Tropical weather analysis - August 21, 2012
Tropical Storm Isaac formed today from former Invest 94L. As of the most recent NHC advisory, the following was posted on the tropical storm:
Wind: 40 mph, with higher gusts
Location: 15.6°N 55.6°W
Movement: W at 18 mph
Pressure: 1006 mb
The center remains a little exposed along the northern edge of the convection due to moderate northeasterly shear, but Isaac appears to be maintaining itself for now.
Figure 1. Latest infrared satellite image of Tropical Storm Isaac. Image credit: NOAA
The SHIPS shows the shear decreasing almost immediately, but water vapor imagery shows a well-defined upper tropospheric cold low over the central Atlantic dropping southward toward the tropical cyclone. The GFS keeps this small low moving rather swiftly toward the south or southwest. Given the relatively zonal flow over the north Atlantic at this time, this seems reasonable. This may act to keep some northerly to northwesterly shear over Isaac for the next day or so. This should prevent Isaac from rapidly intensifying before reaching the Lesser Antilles. More positively, Isaac appears to have mixed out most of the dry air. In about 24 hours, the upper-level environment is forecast to become more favorable -- anticyclonic actually -- and that should allow for some more robust strengthening. Later in the period, possible interaction with the Greater Antilles dictates the intensity forecast remain conservative. However, if Isaac avoids the Greater Antilles, either from south or north, it could easily become a major hurricane. Given the forecast track, I currently see little to prevent Isaac from being a hurricane prior to interaction with Hispaniola. Interests across the eastern Caribbean should carefully monitor the progress of Isaac over the next few days. Residents living in the Lesser Antilles should anticipate the onset of tropical storm conditions Wednesday evening. Interests in Puerto Rico and Hispaniola should also anticipate tropical storm conditions. They should also be preparing for the possibility of hurricane force winds, especially in Hispaniola.
Synoptic data indicates that Isaac is south of a well-established mid-level ridge, which is the steering the system on a generally westward path. This has been the general pattern over this portion of the basin this year. 0z upper air data and water vapor imagery indicates that a trough is undergoing significant amplification over the eastern seaboard and southeastern United States, while a ridge builds over the central plains. However, the data indicates that this trough is moving rather slowly, with a weakness in the ridge not evident until roughly 75W. The models disagree on the depth and amplitude of this trough, as well as how amplified the flow over the central United States becomes over the next few days. Obviously, a more amplified regime would tend to keep the trough in place, while a more progressive flow would tend to prevent the trough from sticking around in any one place for too long. I tend to side with the GFS, which lifts the trough out by Saturday. Just when and where Isaac makes the turn will be critical in determining where US landfall will occur. Looking at the global model fields, a weaker storm will tend to move more westward, spending a longer duration over the Caribbean, while a stronger storm will tend to move poleward, more attuned to the weakness in the western Atlantic ridge. An AMSU microwave overpass taken just after 0z hinted that the cyclone was farther north, closer to 16N. However, this seems to contrast with conventional satellite trends. Probably the best thing for now is to blend those fixes.
While the 0z dynamical guidance has made a significant northward shift, the guidance has shifted in either direction for the last several days. It is unwise to shift one's forecast track in any given direction based on a couple of model shifts. By far the most interesting aspect to note for today is the significant westward shift shown by the ECMWF at 12z. The reason for this drastic shift is not quite clear, but looking at the sea level pressure fields on that model, it appears to forecast a much stronger western Atlantic ridge than the GFS, and a generally more progressive upper air pattern. It also significantly weakens Isaac after it enters the Caribbean Sea. Offshore buoy data over the central and eastern Caribbean Sea shows that the low-level trade winds are still rather strong, perhaps unclimatlogically so for this time of year. This is to be expected during El Nino years, where the Bermuda-Azores ridge tends to be stronger. This would also make sense when one considers the depth of the trough currently over the western Atlantic. It is this sort of synoptic regime that kept Ernesto weak, and that literally obliterated Helene. Perhaps this is why the Euro keeps Isaac weak. For now, I am considering this model to be an outlier, although if it continues with the westward trend, I will need to shift westward as well.
Interests along the southeastern United States coastline should review their hurricane preparedness plans, as Isaac has the potential to be a significant hurricane for that area if interaction with the Greater Antilles is limited.
5-day intensity forecast
INITIAL 08/22 0300Z 35 KT 40 MPH
12 hour 08/22 1200Z 35 KT 40 MPH
24 hour 08/23 0000Z 40 KT 45 MPH
36 hour 08/24 1200Z 45 KT 50 MPH
48 hour 08/25 0000Z 60 KT 70 MPH
72 hour 08/26 0000Z 75 KT 85 MPH...APPROACHING THE COAST OF HAITI
96 hour 08/27 0000Z 65 KT 75 MPH...INLAND OVER EASTERN CUBA
120 hour 08/28 0000Z 65 KT 75 MPH...OVER WATER
5-day track forecast
Figure 2. My 5-day forecast track for Isaac.
Watches and warnings
A TROPICAL STORM WARNING IS IN EFFECT FOR...
* GUADELOUPE AND THE SURROUNDING ISLANDS...AND ST. MARTIN
* ST. KITTS...NEVIS...ANTIGUA...BARBUDA...MONTSERRAT...A ND ANGUILLA
* SABA...ST. EUSTATIUS...AND ST. MAARTEN
* BRITISH VIRGIN ISLANDS
* PUERTO RICO...VIEQUES...CULEBRA...AND THE U.S. VIRGIN ISLANDS
A HURRICANE WATCH IS IN EFFECT FOR...
* PUERTO RICO...VIEQUES...CULEBRA...AND THE U.S. VIRGIN ISLANDS
A HURRICANE WATCH MEANS THAT HURRICANE CONDITIONS ARE POSSIBLE
WITHIN THE WATCH AREA. A WATCH IS TYPICALLY ISSUED 48 HOURS
BEFORE THE ANTICIPATED FIRST OCCURRENCE OF TROPICAL-STORM-FORCE
WINDS...CONDITIONS THAT MAKE OUTSIDE PREPARATIONS DIFFICULT OR
A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.
INTERESTS IN HISPANIOLA AND CUBA SHOULD MONITOR THE PROGRESS OF
FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED
STATES...INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE
MONITOR PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE
FORECAST OFFICE. FOR STORM INFORMATION SPECIFIC TO YOUR AREA OUTSIDE
THE UNITED STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR NATIONAL
A weak area of low pressure in the western Gulf of Mexico has not become any better organized. Satellite images suggest that the system has moved inland, and development is not anticipated.
Probability of development in 48 hours: Near 0%
A tropical wave over the eastern Atlantic ("96L") centered about 600 miles west-southwest of the Cape Verde Islands the potential to become a tropical depression over the next couple of days. Satellite images suggest that the cloud pattern is gradually becoming better organized, although there are no banding features yet.
Figure 3. Latest infrared satellite image of Invest 96L. Image credit: NOAA
I don't really want to speculate too much on 96L right now since there are still a lot of variables with a system so far out at sea. It could become a future player in the steering of Tropical Storm Isaac, but it is far too early to put on any sort of weight on this for now.
Looking at water vapor imagery and satellite, the easterly shear that was hammering the system yesterday does appear to be decreasing, as convection has managed to develop closer to the center relative to yesterday. However, the low-level center still remains on the eastern edge of the convection, which is reminiscent of some residual easterly shear. An anticyclone is forecast to build atop the system over the next several days, and conditions remain favorable for tropical cyclone formation in this area.
Large scale steering analyses suggests that a weakness is present to the north of Invest 96L, probably induced by the upper low currently shearing Isaac. There may be a more west-northwest motion over the next day or so as the low moves off to the southwest. Thereafter, the low is forecast to weaken and fill, and 96L should respond by stabilizing in a more westward trajectory.
Probability of development in 48 hours: 70%