Tropical weather analysis - August 1, 2012
Tropical Depression Five
The tropical wave formerly known as Invest 99L has become a tropical depression. As of the first NHC advisory, the following information was posted on the cyclone:
Wind: 35 mph, with higher gusts
Location: 12.4°N 49.8°W
Movement: WNW at 18 mph
Pressure: 1008 mb
Category: Tropical depression
The depression is rather ragged at the moment, with the center exposed along the northern edge of the convection. Satellite images show that the tropical cyclone is being pummeled by southwesterly shear in association with a large upper low centered several hundred miles to the north. Dry air doesn't currently appear to be an issue, although there is some just to the west of the depression. I should note that although the current cloud pattern is rather ragged looking, it is not uncommon for weak tropical cyclones to look, to put it bluntly, pitiful, at this time of day.
Figure 1. Latest infrared satellite image of Tropical Depression Five. Image credit: NOAA
The intensity forecast is quite problematic. Satellite and water vapor imagery shows that the depression is not currently embedded within a particularly favorable environment, with dry air and southwesterly shear noted in front of it. High cloud motion vectors indicate that this shear is probably not sufficient to decapitate the cyclone, but the shear in combination with the surrounding dry air will likely slow intensification in the short-term. However, the shear is forecast to slowly diminish, and I see faint indications of this on water vapor imagery as well. In particular, conditions should become much more conducive after 24 hours as the ambient upper flow becomes more diffluent. However, there may still be some dry air for the system to contend with in the eastern Caribbean, so rapid intensification seems unlikely for the next few days despite the forecast for a slowly improving upper-tropospheric flow pattern. Some of the models still insist the system will not survive the Caribbean, which is possible if the depression remains disorganized or the shear does not relax as anticipated. However, assuming the current cloud pattern is just a sprawl, slow intensification seems like a more reasonable bet. The SHIPS shows relative humidity values increasing by 72 hours, which is when a more steady strengthening trend may become established. Although the 5-day intensity forecast reflected below does not show a strong hurricane, given the very favorable upper-level environment forecast by the GFS, intensities at longer-ranges may be underestimated. Tropical Depression Five certainly has the potential to become a dangerous hurricane in the western Caribbean Sea next week.
The CMC shows stronger shear, and the ECMWF fails to develop the cyclone throughout its Caribbean cruise. The presumed catalyst for the forecast vertical shear in those models is probably the mid-oceanic trough, which typically lies along and/or north of the Greater Antilles. Given the broad anticyclonic flow over the tropical wave in that region, most of the associated westerly shear is being pushed upstream. Thus, I will go with the GFS for now. However, I will remain conservative until I see how the depression fares against its environment.
5-day intensity forecast
INITIAL 08/02 0000Z 30 KT 35 MPH
12 hour 08/02 1200Z 30 KT 35 MPH
24 hour 08/03 0000Z 35 KT 40 MPH
36 hour 08/03 1200Z 40 KT 45 MPH
48 hour 08/04 0000Z 50 KT 60 MPH
72 hour 08/05 0000Z 55 KT 65 MPH
96 hour 08/06 0000Z 65 KT 75 MPH
120 hour 08/07 0000Z 70 KT 80 MPH
And now it's time to discuss the track. Synoptic data indicates that the depression is located to the south of a low- to mid-level ridge. This should continue the west-northwest motion throughout the forecast period, as the GFS and ECMWF, the ones I'm in greatest agreement with, do not really show the ridge budging all that much. On the other hand, the CMC and NOGAPS still continue to show a poleward track around the periphery of the Atlantic ridge on a trajectory that would take the storm out to sea, or at least toward the southeast United States coast. This seems quite suspect in light of current trends, and the fact that the system is not expected to rapidly intensify. Considerable uncertainty exists after day 5, and at that point it's anyone's guess as to where the cyclone might go. Model guidance suggests that a trough could move from the western United States to the central plains, causing a more northwestward motion by that time. On the other hand, the ridge depicted in these models, particularly within the GFS and ECMWF, is quite strong in the lower troposphere. Thus, if the system miraculously stays weak, a more southern path toward Texas or Mexico is possible.
5-day track forecast
Figure 2. My 5-day forecast track for Tropical Depression Five.
Interests in the southern Windward Islands should closely monitor the progress of this depression. Tropical storm force winds and heavy rainfall are expected to begin overspreading these islands on Friday. Tropical storm watches have already been posted for portions of the Lesser Antilles:
Watches and warnings
SUMMARY OF WATCHES AND WARNINGS IN EFFECT...
A TROPICAL STORM WATCH IS IN EFFECT FOR...
* BARBADOS...ST. VINCENT AND THE GRENADINES...AND DOMINICA
* ST. LUCIA
* MARTINIQUE AND GUADELOUPE
INTERESTS ELSEWHERE IN THE LESSER ANTILLES SHOULD MONITOR THE
PROGRESS OF THIS SYSTEM.
A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.
FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.