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Hurricane Florence Forecast Discussion Number 21
2018-09-04 16:37:32| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Tue Sep 04 2018 386 WTNT41 KNHC 041437 TCDAT1 Hurricane Florence Discussion Number 21 NWS National Hurricane Center Miami FL AL062018 1100 AM AST Tue Sep 04 2018 Florence's structure has gradually increased in organization, with SSMIS passes from a few hours ago revealing the development of a mid-level microwave eye. Dvorak estimates have responded in kind--TAFB is up to T4.0, SAB is at T4.5, and the objective ADT is in between at T4.4. Since there still appears to be moderate southwesterly shear inducing some tilt to the cyclone and disrupting the infrared satellite pattern, the initial intensity is raised conservatively to 65 kt, making Florence a hurricane. The current motion remains west-northwestward, or 295/10 kt, with Florence positioned near the southwestern edge of the subtropical ridge. The steering pattern ahead of Florence is rather complicated and will be evolving over the next few days. A large and complex mid-/upper-level trough located northeast of the Leeward Islands is expected to give way to the development of two upper-level highs centered near the Greater Antilles and southwest of the Azores, with Florence slowing down and turning northwestward between these new features. Despite this complex pattern, the spread in the track models is less than normal, which increases the confidence in the NHC track forecast for the next 5 days. There is some spread which begins to develop around day 5, with the ECMWF model moving Florence a little faster toward the north while the GFS maintains a slower speed and keeps the system to the south. The updated NHC track forecast has been nudged slightly to the east of the previous forecast on days 4 and 5, close to the TVCX consensus but not as far as the HCCA and ECMWF models. There is still too much model spread after day 5 to speculate what Florence might do beyond the official forecast period. Despite Florence becoming a hurricane, the southwesterly shear affecting the cyclone is expected to increase over the next day or two, which should prevent further intensification. In fact, the increasing shear, as well as mid-level relative humidities below 50 percent, should cause weakening between 24 and 72 hours. After 72 hours, decreasing shear and warmer sea surface temperatures should foster some re-intensification, with Florence expected to reattain hurricane intensity by day 5. The intensity guidance is in good agreement, and the NHC forecast is very close to a blend of HCCA, the Florida State Superensemble, and the ICON consensus. FORECAST POSITIONS AND MAX WINDS INIT 04/1500Z 19.7N 42.5W 65 KT 75 MPH 12H 05/0000Z 20.3N 44.0W 65 KT 75 MPH 24H 05/1200Z 21.3N 46.2W 65 KT 75 MPH 36H 06/0000Z 22.4N 48.2W 60 KT 70 MPH 48H 06/1200Z 23.6N 50.0W 55 KT 65 MPH 72H 07/1200Z 25.6N 52.7W 50 KT 60 MPH 96H 08/1200Z 27.5N 55.0W 55 KT 65 MPH 120H 09/1200Z 29.5N 56.5W 65 KT 75 MPH $$ Forecaster Berg
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