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Tropical Storm Florence Forecast Discussion Number 37
2018-09-08 16:53:38| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Sat Sep 08 2018 863 WTNT41 KNHC 081453 TCDAT1 Tropical Storm Florence Discussion Number 37 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Sat Sep 08 2018 Florence has undergone a dramatic structural evolution just within the past 6 hours or so. Vertical shear has decreased just enough for the storm to take on a more symmetric shape, with convection developing in a ring around the low-level center, and an elongated band wrapping around to the northeastern part of the circulation. The convection has waned a little bit in intensity, however, due to the presence of dry air, and Dvorak estimates support maintaining an initial intensity of 55 kt for now. A NOAA P-3 aircraft is approaching Florence now on a research mission and should provide some useful data to better assess the storm's intensity. Recent WindSat microwave data revealed that Florence has a well-defined low-level ring in the 37-GHz channel, which tends to be a harbinger of strengthening when environmental conditions are favorable. Since vertical shear is decreasing and should be 10 kt or less by later today, and Florence is heading toward a deeper pool of warm water over the southwestern Atlantic, a significant phase of intensification is likely to begin by tonight, continuing through Tuesday or Wednesday. In fact, the official intensity forecast explicitly shows rapid intensification occurring between 24 and 48 hours from now, and Florence is expected to become a major hurricane by Monday. The HFIP Corrected Consensus (HCCA) and the Florida State Superensemble (FSSE), both of which tend to do well in these scenarios, are both near the upper end of the guidance suite, especially through day 3. Even by days 4 and 5, the HWRF, HMON, and ICON intensity consensus are near the top end of the guidance, close to HCCA and FSSE. Given the signals in the environment, and the solutions provided by these models, the NHC intensity forecast shows Florence reaching category 4 intensity by day 3 and maintains that through the end of the forecast period. Florence's longer-term motion is 265/6 kt. The cyclone appears to be slowing down as was expected, and this type of motion is likely to continue for the next 24-36 hours. In fact, the track guidance has slowed down during this period, and the updated NHC track forecast is a little slower than the previous one. After 36 hours, the most notable change in the models was a northeastward shift in the 06Z GFS. However, that run appears to be an outlier from the rest of the dynamical models, and its trend opposes the slight westward shift noted in the HCCA and FSSE aids. The updated NHC track forecast is therefore very close to or slightly west of the previous forecast on days 4 and 5. The exact path of Florence as it approaches the southeastern U.S. coastline will depend heavily on the position and strength of the blocking high pressure that is expected to develop north of Bermuda and extend westward over the eastern U.S., and so far there has not been much more clarity on those important details. Key Messages: 1. Florence is forecast to be a dangerous major hurricane near the southeast U.S. coast by late next week, and the risk of direct impacts continues to increase. However, given the uncertainty in track and intensity forecasts at those time ranges, it's too soon to determine the exact timing, location, and magnitude of those impacts. 2. Interests along the U.S. East Coast, particularly from north Florida through North Carolina, should closely monitor the progress of Florence, ensure they have their hurricane plan in place, and follow any advice given by local officials. 3. Large swells are affecting Bermuda and will begin to affect portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 08/1500Z 24.5N 54.3W 55 KT 65 MPH 12H 09/0000Z 24.6N 55.0W 60 KT 70 MPH 24H 09/1200Z 24.6N 56.1W 70 KT 80 MPH 36H 10/0000Z 24.8N 57.5W 85 KT 100 MPH 48H 10/1200Z 25.1N 59.3W 100 KT 115 MPH 72H 11/1200Z 26.3N 64.9W 115 KT 130 MPH 96H 12/1200Z 28.5N 71.5W 125 KT 145 MPH 120H 13/1200Z 31.5N 77.0W 120 KT 140 MPH $$ Forecaster Berg
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