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Tropical Storm Sebastien Forecast Discussion Number 8
2019-11-21 09:32:05| National Hurricane Center (Atlantic)
Issued at 500 AM AST Thu Nov 21 2019 000 WTNT45 KNHC 210831 TCDAT5 Tropical Storm Sebastien Discussion Number 8 NWS National Hurricane Center Miami FL AL202019 500 AM AST Thu Nov 21 2019 Sebastien continues to produce a large area of showers and thunderstorms over its southeastern quadrant with more disorganized convection to the north and west of the center. Despite the fair amount of convection, the cloud pattern lacks banding features and the center of the storm has been challenging to locate. The initial intensity is held at 50 kt based on ASCAT data from several hours ago. This data also indicate that Sebastien's wind field is quite lopsided, with all of its tropical-storm-force winds confined to the eastern side of the circulation. The initial motion of the storm is tough to assess given that there is more than the usual amount of uncertainty in the current position, but my best guess is 015/7 kt. A deep-layer trough over the western Atlantic is approaching Sebastien, and that feature should cause the storm to accelerate to the northeast during the next few days. Although the models agree on the overall scenario, there are major along-track or speed differences among the models, with the GFS/HWRF/HMON being the fastest solutions and the ECMWF being the slowest. In fact, at 48 hours, the spread between the GFS and ECMWF models is more than 700 n mi. The NHC forecast lies roughly near the middle of the guidance envelope and ends up near the UKMET model, but this prediction is of low confidence. Upper-level diffluence, relatively warm SSTs, and a moist environment should allow for Sebastien to strengthen during the next 24 hours or so. In fact, all of the hurricane regional models show significant or even rapid intensification during that time period. Given the asymmetric structure of Sebastien currently, rapid intensification seems unlikely, and the NHC intensity prediction lies closer to the lower end of the guidance in the short term. Beyond 24 hours, Sebastien will be moving into progressively more hostile conditions of increasing shear, drier air, and cooler waters. These conditions should end the opportunity for strengthening and begin the process of extratropical transition. Sebastien is expected to become fully extratropical in about 48 hours, but this could occur sooner if the GFS is correct or later if the ECMWF is correct. The extratropical low is forecast to slowly weaken and dissipate in about 4 days. FORECAST POSITIONS AND MAX WINDS INIT 21/0900Z 23.0N 61.2W 50 KT 60 MPH 12H 21/1800Z 24.5N 59.3W 60 KT 70 MPH 24H 22/0600Z 26.6N 56.4W 70 KT 80 MPH 36H 22/1800Z 29.4N 52.0W 70 KT 80 MPH 48H 23/0600Z 32.4N 47.1W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 24/0600Z 38.6N 34.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 25/0600Z...DISSIPATED $$ Forecaster Cangialosi
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