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Tropical Storm Florence Forecast Discussion Number 35
2018-09-08 04:52:03| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Fri Sep 07 2018 331 WTNT41 KNHC 080251 TCDAT1 Tropical Storm Florence Discussion Number 35 NWS National Hurricane Center Miami FL AL062018 1100 PM AST Fri Sep 07 2018 Florence remains a sheared tropical cyclone. Satellite images indicate that the low-level center is partially exposed on the southwesterly edge of a large convective mass, with the overall circulation somewhat elongated from southwest to northeast. A blend of the latest Dvorak wind speed estimates from TAFB/SAB and the CIMSS SATCON gives a value of 50 kt for this advisory. While the winds at 200 mb are already from an easterly direction near the center of Florence, there is significant shear from northwesterly winds from 300-500 mb, undercutting the outflow layer. This shear is forecast to relax by the global models over the next 36 hours as an anticyclone builds to the north of the storm, which should promote some strengthening by Sunday. After 48 hours, the deep-layer flow becomes easterly near the cyclone, with very little shear while the system is over very warm waters. This pattern favors significant intensification, and most of the guidance brings Florence back to a category 4 hurricane in 4 or 5 days. The intensity forecast is very similar to the last one, and is raised slightly at days 3 and 4 to come into better agreement with the guidance. It is interesting to note that even with a lower initial intensity, the guidance is higher than the last cycle, which speaks to the strength of the signal for intensification in the long range. The initial motion estimate is 265 degrees at 6 kt. Florence is expected to continue moving slowly westward for the next 48 hours under the influence of a weak mid-level ridge over the western Atlantic. By days 4 and 5, an exceptionally strong blocking ridge is forecast to develop between Bermuda and the Northeast U.S. and build westward, keeping Florence on a west-northwestward trajectory with a notable increase in forward speed by the end of the forecast period. It feels like a broken record to mention that the overall guidance envelope keeps shifting southwestward, and the official forecast is moved in that direction. Unfortunately with such a large well-defined steering current from the ridge becoming likely, the extended-range risk to the United States keeps rising, which is confirmed by the majority of the latest ensemble guidance. Key Messages: 1. Regardless of Florence's eventual track, 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. 2. The risk of other direct impacts associated with Florence along the U.S. East Coast next week has increased. However, there is still very large uncertainty in model forecasts of Florence's track beyond day 5, making it too soon to determine the exact location, magnitude, and timing of these impacts. Interests near and along the U.S. East Coast should monitor the progress of Florence through the weekend and ensure they have their hurricane plans in place. FORECAST POSITIONS AND MAX WINDS INIT 08/0300Z 24.8N 53.2W 50 KT 60 MPH 12H 08/1200Z 24.7N 54.1W 50 KT 60 MPH 24H 09/0000Z 24.7N 55.2W 55 KT 65 MPH 36H 09/1200Z 24.7N 56.3W 60 KT 70 MPH 48H 10/0000Z 25.0N 57.8W 70 KT 80 MPH 72H 11/0000Z 25.8N 62.3W 95 KT 110 MPH 96H 12/0000Z 27.5N 68.5W 115 KT 130 MPH 120H 13/0000Z 30.5N 74.5W 115 KT 130 MPH $$ Forecaster Blake
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