Tropical weather analysis - August 27, 2012

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

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

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

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



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

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

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

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

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

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

5-day intensity forecast

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

5-day track forecast



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

Watches and warnings

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

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

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

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

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

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

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

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



Invest 97L

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



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

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

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

Probability of development in 48 hours: 60%



Invest 97E

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



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

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

Probability of development in 48 hours: 60%

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2. caneswatch
5:42 AM GMT on August 27, 2012
You'd be loving it right here now Kori, 60-70 mph winds are starting to roll through.
Member Since: October 8, 2008 Posts: 14 Comments: 4553
1. wxchaser97
5:40 AM GMT on August 27, 2012
Now of course the center moves farther north into the convection which changes everything. Always like reading your blogs Kori even at this late hour, thanks!
Member Since: March 16, 2012 Posts: 127 Comments: 7975

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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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