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Good evening everyone!
Tropical Disturbance INVEST 96L was upgraded to Tropical Depression FOUR a little earlier this evening, and advisories have been initiated by the NHC in Miami. The following information was available as of the 5:00 p.m. EDT advisory:
5:00 PM AST Sat Aug 23
Location: 21.8°N 72.3°W
Moving: NW at 12 mph
Min pressure: 1005 mb / 29.68 in
Max sustained: 35 mph
HURREVAC TRACKING MAP
Current satellite loop imagery of various channels indicate the depression is slowly becoming better organized this evening. If you overlay the Lat and Long. Especially in Shortwave IR, you can pretty much see and follow the center position where the NHC has it mentioned. However, based on some careful, closeup analysis, there appears to still be some competition of another vortex to the SE (at least of the time of analysis). As stated in the NHC Forecast Discussion, given the center has just become better defined, it is possible we have a broader center than what may appear. However, at the moment, 850 mb vorticity does pretty well match up with the NHC position.
The depression is under the influence of an upper level anticyclone just to the SW of the center of circulation. While this is not optimal, it is positioned in such a way as to allow for divergence aloft, thereby ventilating the depression and establishing outflow channels pretty much in all quads of the system as shown by the arrows drawn around the upper level wind map.
The current wind shear forecast indicates upper level winds to remain conducive for further develop, however this should be somewhat slow for the next 12-18 hours. Upper level winds become a little more favorable as this reaches the central Bahamas. If you note in the Zonal Shear Map, you’ll notice a purple line running over the northern Bahamas…this shows ZERO zonal shear. You’ll also note the clockwise flow…this is the upper level anticyclone that will continue to ventilate the depression. Based on these conditions, and higher ocean heat content, I agree with the NHC intensity forecast at the moment. However, should this become better organized sooner, I would not rule out a hurricane closer to the 90 MPH range near the end of the 5 day forecast period.
Tropical Storm Warnings have been issued, and residents of the Bahamas should follow action as directed by their local NWS office statements. Until we have better confidence in the track, residents along the Florida east coast should monitor the progress of this system.
The depression is moving to the NW, according to the advisory. Again, this is not totally certain as ascertained by the NHC. Satellite loop imagery close ups indicate the system may have a slightly more westward component to it at the moment, and may still be moving closer to ta 285 degree motion.
The depression is however moving in the direction toward a current weakness in the ridge, which has strengthened a little more during the day. A somewhat of a westerly flow is still noted just west of the Bahamas, but may be weakening. Based on my analysis, I believe the depression MAY move a little more to the left, before a solid NW motion ensues. Based on the steering layers forecast maps valid for 00Z tonight, I have to concur with the NHC track at the moment, however I do believe the Dynamic Model guidance may shift left somewhat. Given the uncertainty mentioned by the NHC as far as track, and one steering layer forecast maps indicates the system could just have the western fringes affect the FL east coast…with all things equal, given this uncertainty, I am going left of the NHC forecast track, and just to the left edge of the TVCA/TVCN consensus models at the moment. Again, these may change due to the uncertainty of the LLC initialization. One steering forecast solution builds the ridge briefly north of the system, which would allow for a continued westward component…however both forecasts indicate the weakness to really open up after the 5 -6 day period, and the system should begin to recurve.
I will be off tomorrow, but will try to have an update in the evening.
Have a blessed evening!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST / SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS
OFFICIAL SKYWARN SPOTTER (ADVANCED)