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Tropical Storm Sebastien Forecast Discussion Number 7
2019-11-21 03:54:03| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Nov 20 2019 378 WTNT45 KNHC 210253 TCDAT5 Tropical Storm Sebastien Discussion Number 7 NWS National Hurricane Center Miami FL AL202019 1100 PM AST Wed Nov 20 2019 Deep convection has intensified near and to the southeast of the center of Sebastien this evening, and a recent partial ASCAT pass suggests the center is still on the edge of the thunderstorms. Satellite intensity estimates have risen slightly, so the initial wind speed is set to 50 kt, on the high side of the recent values. The storm continues to move slowly to the north tonight. Increasing deep-layer flow ahead of a mid-latitude trough should turn Sebastien northeastward overnight and cause the cyclone to further accelerate by late Thursday and Friday. The models aren't in great agreement on how quickly the storm will move to the northeast, however, with the model guidance having a stronger storm generally moving faster to the northeast. Since Sebastien isn't expected to get that intense, it makes physical sense to avoid the fastest solutions. Thus, the new forecast is slower than the last one, closer to the model consensus than the GFS-based guidance. The environment is forecast to become more conducive for strengthening during the next 24 hours as significant upper-level divergence increases near Sebastien. Combined with low or moderate shear and warm waters, these conditions should support further intensification, and the new NHC forecast now shows Sebastien as a hurricane for a short period of time. Notably, this forecast is still on the conservative side of the guidance, with all of the regional hurricane models showing Sebastien becoming a fairly potent hurricane in a day or two. This doesn't seem likely after examining the model initial structure of the HWRF/HMON models, which show a much more vertically aligned cyclone than Sebastien appears to be now, so the NHC forecast is only a bit higher than the last one. In a couple of days, the storm will be moving over cooler waters, with increasing shear and baroclinicity. That should facilitate Sebastien's transition to a non-tropical cyclone by the end of the work week, but note that the extratropical transition has been shifted back about a day due to the stronger-than-forecast cyclone likely staying a little more separate from an incoming cold front. FORECAST POSITIONS AND MAX WINDS INIT 21/0300Z 22.2N 61.5W 50 KT 60 MPH 12H 21/1200Z 23.3N 60.5W 55 KT 65 MPH 24H 22/0000Z 25.5N 57.7W 65 KT 75 MPH 36H 22/1200Z 28.0N 53.9W 65 KT 75 MPH 48H 23/0000Z 31.0N 49.0W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 24/0000Z 37.0N 38.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 25/0000Z...DISSIPATED $$ Forecaster Blake
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