TROPICAL CYCLONE FORMATION: T.S. FIONA / INVEST 99L (60%)/ EATL DIST (70%)
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(T. F. “Storm” Walsh III)
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CURRENT 2016 ATLANTIC TROPICAL CYCLONE TOTALS:
TOTAL STORMS: 6
INTENSE HURRICANES: 0
StormW’s Seasonal Hurricane Forecast:
TOTAL STORMS: 13-15
INTENSE HURRICANES: 3-4
Good evening everyone!
This evenings forecast will focus on Tropical Disturbance INVEST 99L, as Tropical Storm Fiona continues to move to the NW, and this track is forecast to continue during the next 4-5 days, along with her forecast to weaken to a remnant low. One thing about Fiona however is, the remnant may not re-curve right away, as in 4 days, steering currents become weak, and forecast steering shows the remnant may begin to head back to the west for a brief period. The EATL Disturbance has just exited the African coast, and forecast steering indicates this should perform almost an immediate re-curve.
Satellite loop images indicate INVEST 99L is not very well organized at the moment. Earlier this afternoon, 99L appeared to have had convection coalesce symmetrically. However, since then, the reported center is located to the north of the heavy convective area. Based on the 18Z ATCF BTK product, the following was available on INVEST 99L:
2:00 PM AST Sat Aug 20
Location: 12.0°N 38.0°W
Moving: W at 17 mph
Min pressure: 1009 mb / 29.80 in
Max sustained: 30 mph
Upon analysis of water vapor loop imagery, and MIMIC TPW animation, 99L appears to have taken in some drier air. However, you’ll note in the MIMIC TPW loop, it appears that 99L “”ate” the dry air. The drier air appears north of the TPW associated with 99L as a blue blip…a few frames later, you can see 99L eat the blip.
Analysis of the 850 mb vorticity product does agree with the reported center location. Based on satellite loop imagery, lower convergence, and upper divergence products, It would not surprise me to see a possible center reformation try to occur under the heavy convection.
99L has been under some slight shear during the past few hours, as the orientation of the upper level anticyclone had been such that the upper level flow has not been optimal. The recent shear product shows the center of the upper level anticyclone has inched closer to the reported center, while the heavy convection is under 20 knots of easterly shear. This may continue for the next 24 hours, possibly 36. However, 42 hours into the forecast period from 12Z, information from the NOAA RUC HFIP site shows both the GFS and ECMWF in agreement of wind shear falling to 10 knots and below. Analysis of the GFS wind shear forecast map indicates the upper level anticyclone may be close to, or centered over the disturbance by that time. Given this premise, and seeing how 99L has a lot of high TPW content, I believe INVEST 99L should start to become better organized, and begin slowly develop. Another factor in this slow process is, this system has a very large circulation, so it is going to take some time for it to consolidate. IF the center does in fact reform under the convection, development could begin to occur sooner. If not, then I feel the current intensity guidance may be a bit quick in bring this to T.S. status. I would mention however, the 3 models most widely used in intensity guidance (SHIPS, DSHP, and LGEM) have been consistent in bring this system to CAT 2 Hurricane status in 5 days. This may be overdone, as 99L is still lacking a closed LLC. This means the models do not have a closed, warm core center to initialize. Based on this, dynamic model track guidance and intensity guidance should be considered PRELIMINARY, and considered only an approximation. However, should 99L mix out the drier air, the GFS wind shear forecast not only calls for the upper level anticyclone to develop over the system, the upper feature becomes pretty textbook, and moves in tandem with 99L. Base on this premise, a hurricane within 5-7 days cannot be ruled out at the moment.
INVEST 99L continues to move toward the west, and I expect this motion to continue during the next 48-60 hours, before beginning to move more toward the WNW. This motion will be in response to a break in the subtropical ridge near approximately 28-30 N;62W. At the moment, based on analysis of the forecast steering layers maps, valid for 00Z this evening, the flow on the southern periphery of the ridge may be strong enough as to not allow the break to pull 99L north. The forecast weakness in the ridge appears to be weak enough to only impart a WNW motion on 99L. This is currently supported by the GFS Ensemble members, and dynamic model guidance based on the 18Z ATCF update. The two more accurate models TVCN/TVCA continue to bring 99L into the Caribbean, and then over Hispaniola. Some of the other modeling is slightly to the right. I currently prefer the TVCA/TVCN consensus. Should this motion occur, after that, steering will be tricky…as the steering layers forecast models only go out 6 days. The possibility at the moment suggests that the FL. Peninsula may be threatened, while another probability is the U.S. east coast could be threatened. My thoughts on this are based on what the steering forecast maps indicate. It is the unknown variable of whether or not the western ridge forecast for over the Gulf coast states in 5 days extends eastward enough to help steer 99L more toward the west, or whether or not the flow around the very edge weakness allows it to move further northward, closer to the eastern Seaboard. Again, model guidance is still preliminary, however the general consensus is that a possibility for 99L to become a U.S threat exists at the moment.
EARLY MODEL CONSENSUS 18Z
This is not to alarm anyone, however it is my recommendation that residents of the FL. Peninsula, and along the eastern Seaboard monitor this system closely. Although these uncertainties, this would be an excellent time to review hurricane preparedness plans in these areas.
I intend to have another update tomorrow late afternoon or early evening.
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS