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Tropical Storm Sebastien Forecast Discussion Number 9
2019-11-21 15:49:08| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Thu Nov 21 2019 000 WTNT45 KNHC 211449 TCDAT5 Tropical Storm Sebastien Discussion Number 9 NWS National Hurricane Center Miami FL AL202019 1100 AM AST Thu Nov 21 2019 Sebastien continues to produce a large area of deep convection over the eastern semicircle of the circulation, but the banding features are not very well defined. The intensity estimate remains at 50 kt based on a blend of subjective Dvorak estimates from TAFB and SAB. A diffluent upper-level wind environment and warm waters are expected to bring about some strengthening through tonight, however the window of opportunity for strengthening will probably close tomorrow morning. After that time, drier air and strong shear should cause gradual weakening. The official intensity forecast is a little above the model consensus. In a couple of days, the GFS shows Sebastien becoming embedded in a frontal zone, signifying the transition to an extratropical cyclone as shown in the NHC forecast. The ECMWF model suggests that this event could occur later, but the current close proximity of a front to Sebastian argues for extratropical transition to occur within 48 hours. The circulation is a bit elongated from south to north, and the center remains difficult to locate. This results in a greater than normal uncertainty in the initial motion, and my best guess is 030/7 kt. A turn toward the northeast, with acceleration, is expected during the next couple of days due to the influence of a mid-level trough. There is considerable along-track model spread, with the ECMWF forecast being much slower than the majority of the guidance. The official forecast is somewhat slower than the dynamical model consensus out of respect for that typically reliable model. FORECAST POSITIONS AND MAX WINDS INIT 21/1500Z 23.5N 60.5W 50 KT 60 MPH 12H 22/0000Z 25.0N 58.3W 65 KT 75 MPH 24H 22/1200Z 27.2N 55.0W 70 KT 80 MPH 36H 23/0000Z 30.0N 50.4W 65 KT 75 MPH 48H 23/1200Z 33.2N 45.2W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 24/1200Z 39.7N 31.5W 35 KT 40 MPH...POST-TROP/EXTRATROP 96H 25/1200Z...DISSIPATED $$ Forecaster Pasch
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