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Tropical Storm Florence Forecast Discussion Number 17
2018-09-03 16:43:24| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Mon Sep 03 2018 541 WTNT41 KNHC 031443 TCDAT1 Tropical Storm Florence Discussion Number 17 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Mon Sep 03 2018 While Florence's structure improved overnight, the cloud tops have warmed and the deep convection has thinned during the past several hours. The 12Z satellite intensity estimates range from 55 to 65 kt, but given recent trends the initial intensity is set at the low end of that range at 55 kt, although this is quite uncertain given the recent fluctuations in the cloud pattern. UW-CIMSS satellite diagnostics indicate that around 20 kt of southwesterly shear is affecting Florence, while the SHIPS analysis based on the GFS fields shows only about 10 kt. SSTs warm from this point forward along the forecast track, but shear is expected to be steady or strengthen, and the mid-level relative humidity values decrease to around 50 percent during the next 48 to 72 hours. Given these mixed factors, the NHC intensity forecast shows some possibility for strengthening in the next 12 hours, followed by a slow decay through 72 hours. Some restrengthening is forecast late in the period as SSTs warm above 28C and the atmospheric moisture increases. The NHC forecast is close to or a bit above the latest IVCN consensus aid and about 5 kt above the previous NHC forecast through 96 hours. The initial motion estimate is 285/14. Florence will be steered generally west-northwestward for the next 72 hours by the Atlantic subtropical ridge, followed by a northwestward turn at days 4 and 5. While there is large spread in the guidance between the HWRF on the right and the UKMET on the left, the GFS, ECMWF, and their ensemble means are more tightly clustered near the middle of the guidance envelope. Since the overall track forecast reasoning has not changed, the new NHC forecast remains near the middle of the guidance. This forecast is a bit north of the previous NHC track given the initial position and lies a little south of the consensus aids to reflect less influence of the outlier HWRF model to the north. FORECAST POSITIONS AND MAX WINDS INIT 03/1500Z 18.3N 38.7W 55 KT 65 MPH 12H 04/0000Z 18.6N 40.7W 60 KT 70 MPH 24H 04/1200Z 19.1N 43.0W 60 KT 70 MPH 36H 05/0000Z 19.9N 45.3W 55 KT 65 MPH 48H 05/1200Z 20.8N 47.6W 50 KT 60 MPH 72H 06/1200Z 23.0N 52.2W 45 KT 50 MPH 96H 07/1200Z 25.0N 55.0W 50 KT 60 MPH 120H 08/1200Z 27.0N 58.0W 55 KT 65 MPH $$ Forecaster Brennan
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