Tropical weather analysis - August 3, 2012
Ernesto
Tropical Storm Ernesto is nearing the Windward Islands, and is located very near Barbados. As of the latest NHC advisory, the following information was posted on the cyclone:
Wind: 50 mph, with higher gusts
Location: 13.4°N 58.3°W
Movement: W at 22 mph
Pressure: 1005 mb
Category: Tropical storm
While by no means fabulous, the satellite presentation is slowly improving, with new convective cells developing closer to the low-level center.
Figure 1. Latest infrared satellite image of Tropical Storm Ernesto. Image credit: NOAA
Westerly to southwesterly wind shear associated with an anticyclone over Venezuela is still affecting Ernesto. The SHIPS models shows this shear decreasing over the next 12-24 hours, which if it verifies, should allow Ernesto to strengthen. Current large-scale trends appear to support this. Additionally, water vapor and CIMSS Total Precipitable Water (TPW) imagery shows little evidence of any nearby dry air. Taking this into consideration, there seems to be little reason to assume the cyclone will not continue intensifying. Ernesto has escaped the influence of the upper low near 27.5N 45.0W that had been slinging westerly shear, and from this point onward the upper flow looks relatively favorable up to 75W. I note however, that there appears to be a bit of southeasterly shear over the waters of the eastern Caribbean. These are not trade winds, but are likely associated with the South American anticyclone. Should this feature fail to weaken or collocate with Ernesto, there could be a short-term struggle for intensification. However, since this could just as easily be part of the outflow related to Ernesto, I am not going to count this as a deterrent to strengthening.
The upper-level tropospheric wind pattern over the Caribbean appears conducive for at least steady strengthening over the next couple of days. When Ernesto approaches the longitude of Jamaica in about three days, a very favorable upper-level environment awaits the tropical cyclone. Thus, at longer ranges, there is definitely the possibility of rapid intensification. But until I can get a little more confidence in how the shear pattern is going to evolve, I will remain conservative. Ernesto has the potential to be a major hurricane in the Gulf of Mexico beyond day five.
5-day intensity forecast
INITIAL 08/03 0600Z 45 KT 50 MPH
12 hour 08/03 1800Z 50 KT 60 MPH
24 hour 08/04 0600Z 50 KT 60 MPH
36 hour 08/04 1800Z 55 KT 65 MPH
48 hour 08/05 0600Z 60 KT 70 MPH
72 hour 08/06 0600Z 70 KT 80 MPH
96 hour 08/07 0600Z 75 KT 85 MPH
120 hour 08/08 0600Z 80 KT 90 MPH
The track forecast is still a bit of a challenge, primarily due to how much strength Ernesto will gain while moving across the Caribbean. The global models suggest that the strong subtropical ridge to the north of the system will remain intact throughout the next five days, steering the system generally toward the west-northwest. By Tuesday, an upstream trough is forecast by all of the models to begin developing over the southern plains. That favors a slowing of the forward speed, which is supported by the models. Ordinarily this would be the harbinger of recurvature, but the GFS and ECMWF still insist on taking the system westward toward Mexico and/or the western Gulf at that point. However, examination of the lower- to middle-tropospheric fields within those models shows that the depicted ridge in much weaker at 500 mb than it is at 850 mb. Hence, these more westward forecasts are likely predicated on a weaker system moving in tandem with the lower-tropospheric flow. Since Ernesto is expected to be a fairly strong system, I consider this scenario to be unlikely (but still possible if Ernesto struggles).
As a course of least regret, I will not deviate too far east of the model consensus -- yet. However, I anticipate that they will come more east over the next couple of days. A significant slowing of the forward speed is possible by day five and beyond as the large-scale steering weakens with the approach of the aforementioned trough. Given the inherent uncertainties ever present in intensity forecasts, which the track is contingent upon, confidence in the forecast track beyond day five is not particularly high.
Interests in the Windward Islands and western Caribbean should closely monitor the progress of Ernesto. The Barbados radar already shows rainbands beginning to affect portions of the Lesser Antilles within the tropical storm warning area. Preparations to protect life and property should be completed quickly before conditions begin to deteriorate.
5-day track forecast
Figure 2. My 5-day forecast track for Ernesto.
Watches and warnings
THE GOVERNMENT OF TRINIDAD AND TOBAGO HAS ISSUED A TROPICAL STORM
WATCH FOR GRENADA AND ITS DEPENDENCIES.
SUMMARY OF WATCHES AND WARNINGS IN EFFECT...
A TROPICAL STORM WARNING IS IN EFFECT FOR...
* BARBADOS...ST. VINCENT AND THE GRENADINES...AND DOMINICA
* ST. LUCIA
* MARTINIQUE AND GUADELOUPE
A TROPICAL STORM WATCH IS IN EFFECT FOR...
* GRENADA AND ITS DEPENDENCIES
A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN
THE NEXT 12 TO 24 HOURS.
A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...IN THIS CASE WITHIN THE NEXT
12 TO 24 HOURS.
INTERESTS ELSEWHERE IN THE LESSER ANTILLES SHOULD MONITOR THE
PROGRESS OF THIS SYSTEM.
FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.
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