flow north of the subtropical ridge axis. The global models are in good agreement on this, although there are still some differences, albeit less substantial, in regards to where Nadine ends up at longer ranges. But since the GFS and ECMWF have come into better agreement, I will tend to follow them instead of the generally less reliable models. My forecast is pretty similar to the National Hurricane Center one, albeit slower at day five due to the models unanimously slowing the forward speed.

5-day intensity forecast

INITIAL 09/14 0600Z 60 KT 70 MPH
12 hour 09/14 1800Z 60 KT 70 MPH
24 hour 09/15 0600Z 65 KT 75 MPH
36 hour 09/15 1800Z 65 KT 75 MPH
48 hour 09/16 0600Z 70 KT 80 MPH
72 hour 09/17 0600Z 75 KT 85 MPH
96 hour 09/18 0600Z 75 KT 85 MPH
120 hour 09/19 0600Z 70 KT 80 MPH

5-day track forecast



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



Kristy

Tropical Storm Kristy continues to march across the Pacific well south of Baja. As of the latest NHC advisory, the following was posted on the storm:

Wind: 50 mph, with higher gusts
Location: 19.7°N 111.1°W
Movement: WNW at 10 mph
Pressure: 1000 mb

Earlier microwave data suggested that the center was still to the north of the convection, but recent satellite images indicate that Kristy may be making a bit of a comeback. However, the convection remains pretty asymmetric.



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

While the shear is forecast to decrease -- in fact it may be doing so now -- this will be concurrent with passage over cooler waters. Indeed, analysis of water temperatures in the region suggests that Kristy will cross the 26C isotherm in about 12 hours. Thus, the current resurgence of convection is very likely to be temporary, and was probably aided by diurnal processes. Kristy is expected to become a remnant low in about three days, possibly sooner.

Kristy is moving toward a break in the Pacific subtropical high as a trough amplifies offshore the western United States, which should cause a bend toward the northwest, or at least ocntinue the west-northwest motion. In about three days, Kristy is expected to slow and turn north then east as a decaying remnant low.

5-day intensity forecast

INITIAL 09/14 0600Z 45 KT 50 MPH
12 hour 09/14 1800Z 45 KT 50 MPH
24 hour 09/15 0600Z 40 KT 45 MPH
36 hour 09/15 1800Z 40 KT 45 MPH
48 hour 09/16 0600Z 35 KT 40 MPH
72 hour 09/17 0600Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/18 0600Z 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
120 hour 09/19 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW

5-day track forecast



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



Invest 91E

An area of disturbed weather located several hundred miles southwest of Tropical Storm Kristy is producing disorganized showers and thunderstorms. The system is experiencing some northeasterly shear, but this should steadily abate. Upper-level winds favor additional development, although the GFS thinks the flow will be more diffluent than anticyclonic.

This low is expected to move generally westward over the next few days.

Probability of development in 48 hours: 50%

A second area of disturbed weather is located west of disturbance 91E, around 130W. While upper-level winds are fairly light at the moment, water vapor imagery shows southwesterly shear approaching the system. The GFS repsonds to this by increasing the upper flow over the system over the next few days. I do not expect significant development from this area.

Probability of development in 48 hours: 20%

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Storm Nadine Tropical Storm Kristy Invest 91E

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Tropical weather analysis - September 12, 2012

By: KoritheMan, 5:39 AM GMT on September 12, 2012

Nadine

Tropical Storm Nadine formed this evening over the central Atlantic. As of the most recent NHC advisory, the following information was available on the tropical storm:

Wind: 40 mph, with higher gusts
Location: 17.8°N 45.2°W
Movement: WNW at 15 mph
Pressure: 1004 mb

Satellite images show an improvement in the cloud pattern of the tropical cyclone. Central convection has increased significantly, and convective bands are noted to the north and south of the center, respectively. Upper-level outflow is well-defined in all quadrants except the east.



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

I see no obvious reason why Nadine should not continue to strengthen. The shear is low, the water will be getting progressively warmer, and water vapor imagery does not currently show much dry air affecting the cyclone. Indeed, the SHIPS rapid intensification parameter at 0z shows a 44% chance of a 25 kt increase in the next 24 hours. Given the much improved convective structure, vertical structure, and outflow pattern, this could happen. However, tropical cyclones do not always rapidly intensify in seemingly favorable environments. In fact, they sometimes don't intensify at all. And since there is little skill in predicting these sort of events, I will not explicitly indicate rapid intensification. But again, given the environment, it's certainly possible, especially given the rapidity in which the central core has organized over the last several hours. I'll admit my intensity forecast could be conserative, and is likely too low if current trends continue.

By Friday, the GFS and SHIPS suggest an increase in westerly shear as an upper-level trough amplifies to the west of the tropical cyclone. This should cause Nadine to weaken at that time. There are indications in the GFS vertical shear fields that the upper flow could gradually become more favorable after that time as Nadine moves over high latitudes. This could allow for some reintensification beyond day five, although this is uncertain since the cold front that recurved Leslie and Michael was quite strong, and strong subsident flow is noted behind it. This could catch up with Nadine at longer ranges.

Recent satellite fixes indicate that Nadine might have turned back to the northwest. This is consistent with the global and dynamical models, which move Nadine through a weakness in the subtropical ridge. In about three days, an upper low is forecast to develop over the central Atlantic near Bermuda, which should reinforce the weakness and move Nadine out to sea. After 72 hours, there are large disagreements amongst the various model members as to the extent in which Nadine interacts with the low. The GFS, GFDL, and HWRF are faster, while the ECMWF and CMC are slower. Given the splendid performance of the GFS this year, as well as the fact that the synoptic pattern is generally less variable over the open Atlantic than closer to the United States (as we saw with Isaac), I am going to side with the faster solution.

5-day intensity forecast

INITIAL 09/12 0300Z 35 KT 40 MPH
12 hour 09/12 1200Z 40 KT 45 MPH
24 hour 09/13 0000Z 50 KT 60 MPH
36 hour 09/13 1200Z 60 KT 70 MPH
48 hour 09/14 0000Z 70 KT 80 MPH
72 hour 09/15 0000Z 75 KT 85 MPH
96 hour 09/16 0000Z 70 KT 80 MPH
120 hour 09/17 0000Z 65 KT 75 MPH

5-day track forecast



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



Invest 90E

A well-defined area of low pressure in the eastern Pacific located several hundred miles south of Acapulco continue to show signs of organization. A ball of convection encompasses the center, but low cloud lines suggest that the central gyre remains displaced a little to the east of the deepest thunderstorms, possibly due to a little bit of easterly shear, as evidenced by the squashed outflow pattern in the eastern semicircle.



Figure 3. Latest infrared satellite image of Invest 90E. Image credit: NOAA

Only a slight increase in organization would result in the formation of a tropical depression.

Upper-level winds are expected to remain favorable for the next three days (although the GFS suggests that might not quite be anticyclonic). After that, the system is forecast to cross the 26C isotherm, encounter drier air, and come under increasing northwesterly shear due to a strong upper-level high over the central Pacific. This should induce weakening beyond that time.

The system is expected to continue moving generally toward the west-northwest. A turn to the northwest is expected in about three to four days as a weak upper trough develops over California. While the GFDL suggests a threat to Baja, this solution has been discontinued since that model is an outlier, and tends to perform poorly with eastern Pacific tropical cyclones.

Probability of development in 48 hours: 90%


Elsewhere

Elsewhere in the Atlantic, the ECMWF has continued to suggest the possibility of a broad area of low pressure forming in the western or central Gulf of Mexico in about three to four days. I am unsure what this is supposed to come from -- there are no tropical waves apparent on satellite imagery, or the National Hurricane Center surface map. Looking at the 850 mb vorticity fields in that model, it seems to come from the cold front now moving across the upper midwest. The parent upper low is centered over Manitoba, and this trough appears quite vigorous. In fact, it may be trying to take on a negative (northeast-southwest) tilt. This suggests that the Euro's prognostication of homegrown development may not be too far off the mark. However, it lacks support from the GFS, and the other models have been scanty with it at best.

The synoptic pattern favors a threat to the northern Gulf Coast with any system that does develop. The upper tropospheric flow on the GFS does not look especially favorable, and this system will likely be a sheared one if it does develop, with the associated precipitation shield well east or northeast of the center.

2012 Atlantic hurricane season Tropical Storm Nadine Invest 90E 2012 East Pacific hurricane season

Updated: 6:48 AM GMT on September 12, 2012

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Tropical weather analysis - September 11, 2012

By: KoritheMan, 9:07 AM GMT on September 11, 2012

Leslie

Leslie is racing toward Newfoundland with near hurricane force winds. As of the latest NHC advisory, the following was posted on the storm:

Wind: 70 mph, with higher gusts
Location: 45.7°N 56.4°W
Movement: NNE at 40 mph
Pressure: 968 mb

Leslie's appearance resembles that of a frontal low on satellite images, with an elongated band of convection well-removed from the center. There is also a large frontal-type band extending well north of the center. This means that Leslie is attached to a front.



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

Leslie is currently over SSTs of about 18C, according to offshore buoy observations off the coast of Newfoundland. In addition, water vapor imagery shows fast sothwesterly upper-level flow afflicting the system, indicative of strong vertical shear. This flow is pushing subsidence into the system. This cold air behind the upper-level trough that Leslie is now attached to will combine with the cold underlying water temperatures to bring about the cessation of any tropical characteristics Leslie might have had.

Regardless of the technical status of the system, Leslie is still expected to be a vigorous low pressure system at landfall. Station 44139, located between the coast of Nova Scotia and Newfoundland, reported sustained winds of 50 mph with a gust to 70 mph at 0500 UTC local time. The buoy also reported an east wind, along with a 973 mb central pressure that was rapidly falling. This suggests that the center was quickly approaching the buoy. Since that time, satellite imagery suggests that the center passed directly over the buoy. The convective pattern suggests that the buoy could have missed the strongest winds, and wind gusts to hurricane force are still possible over sections of the Avalon Peninsula, although the duration of these events may be limited due to lack of convection near the center to sufficiently mix down the winds from aloft to the surface.

5-day intensity forecast

INITIAL 09/11 0900Z 60 KT 70 MPH
12 hour 09/11 1800Z 60 KT 70 MPH...EXTRATROPICAL
24 hour 09/12 0600Z 60 KT 70 MPH...EXTRATROPICAL
36 hour 09/12 1800Z 55 KT 65 MPH...EXTRATROPICAL
48 hour 09/13 0600Z 50 KT 60 MPH...EXTRATROPICAL
72 hour 09/14 0600Z...ABSORBED BY LARGE FRONTAL ZONE

I will not bother drawing a track forecast for this system, as given the limitations of the map I'm using, I would only be enabled to draw a forecast point out to 12 hours. Hardly ideal.

Watches and warnings

A HURRICANE WATCH IS IN EFFECT FOR...
* NEWFOUNDLAND FROM STONES COVE TO CHARLOTTETOWN

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* NEWFOUNDLAND FROM INDIAN HARBOUR... TO TRITON

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



Michael

Michael is weakening. As of the latest NHC advisory, the following was posted on the storm:

Wind: 65 mph, with higher gusts
Location: 37.1°N 47.7°W
Movement: N at 18 mph
Pressure: 994 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

The cyclone is quickly succumbing to northerly shear. All of the convection is south of the center, which is now partially exposed.



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

Water vapor imagery shows even stronger northerly shear about to affect Michael. That along with cooler waters should continue the weakening trend, and Michael may weaken faster than shown below. In about 24 hours, Michael is forecast to lose tropical characteristics. Just after 48 hours, the small cyclone is expected to become absorbed by a very large extratropical low pressure system over the north Atlantic.

Recent satellite fixes suggest that Michael has turned northward as a deep-layer trough to the west weakens the ridge downstream of the tropical cyclone. This is consistent with forecasts from the global models over the last several days. The storm is expected to gradually turn northeast as deep-layer southwesterly flow increases over the storm.

5-day intensity forecast

INITIAL 09/11 0900Z 55 KT 65 MPH
12 hour 09/11 1800Z 45 KT 50 MPH
24 hour 09/12 0600Z 35 KT 40 MPH...EXTRATROPICAL
36 hour 09/12 1800Z 35 KT 40 MPH...EXTRATROPICAL
48 hour 09/13 0600Z 30 KT 35 MPH...EXTRATROPICAL
72 hour 09/14 0600Z...ABSORBED BY LARGE FRONTAL ZONE

5-day track forecast



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



Invest 91L

A tropical wave in the central Atlantic continues to show signs of organization, and only a slight increase in thunderstorm activity would result in the formation of a tropical depression.

The satellite signature of the wave is not impressive. While the low-level center is well-defined, the central convection consists of a small ball of convection. A large convective band is found in the western semicircle, well-removed from the center. This could be partly responsible for the current lack of convection near the actual vortex.



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

Upper-level winds ahead of the systme are favorable for strengthening; an upper low developing between Bermuda and the wave is creating a strong poleward outflow channel to the north of the system. Water vapor imagery also shows less dry air than the system has been dealing with over the last couple days. The SHIPS shows westerly shear increasing over the system in about 72 hours, possibly in response to a deep-layer trough forecast to amplify to the west of the tropical wave over the central Atlantic. The GFS also shows it to an extent, but is quick to rebuild the anticyclone. I will go with the GFS solution of less shear, as the 300 mb fields in that model do not support anything too terribly threatening in the shear department. Thus, I expect a hurricane in about three or four days. However, this forecast may require revision in my next entry.

The low is expected to move west-northwest for the next 36-48 hours. After that, a turn to the northwest and north is expected as the cyclone feels the deep southerly flow associated with the aforementioned trough. The global models are, in general, in agreement on this. However, distinct differences are apparent between the GFS and ECMWF regarding forward speed, and how quickly the wave turns northeast in the long-range. The Euro shows the system slowing after day four, presumably in response to a baroclinic development in the central Atlantic. The GFS keeps the system moving more steadliy to the northeast. It's probably best to blend these extremes and call for a slow northward motion at the end of the period.

Probability of development in 48 hours: 90%



Invest 90E

A tropical disturbance in the eastern Pacific off is showing signs of organization. Shower activity has increased significantly near the low-level center, which appears to be becoming better defined. Environmental conditions favor additional development, and a tropical depression could form over the next day or two as the system moves off toward the west-northwest.

Probability of development in 48 hours: 70%

Tropical Storm Leslie Invest 90E 2012 East Pacific hurricane season Invest 91L Tropical Storm Michael 2012 Atlantic hurricane season

Updated: 9:12 AM GMT on September 11, 2012

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Tropical weather analysis - September 10, 2012

By: KoritheMan, 5:47 AM GMT on September 10, 2012

Leslie

After lashing Bermuda with tropical storm force winds, Leslie is moving away from the island. As of the latest NHC advisory, the following information was available on the tropical storm:

Wind: 60 mph, with higher gusts
Location: 34.4°N 61.8°W
Movement: NNE at 16 mph
Pressure: 988 mb

Leslie has changed little in organzation over the last several hours, except that convection has decreased, particularly in the western and southern quadrants. This may be due to dry air in the middle troposphere, as denoted by upper air data from Bermuda.



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

Satellite and microwave data suggests that the cyclone lacks an inner core, and I am becoming doubtful it ever regains hurricane strength, especially since it will be interacting with a large frontal zone over the next couple of days. While some of the models still signify reintensification, they are slowly coming down. I will forecast a little bit of strengthening, but not much. While trough interaction does occasionally help to trigger intensification, it is difficult to predict the actual timing and specific mechanisms that govern such events. My current thinking is that the trough is too close to the tropical cyclone. Global model fields suggest that Leslie will become extratropical in about 48 hours, and become absorbed by a larger extratropical low over the North Sea just beyond day four.

Water vapor imagery shows the aforementioned trough amplifying to the west of Leslie. The global models respond to this pattern by forecasting a north-northeast motion toward the Avalon Peninsula of Newfoundland. Thereafter, the system is expected to accelerate east-northeast, then eastward in the high-latitude westerlies over the north Atlantic. The developing synoptic pattern is consistent with this trend.

Due to limitations in the map I use to make track forecasts, I will only be able to include forecast points out to 36 hours.

5-day intensity forecast

INITIAL 09/10 0300Z 50 KT 60 MPH
12 hour 09/10 1200Z 50 KT 60 MPH
24 hour 09/11 0000Z 55 KT 65 MPH
36 hour 09/11 1200Z 55 KT 65 MPH...INLAND ALONG AVALON PENINSULA
48 hour 09/12 0000Z 55 KT 65 MPH...EXTRATROPICAL
72 hour 09/13 0000Z 50 KT 60 MPH...EXTRATROPICAL
96 hour 09/14 0000Z 45 KT 50 MPH...EXTRATROPICAL
120 hour 09/15 0000Z....ABSORBED BY EXTRATROPICAL LOW OVER NORTH SEA

5-day track forecast



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



Michael

Tenacious Michael hangs onto hurricane status. As of the latest NHC advisory, the following was posted on the hurricane:

Wind: 85 mph, with higher gusts
Location: 33.6°N 44.4°W
Movement: W at 6 mph
Pressure: 983 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

The cloud pattern remains fairly well-organized, although the eyewall is open to the northwest. This is confirmed by recent microwave data as well, which shows very little convection in that quadrant.



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

Based on the convective pattern, it is obvious that Michael is experiencing northerly shear. This has been well anticipated by the global models. In about a day or so, the upper tropospheric flow is forecast to increase and transition to a more southwesterly regime ahead of a deep-layer trough, the same one recurving Leslie. In combination with cooler waters, that should prove sufficient to hasten the weakening trend, and ultimately presage extratropical transition. By day four, the hurricane is forecast to become absorbed by the large extratropical low forecast to be over the north Atlantic.

Michael is moving westward as a shortwave mid-level ridge builds in to the north of the hurricane in the wake of a trough. However, the hurricane should soon feel the southerly flow associated with Leslie and the frontal zone encroaching on it. As a result, Michael is expected to turn to the west-northwest, then northwest, followed by a gradual recurve to the northeast as it becomes fully captured by the trough. My forecast track is in excellent agreement with that from the National Hurricane Center based on the evolving large-scale pattern. This is a very straightforward track forecast.

5-day intensity forecast

INITIAL 09/10 0300Z 75 KT 85 MPH
12 hour 09/10 1200Z 70 KT 80 MPH
24 hour 09/11 0000Z 60 KT 70 MPH
36 hour 09/11 1200Z 50 KT 60 MPH
48 hour 09/12 0000Z 40 KT 45 MPH...EXTRATROPICAL
72 hour 09/13 0000Z 35 KT 40 MPH...EXTRATROPICAL
96 hour 09/14 0000Z...ABSORBED BY FRONT

5-day track forecast



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



Invest 91L

A tropical wave in the eastern Atlantic centered several hundred miles west of the Cape Verde Islands is producing a concentrated area of showers and thunderstorms. The deepest convection appears to be confined to the west side of the wave axis.



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

Earlier ASCAT data suggested a fairly well-defined surface circulation, but the low cloud motions on satellite imagery are less conclusive. My guess is, if a closed surface circulation exists, it is currently very small.

Upper-level winds are conducive for development, and the GFS forecasts an anticyclone to build over the budding disturbance over the next day or so. The net result should be organization, and this system is likely to become a tropical depression over the next day or two.

The large scale pattern over the Atlantic favors a sharp recurvature well north of the Leeward Islands.

Probability of development in 48 hours: 70%



Eastern Pacific disturbance

A large area of disturbed weather a few hundred miles south-southwest of the Gulf of Tehuantepec is producing disorganized showers and thunderstorms over the far eastern Pacific waters and adjacent land areas of southeast Mexico. Upper-level winds appear conducive for gradual development of this low over the next couple days as it moves west-northwestward. The only negative I can see is the large size of the disturbance. The GFS and ECMWF develop this system into a tropical cyclone over the next several days, with the ECMWF being the faster of the two.

It does not currently appear that the system will threaten any land areas, although it may pass near Socorro Island in about four or five days.

Probability of development in 48 hours: 40%

2012 Atlantic hurricane season Tropical Storm Leslie Hurricane Michael Invest 91L

Updated: 5:57 AM GMT on September 10, 2012

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Tropical weather analysis - September 8, 2012

By: KoritheMan, 2:42 AM GMT on September 09, 2012

Leslie

Leslie continues to meander, but appears to be slowly moving away from its cold wake. As of the latest NHC advisory, the following was available on the cyclone:

Wind: 65 mph, with higher gusts
Location: 30.1°N 62.6°W
Movement: N at 8 mph
Pressure: 988 mb



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

There is still little evidence of an inner core with Leslie, and based on the satellite signature the radius of maximum winds are probably still on the order of about 75 nautical miles.

Analysis of water vapor imagery suggests that the shear over the system has decreased. Additionally, buoy data suggests that offshore water temperatures near Bermuda are around 84F. The low shear combined with the warm waters should favor intensification, although ambient dry air may still be a problem, especially considering how large Leslie's circulation is. It no longer appears likely, even given a remote chance, that Leslie will become a major hurricane. Based on SST analyses and the global model isobaric fields, Leslie is expected to lose tropical characteristics in about three days. While the models do not suggest reintensification as a post-tropical cyclone, they suggest that Leslie could be a rather vigorous extratropical low over the north Atlantic for a few days subsequent to transition.

As a large-scale trough amplifies over the western Atlantic, the surrounding steering flow has become better defined. Consequently, the tropical storm has responded by turning north with some acceleration. The global models are in good agreement on the track, taking the system well east of Bermuda tomorrow, and accelerating Leslie northeastward after that. It is possible that Leslie will make landfall along the southeastern tip of Nova Scotia as an extratropical cyclone in about four days.

My forecast is similar to but a little east of the National Hurricane Center.

5-day intensity forecast

INITIAL 09/09 0300Z 55 KT 65 MPH
12 hour 09/09 1200Z 60 KT 70 MPH
24 hour 09/10 0000Z 65 KT 75 MPH
36 hour 09/10 1200Z 70 KT 80 MPH
48 hour 09/11 0000Z 75 KT 85 MPH
72 hour 09/12 0000Z 70 KT 80 MPH...EXTRATROPICAL
96 hour 09/13 0000Z 60 KT 70 MPH...EXTRATROPICAL
120 hour 09/14 0000Z 45 KT 50 MPH...EXTRATROPICAL

5-day track forecast



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

A tropical storm warning remains in effect for Bermuda, although I doubt wind gusts above 40 mph will be felt on that island.



Michael

Michael continues as a hurricane. As of the latest NHC advisory, the following was available on the hurricane:

Wind: 100 mph, with higher gusts
Location: 33.4°N 42.4°W
Movement: NNW at 6 mph
Pressure: 975 mb
Category: 2 (Saffir-Simpson Hurricane Scale)

Michael hasn't changed much this evening, except that the central dense overcast has shrunk. The eyewall convection continues to fluctuate, but in general the northwestern eyewall appears to be opened.



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

Water vapor imagery shows northwesterly shear impinging on the western side of the small hurricane, associated with the backside of a mid- to upper-level trough passing to the north. However, the flow to the north of the hurricane is becoming zonal, signifying departure of the trough. As this feature lifts out, so will the shear. In about 72 hours, however, the GFS/SHIPS forecast a marked decrease in northerly shear as Michael begins to feel outflow from what should be an intensifying Hurricane Leslie. The shear is eventually forecast to transition to a more eastward direction and increase even further. The end result should be rapid weakening, especially given Michael's small size.

Most of the models are now suggesting that the hurricane will become absorbed by the much larger circulation of Leslie later in the forecast period, and as a course of least regret, I now will as well.

The hurricane should soon turn to the west-northwest under the influence of a building mid-level ridge. By late Monday, a turn back to the northwest and north is expected as Michael begins to interact with Leslie. This track is in good agreement with the global models.

5-day intensity forecast

INITIAL 09/09 0300Z 85 KT 100 MPH
12 hour 09/09 1200Z 85 KT 100 MPH
24 hour 09/10 0000Z 80 KT 90 MPH
36 hour 09/10 1200Z 75 KT 85 MPH
48 hour 09/11 0000Z 70 KT 80 MPH
72 hour 09/12 0000Z 55 KT 65 MPH
96 hour 09/13 0000Z...ABSORBED BY HURRICANE LESLIE

5-day track forecast



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



Invest 91L

A tropical wave in the eastern Atlantic centered in the vicinity of the southern Cape Verde Islands ("91L") is producing disorganized showers and thunderstorms. Microwave and satellite data suggest that the system isn't too well-organized yet. However, upper-level winds appear favorable enough -- at least diffluent -- to allow for gradual development of this wave as it moves westward. In a few days, the system is expected to turn toward the west-northwest admist a break in the subtropical ridge. This wave looks like another one for the fishes. However, the recurve pattern this year seems to be a little weaker than the last few years, and I don't think the United States is quite done yet. We will need to watch any system that comes out of the Caribbean or Gulf.

Probability of development in 48 hours: 40%

2012 Atlantic hurricane season Tropical Storm Leslie Hurricane Michael Invest 91L

Updated: 2:44 AM GMT on September 09, 2012

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Tropical weather analysis - September 7, 2012

By: KoritheMan, 7:10 AM GMT on September 07, 2012

Leslie

Leslie refuses to move. As of the latest NHC advisory, the following information was posted on the hurricane:

Wind: 75 mph, with higher gusts
Location: 26.5°N 62.2°W
Movement: Stationary
Pressure: 985 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Looks can certainly be deceiving. While Leslie has a well-organized appearance on satellite images, earlier microwave data suggested that the lower- and mid-level circulations continue to have an eastward displacement, which implies that the hurricane is still under some westerly shear.



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

The SHIPS and GFS still insist on a relaxation of the shear, but so far this has not materialized. An upper low north of the Bahamas is forecast to decay over the next 12-24 hours. There is some evidence of this on water vapor images. Assuming that this trend continues, the shear should decrease over the hurricane, and it would have an opportunity to strengthen. However, microwave data suggest that Leslie is a large hurricane that lacks an inner core. Such systems generally take awhile to spin up (and sometimes never do), as we saw with Isaac. In addition, NOAA buoy 41049 located very near the cyclone center indicates that water temperatures have dropped significantly under the hurricane -- as much as 3F -- over the last 24 hours, and are now below 26C. No doubt, this is due to the continued quasi-stationary movement of the storm. Until Leslie begins to move, it will probably not strengthen. The global models suggest this should happen over the next 24-36 hours as an upper trough over the central United States swings eastward. While not explicitly shown, I would not be surprised to see Leslie briefly drop below hurricane strength over the next 24 hours due to the cooling SSTs. After 72 hours, the GFS shows Leslie losing its anticyclone, and southwesterly shear increasing over the hurricane. That, in combination with progressively cooler sea surface temperatures should lead to weakening during the latter portion of the forecast period. Beyond day five, Leslie will likely become extratropical due to very cold SSTs.

Leslie remains in an environment of weak steering. UW-CIMSS analysis shows that the hurricane is sandwiched between two anticyclones, which is resulting in little movement. However, the models suggest that the easternmost anticyclone will remain stronger, which fits well with synoptic trends over the United States. The models respond to this pattern by forecasting a slow, mostly northward motion through the next day or so. Thereafter, the hurricane is forecast to slowly pick up speed as the aforementioned trough amplifies. A greater degree of acceleration is shown near the end of the forecast period as Leslie becomes fully embedded in the high-latitude westerlies. In general, the guidance has trended eastward over the last 24 hours, and it now appears that Leslie will miss Bermuda by a fairly wide margin. However, the wind field is large enough that tropical storm force winds may still occur on that island, particularly if the track shifts westward. For that reason, a tropical storm watch has been posted for that island.

In the meantime, Leslie's large wind field will continue to generate substantial waves and rip currents across much of the United States east coast, Bermuda, and Atlantic Canada over the next several days.

My forecast track is a little slower than the guidance.

5-day intensity forecast

INITIAL 09/07 0600Z 65 KT 75 MPH
12 hour 09/07 1800Z 65 KT 75 MPH
24 hour 09/08 0600Z 65 KT 75 MPH
36 hour 09/08 1800Z 70 KT 80 MPH
48 hour 09/09 0600Z 80 KT 90 MPH
72 hour 09/10 0600Z 90 KT 105 MPH
96 hour 09/11 0600Z 80 KT 90 MPH
120 hour 09/12 0600Z 65 KT 75 MPH

5-day track forecast



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



Michael

After briefly becoming the first major hurricane of the season early on Thursday, Michael has steadliy weakened. As of the latest NHC advisory, the following was posted on the hurricane:

Wind: 105 mph, with higher gusts
Location: 30.8°N 40.8°W
Movement: NNE at 5 mph
Pressure: 970 mb
Category: 2 (Saffir-Simpson Hurricane Scale)

The earlier erosion of the inner core convection appears to have stopped for now, with satellite images showing a more mature CDO. However, the eye has continued to contract, and is fairly ragged at the moment. Earlier microwave data also suggested that the eyewall was open to the northeast. All of this data points to a hurricane that is slowly weakening.



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

Water vapor imagery suggests that Michael is approaching a wall of northerly shear emanating from the backside of a mid- to upper-level trough located to the north of the hurricane. The GFS suggests that shear levels will oscillate between favorable and unfavorable over the next 72 hours, after which an abrupt increase in northerly shear is forecast as the tiny hurricane moves closer to the much larger circulation of Hurricane Leslie. Given the small size of the vortex, it is possible that Michael will weaken more quickly than indicated below.

Based on UW-CIMSS steering analysis and water vapor imagery, which shows the trough bypassing the hurricane, I agree with the National Hurricane Center forecast track, albeit a little to the east for the first 12 hours.

5-day intensity forecast

INITIAL 09/07 0600Z 90 KT 105 MPH
12 hour 09/07 1800Z 80 KT 90 MPH
24 hour 09/08 0600Z 75 KT 85 MPH
36 hour 09/08 1800Z 65 KT 75 MPH
48 hour 09/09 0600Z 60 KT 70 MPH
72 hour 09/10 0600Z 50 KT 60 MPH
96 hour 09/11 0600Z 40 KT 45 MPH
120 hour 09/12 0600Z 30 KT 35 MPH

5-day track forecast



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



Invest 90L

A weak area of low pressure in the northern Gulf of Mexico located about 60 miles southeast of the Mouth of the Mississippi River remains disorganized. While this low is producing shower activity, it is poorly-organized, and displaced well to the southwest of the low-level center due to persistent northeasterly shear. Surface observations still indicate a fairly robust surface circulation, however, and winds to about 30 knots are probably still occurring in the deep convection to the south based on earlier oil rig reports.

Upper-level winds could improve in about 36 hours, and the system still has the opportunity to develop before it reaches the Florida peninsula in about three days. The global models are in good agreement on this. While some of the statistical models have trended toward Louisiana in the last two cycles, the more reliable dynamical consensus continues to indicate a turn toward Florida. However, the statistical models could turn out to be right if the ridge -- which appears quite strong at the moment -- does not appreciably weaken over the next day or two, or if 90L moves more southwestward than indicated. Currently, it is not forecast to get below about 27.5N as per the global models.

There were some faint indications in the global models earlier on Thursday that the trough could bypass the system once it is in the western Atlantic, and the models responded by showing some 850 mb vorticity moving westward across Florida and into the Gulf of Mexico. However, the 0z camp has become less aggressive on this.

Probability of development in 48 hours: 30%



New African wave

A tropical wave has just emerged off the west coast of Africa. The GFS and ECMWF suggest this system could become a tropical depression over the next few days. Environmental conditions are favorable for gradual development of this wave as it moves generally westward over the next 48 hours.

Probability of development in 48 hours: 10%

2012 Atlantic hurricane season Hurricane Leslie Hurricane Michael Invest 90L

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Tropical weather analysis - September 5, 2012

By: KoritheMan, 2:18 AM GMT on September 06, 2012

Leslie

After many days of dancing and prancing, Leslie has become a hurricane, the sixth of the 2012 season. As of the latest NHC advisory, the following was posted on the system:

Wind: 75 mph, with higher gusts
Location: 25.9°N 62.7°W
Movement: N at 2 mph
Pressure: 987 mb
Category: 1 (Saffir-Simpson Hurricane Scale)

Satellite images show a storm that finally seems to be organizing. Numerous spiral bands are evident, including an especially large one to the east. The central dense overcast appears to be getting more symmetrical as well. Finally, upper-level outflow has improved exponentially to the west. This is something that was lacking over the last several days, and it shows that the shear is finally beginning to lessen. In fact, based on the cloud pattern, I feel Leslie is finally on her way to establishing the much discussed anticyclone. If true, she could begin to strengthen a little more quickly. although consolidation of her large circulation may prevent a quicker pace of intensification in the near-term.



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

A TMI overpass just after 2100 UTC suggested that the low- and mid-level centers were displaced by about 25 miles, probably a symptom of continued west-northwesterly shear. However, that was several hours ago, and with the improving cloud pattern since that time, I imagine the lower and middle tropospheric circulations may finally be starting to come together as one. The improving vertical structure suggests additional strengthening is probably imminent. Given the improving organizaton and the very favorable upper air environment forecast by the GFS -- which seems to finally be materializing -- I will go a little higher than the National Hurricane Center, forecasting Leslie to peak as a low-end major hurricane. By Monday, the GFS suggests that the anticyclone may become a bit dislodged. Concurrent with passage over cooler waters, this should cause some weakening on day five.

Water vapor imagery suggests Leslie remains embedded in a region of weak steering currents. However, a shortwave trough is seen amplifying over the eastern United States, which should act to weaken the ridge over the next day or two. While this trough is forecast to dissipate in about three days, Leslie should have progressed far enough to the north by then so that another trough forecast to arrive by the weekend completely picks it up. Bermuda is in the crosshairs, and could take a direct hit. Conditions don't look quite the same as they did with Hurricane Igor two years ago, so I find it unlikely that they get lucky with another weakening storm. A Fabian type scenario seems more realistic this time around. Fabian was the only Bermuda hurricane to have its name retired. However, as I understand it, building codes on Bermuda are extremely strict, so all things considered they should fare pretty well. They got by with only a handful of deaths from Fabian. While I realize death isn't a statistic, I would rather five die than five thousand. I am not concerned about a catastrophe on that island, but it could cause considerable disruption to the electrical grid on the island. Based on the forecast track, a hurricane watch will likely be issued for the island tomorrow evening.

Large waves and dangerous rip currents will continue impacting much of the United States east coast and Bermuda in the interim. These conditions will gradually spread northward to portions of Atlantic Canada by the weekend.

5-day intensity forecast

INITIAL 09/06 0000Z 65 KT 75 MPH
12 hour 09/06 1200Z 65 KT 75 MPH
24 hour 09/07 0000Z 70 KT 80 MPH
36 hour 09/07 1200Z 75 KT 85 MPH
48 hour 09/08 0000Z 85 KT 100 MPH
72 hour 09/09 0000Z 95 KT 110 MPH
96 hour 09/10 0000Z 100 KT 115 MPH
120 hour 09/11 0000Z 90 KT 105 MPH

5-day track forecast



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



Michael

While not officially pending the new advisory, Michael has become a hurricane according to the 0z ATCF estimate. As of the 5:00 PM NHC advisory, the following was posted on the system:

Wind: 70 mph, with higher gusts
Location: 28.8°N 42.7°W
Movement: NE at 7 mph
Pressure: 994 mb

Michael is a small but well-organized hurricane, with a symmetrical CDO and a small eye admist that CDO. An upper-level trough to the northwest of the hurricane is providing a poleward outflow channel, which is undoubtedly aiding the intensificaton process.



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

Upper-level winds are forecast to remain favorable for additional strengthening, especially by 24 hours. The net result should be intensification. Depending on how much organizing Michael does over the next day or so, there is a chance of a major hurricane. Some of the intensity models do bring it close to this threshold over the next 48 hours.

Interestingly, the models seem to want to weaken Michael after 48 hours, and I am not sure why. All I can think of is that the underlying oceanic heat content drops off sharply after that time, and becomes virtually nonexistent. However, it doesn't always take deep warm waters to generate a strong hurricane, and I can't just ignore the lack of vertical shear in the GFS. By day four, Michael is expected to move close enough to Hurricane Leslie that northerly shear associated with the outflow of the latter will begin to weaken the small hurricane. It is possible that Michael will weaken quicker than indicated here at longer ranges, given the small circulation envelope.

Michael is south of a strong upper-level trough. The global models suggest the trough will have the capacity to continue moving Michael on its current northeastward trajectory for about another 24-36 hours before it begins to slow down and turn northwestward under a building ridge. There is some disagreement amongst the models as to what happens to Michael after that. The ECMWF is more westward, while the GFS is more northward. The CMC actually suggests that the trough will capture the hurricane and move Michael northeast. For now I will blend the GFS and ECMWF.

5-day intensity forecast

INITIAL 09/06 0000Z 65 KT 75 MPH
12 hour 09/06 1200Z 70 KT 80 MPH
24 hour 09/07 0000Z 80 KT 90 MPH
36 hour 09/07 1200Z 85 KT 100 MPH
48 hour 09/08 0000Z 90 KT 105 MPH
72 hour 09/09 0000Z 95 KT 110 MPH
96 hour 09/10 0000Z 85 KT 100 MPH
120 hour 09/11 00000Z 75 KT 85 MPH

5-day track forecast



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



Invest 90L

An area of low pressure over the north-central Gulf of Mexico is producing limited shower activity. Strong northerly shear is inhibiting organization, and any development is expected to be slow, as the GFS and SHIPS do not indicate much relaxation of this shear until around 48 hours. However, 90L may stay just west of the strongest shear, so it may be able to organize a bit. There is some also dry air that may interfere with organization.



Figure 5. Latest infrared satellite image of Invest 90L. Image credit: NOAA

Based on radar and satellite fixes, there is very little large scale rotaton with this system, and no organization to the precipitaton echoes on the Mobile radar. Any rotation would appear to be confined about 75 miles southeast of the southern tip of Louisiana. The global models suggest that 90L will move slowly west-southwest to southwest over the next couple of days before gradually turning east and northeast across the upper Florida peninsula due to a deep-layer trough forecast to amplify over the central and southern United States. This front appears strong enough to generate a little bit of cold air advection subsequent to its passage, which is a little unusual for early September. Right now the best analog I can find for 90L is Tropical Storm Henri of 2003:



Figure 6. Track of Tropical Storm Henri in 2003.

I would be surprised to see 90L become anything more than a weak to moderate tropical storm, although greater intensification is possible in the western Atlantic if the front doesn't contain too much baroclinicity.

Probability of development in 48 hours: 50%

2012 Atlantic hurricane season Hurricane Leslie Tropical Storm Michael Invest 90L

Updated: 2:21 AM GMT on September 06, 2012

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Tropical weather analysis - September 4, 2012

By: KoritheMan, 10:32 PM GMT on September 04, 2012

Leslie

Tenacious Leslie continues to hang on amidst strong vertical shear. As of the latest NHC advisory, the following was posted on the cyclone:

Wind: 65 mph, with higher gusts
Location: 25.3°N 62.6°W
Movement: N at 3 mph
Pressure: 994 mb

After looking rather sickly this morning, convection has returned to the center, which based on visible satellite images appears to be located under the convection.



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

While there is still shear over Leslie and the center could easily become exposed again, the strongest shear is located well north of the tropical storm based on analysis of water vapor imagery. Leslie is moving slow enough that it should not feel this shear. The global models still insist on a more favorable upper air pattern developing over the tropical cyclone during the next couple of days. One inhibiting factor that counteract the anticyclonic wind pattern is the continued slow motion of the cyclone; most of the global models show Leslie moving little over the next three days, which could create cold water upwelling underneath the cyclone vortex. But since the underlying ocean is fairly warm for this part of the Atlantic, I am unsure how much of a negative factor this will actually be. By the weekend, Leslie is forecast to begin moving again. With the maintenance of the very favorable upper-level pattern previously alluded to, the cyclone is forecast to strengthen, and it could do so more quickly than I am indicating. Leslie easily has a chance to become a major hurricane. This is could occur very near Bermuda, and interests there should carefully monitor Leslie's progress.

Leslie is caught within a very complex steering pattern. Water vapor and 12z upper air data over the United States show an amplifying shortwave trough over the Ohio Valley and mid-Atlantic. Downstream from the trough, there is evidence of a building low- to mid-level ridge in the wake of a trough that has bypassed Leslie's longitude. This type of pattern usually means slow movement, and that is what will be forecast.

Recent satellite fixes suggest that Leslie might have become quasi-stationary again, with perhaps a very slow northward drift. This general motion is forecast to continue over the next few days, with a gradual turn to the north-northwest later in the period. Beyond Sunday, the system is forecast to come under the influence of a deep-layer trough forecast to move from the mid-Atlantic states into the waters of the western Atlantic. This trough should be sufficient to recurve Leslie. However, depending on the timing and amplitude of the trough, as well as Leslie's position in relation to it, the storm could pose a threat to New England or Atlantic Canada. None of the models are indicating the former, but they do suggest the latter. Interests in both these areas should follow Leslie's progress carefully through the early part of next week.

5-day intensity forecast

INITIAL 09/04 2100Z 55 KT 65 MPH
12 hour 09/05 0600Z 55 KT 65 MPH
24 hour 09/05 1800Z 60 KT 70 MPH
36 hour 09/06 0600Z 65 KT 75 MPH
48 hour 09/07 1800Z 70 KT 80 MPH
72 hour 09/08 1800Z 75 KT 85 MPH
96 hour 09/09 1800Z 80 KT 90 MPH
120 hour 09/10 1800Z 90 KT 105 MPH

5-day track forecast



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



Michael

Tropical Storm Michael formed from what was Tropical Depression Thirteen yesterday. As of the latest NHC advisory, the following was posted on the storm:

Wind: 50 mph, with higher gusts
Location: 27.5°N 43.7°W
Movement: NNW at 7 mph
Pressure: 1006 mb

Satellite images reveal that Michael is a rather small tropical storm, and would likely have been missed prior to the advent of the satellite era. It still doesn't beat horizontally-challenged Marco from 2008 though, which was probably no more than 75 miles across. Although cloud tops have warmed, an SSMI overpass from 1830z suggested a well-defined curved banding pattern, as well as the beginnings of a warm spot, possibly a mid-level eye. However, this feature has not apparent on conventional satellite images, and I am uncomfortable lableing this an eye for now. In any event, Michael is clearly strengthening.



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

Upper-level outflow is confined to the eastern portion of the circulation due to continued northwesterly shear. The global models show an increase in this shear over the next 24-48 hours as an upper-level trough to the north of Michael moves eastward. Since Michael is a small storm, it will be highly susceptible to any increase in shear. In fact, I would not be surprised to see it weaken a little, but a more conservative wager would be to indicate little change. Beyond 48 hours, the upper-level winds are forecast to relax, and a small anticyclone is forecast to move in tandem with the tropical cyclone. This should promote some strengthening later in the forecast period. In the mean time, a little additional intensification is forecast before the upper flow becomes prohibitive.

Water vapor and UW-CIMSS steering analysis suggests that Michael is wedged between an approaching trough to the west and a mid-level ridge to the east. This pattern should result in a north-northwestward motion in the short-term, followed by a gradual poleward bend as the trough becomes the dominant steering influence. In about three days, the trough is forecast to bypass Michael, leaving the cyclone in a region of weak steering. At that point, the primary steering mechanism will be a narrow mid-level ridge, but moreso the circulation of Leslie, which is forecast to be a large hurricane at that time. Michael should respond by turning slowly west-northwestward around the eastern side of the cyclone vortex. In general the models agree on this, but Michael has to stairstep to get there. This kind of squashed spider consensus usually indicates slow movement, as well as a big forecasting headache. My forecast track is pretty similar to that of the National Hurricane Center.

Large swells will continue affecting much of the US east coast, Bermuda, and Atlantic Canada throughout much of this week.

5-day intensity forecast

INITIAL 09/04 2100Z 45 KT 50 MPH
12 hour 09/05 0600Z 50 KT 60 MPH
24 hour 09/05 1800Z 50 KT 60 MPH
36 hour 09/06 0600Z 50 KT 60 MPH
48 hour 09/07 1800Z 50 KT 60 MPH
72 hour 09/08 1800Z 55 KT 65 MPH
96 hour 09/09 1800Z 60 KT 70 MPH
120 hour 09/10 1800Z 65 KT 75 MPH

5-day track forecast



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



Remnants of Isaac

Isaac's hatred for the northern Gulf Coast continues. Even in death, he refuses to quit. A satellite archive from the last few days shows that a piece of the mid-level energy of former Hurricane Isaac detached from the main frontal zone, moved eastward across Arkansas into Tennessee, where it gradually turned southward. Now, it is located over southeastern Alabama, where it is bringing rain and severe thunderstorms. Surface observations do not indicate a well-defined circulation, and if it is going to regenerate, it will probably take a little time to do so. Isaac should be over the Gulf of Mexico in about 6-12 hours.

Upper air data, water vapor images, and real time steering data from UW-CIMSS shows that a mid-level ridge is building over the central United States as a weak shortwave moves by to the north. A break in the ridge is noted over the eastern United States from about 83W eastward. However, the flow over the central and eastern US is highly zonal, indicating a weak trough. We saw this with the same trough that brought Isaac to Louisiana. The global models have successfully identified this trough as a weak one, and it is forecast to bypass Isaac over the next day or so, and Isaac will have to wait for another trough developing off the Pacific northwest, forecast to arrive by the weekend. The result will be weak steering currents. This portion of the forecast is critical in determining which section of the Gulf Coast feels Isaac's second go round. If the storm moves significantly south and west, it could make landfall along the northern Gulf Coast as the trough picks it up. Alternatively, if the system doesn't gain any significant westward longitude before the arrival of the trough, it will move across the Florida peninsula and into the western Atlantic. This seems to be the most likely scenario for now based on the consensus of the global models and Isaac's close proximity to the east coast trough. However, they tend to perform poorly with small entities, and this is Isaac, so you never know. The synoptic setup here is similar to what we saw with Debby back in June, and it's essentially a battle between the ridge over the central US and the trough over the east.

Based on the depth of the upper low over the Bahamas, should Isaac become relatively deep over the next day or two, it could move southwestward around the western periphery of said low, which has a strong signature at 500 mb and above. This would place the northern Gulf Coast at greater risk.

Strong northerly shear is forecast to affect the northeastern Gulf waters for the next few days, in association with the upper low and the central US ridge. However, if Isaac moves southwest, it will be moving in the same direction as the shear vector, which would likely dampen the blow. Vertical shear will be less the farther west Isaac goes.

Probability of development in 48 hours: 10%

2012 Atlantic hurricane season Tropical Storm Leslie Tropical Storm Michael Remnants of Isaac

Updated: 10:49 PM GMT on September 04, 2012

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Tropical weather analysis - September 3, 2012

By: KoritheMan, 3:18 AM GMT on September 04, 2012

Leslie

Leslie continues to move across the western Atlantic as a tropical storm. As of the latest NHC advisory, the following was posted on the cyclone:

Wind: 65 mph, with higher gusts
Location: 24.0°N 63.1°W
Movement: Stationary
Pressure: 998 mb

Leslie remains sprawling and disorganized, with little evidence of banding within the large convective cloud shield that accompanies the tropical cyclone. Earlier microwave data suggested that the low-level center was exposed to the northwest of the deep convection due to continued northwesterly shear. This assumption is confirmed by conventional satellite images. Interestingly, the earlier referenced microwave pass showed what looked like an eye feature in the 37 GhZ channel. However, this central feature was much farther into the deep convection, and it would be unusual to have a low-level eye at the bottom half of the troposphere, while having no such signature or hint at the upper half of the troposphere. I have never seen an eye evident at the 37 GhZ channel in those images before the 85 GhZ one. The former is sensitive to the lower troposphere, while the latter is more sensitive to the middle and upper troposphere. Still rather interesting to note, though.



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

The SHIPS and the global models slowly relax the shear over the next few days, especially beyond 72 hours, when a large anticyclone is forecast to coexist with Leslie. This synoptic evolution favors intensification, and despite its struggles, Leslie is still expected to become a hurricane. There is a chance that it could become a major hurricane at longer ranges, but this is not being explicitly indicated at this time.

Water vapor images show that the initial trough has bypassed Leslie, leaving the tropical storm in an area of weak steering between two ridges -- one over the central Atlantic rebuilding in the wake of the trough, and another off the east coast of the United States. There is a small upper low centered across the northwestern Bahamas. For now, this feature is not expected to have much influence on steering. In about three days, however, the models show it moving much closer to Leslie's vortex. At that point the storm is forecast to be deep enough and in close enough proximity to the low to begin moving again.

It should be noted that the UKMET and ECMWF models show a more prolonged north-northwest to northwestward motion through the weekend. This is in contrast to the GFS, which barely shows the cyclone moving near Bermuda before being picked up by a trough. There is some hinting from the CMC and NOGAPS that Leslie could pose a long-range threat to the mid-Atlantic or New England. However, none of the more reliable global models -- namely the GFS and ECMWF -- show this. Nonetheless, there is enough fragility in the pattern and disagreement amongst the models so that interests in these areas should monitor the progress of Leslie.

Regardless, the cyclone is forecast to grow considerably. The prolonged fetch of strong winds associated with the circulation of Leslie is expected to generate large swells and rip currents along much of the east coast of the United States over the next week. This wave action could eventually find its way to sections of Atlantic Canada as the cyclone begins a more definitive movement. Bermuda will also be impacted by high swells, and the cyclone could pass very close to that island over the next 5 - 7 days. Interests there should be preparing for an intensifying hurricane.

Given the split in the model guidance, the best I can do is average the solutions until Leslie responds to the upper low. This leads to a track forecast that is similar to but slower than the National Hurricane Center. There is not a lot of confidence to this forecast.

5-day intensity forecast

INITIAL 09/04 0000Z 50 KT 60 MPH
12 hour 09/05 1200Z 50 KT 60 MPH
24 hour 09/06 0000Z 55 KT 65 MPH
36 hour 09/06 1200Z 60 KT 70 MPH
48 hour 09/07 0000Z 65 KT 75 MPH
72 hour 09/08 0000Z 70 KT 80 MPH
96 hour 09/09 0000Z 80 KT 90 MPH
120 hour 09/10 0000Z 85 KT 100 MPH

5-day track forecast



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



Tropical Depression Thirteen

A small area of low pressure that originated from an upper-level trough spawned a small tropical depression over the central Atlantic today. As of the latest NHC advisory, the following was availble:

Wind: 35 mph, with higher gusts
Location: 25.9°N 42.8°W
Movement: NW at 5 mph
Pressure: 1012 mb

Convection has developed closer to the low-level center over the past couple of hours, but it remains fairly shallow.



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

The large scale environment does not appear especially conducive for intensification. Northwesterly shear is forecast to persist, and there is some dry air in the cyclone's surroundings. Also, there is little evidence of anticyclonic outflow at this time. However, only a small increase in organization could result in the system becoming a tropical storm. This system is forecast to dissipate in a few days. Small systems like this one are highly susceptible to shear, so even a subtle increse in the vertical shear could result in the system being incapacitated quicker than I am indicating.

Given the short history of the tropical cyclone, initial movement is difficult to determine. Based on shortwave infrared fixes, it looks to me as if the system is beginning to turn to the west-northwest. This is consistent with the overall pattern, with the cyclone located between an upper trough to the northwest and a mid-level ridge to the east. As the trough amplifies, the cyclone is expected to turn north, then northeast with a slight increase in forward speed. My track is to the right of the one from the National Hurricane Center, predicated on the assumption that the depression will feel greater influence from the trough. Alternatively, the cyclone is small enough that it may not fully respond to the trough, and instead simply meander.

5-day intensity forecast

INITIAL 09/04 0000Z 30 KT 35 MPH
12 hour 09/04 1200Z 35 KT 40 MPH
24 hour 09/05 0000Z 35 KT 40 MPH
36 hour 09/05 1200Z 30 KT 35 MPH
48 hour 09/06 0000Z 30 KT 35 MPH
72 hour 09/07 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/08 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
120 hour 09/09 0000Z...DISSIPATED

5-day track forecast



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



John

John continues to move across the open Pacific. As of the latest NHC advisory, the following was posted on the storm:

Wind: 35 mph, with higher gusts
Location: 21.7°N 114.6°W
Movement: NW at 13 mph
Pressure: 1003 mb

John is experiencing northeasterly shear. Satellite images show the low-level center to be exposed well to the northeast of the shapeless and pulsating convection.



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

John does not have long to live now that the low-level center is racing northwestward away from the convection. While the shear is forecast to decrease as John moves away from the axis of the upper-level ridge, water vapor imagery shows dry air in the immediate path of the tropical cyclone. Naturally, cool sea surface temperatures will coexist with the subsident airmass. The combination of these features should quickly obliterate John, and the cyclone is forecast to degenerate into a remnant low later on Tuesday.

John remains south of a weakness in the ridge over the southwestern United States as an upper trough amplifies off the California coast. This evolution should cause a general northwest motion until the system becomes a remnant low. Thereafter, the cyclone is forecast to turn west-northwest as it becomes a shallower vortex.

5-day intensity forecast

INITIAL 09/04 0000Z 30 KT 35 MPH
12 hour 09/04 1200Z 30 KT 35 MPH
24 hour 09/05 0000Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
36 hour 09/05 1200Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
48 hour 09/06 0000Z 20 KT 35 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/07 0000Z 20 KT 25 MPH...POST-TROPICAL/REMNANT LOW
96 hour 09/08 0000Z...DISSIPATED

5-day track forecast



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

2012 Atlantic hurricane season 2012 East Pacific hurricane season Tropical Storm Leslie Tropical Depression Thirteen Tropical Storm John

Updated: 3:28 AM GMT on September 04, 2012

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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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