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Tropical Storm Sebastien Forecast Discussion Number 10
2019-11-21 21:32:13| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Nov 21 2019 000 WTNT45 KNHC 212032 TCDAT5 Tropical Storm Sebastien Discussion Number 10 NWS National Hurricane Center Miami FL AL202019 500 PM AST Thu Nov 21 2019 Convection associated with Sebastien has become better organized during the past several hours, as a band has formed near and over the center in the eastern semicircle. Overall, the cloud pattern remains elongated from north-to-south as the storm is interacting with a cold front not far to its northwest. Scatterometer data received just after the last advisory indicated 45-50 kt winds, and based on this and current satellite intensity estimates the initial intensity remains 50 kt. The cyclone has turned northeastward with the initial motion now 050/8. Southwesterly mid-latitude flow should steer Sebastien or its remnants quickly northeastward, and the guidance is in good agreement on the direction of movement during the next several days. However, there is a significant spread in the forward speed, with the new ECMWF run being much slower than the other guidance. The new track forecast will be only slightly slower than the previous forecast, but adjustments to the forecast forward speed may be necessary later. Sebastien is in an apparently unfavorable environment for development due to strong shear, decreasing sea surface temperatures along the forecast track, and an expected extratropical transition as it merges with the front. The intensity guidance is rather divergent during the first 24 h, with the SHIPS and LGEM models showing weakening while the dynamical guidance continues to forecast strengthening despite the environment. After that time, all guidance indicates that Sebastien should weaken. The intensity forecast continues the trend of the previous forecast in showing some strengthening during the first 24 h in agreement with the dynamical models, followed by weakening and extratropical transition by 48 h. Given how close the cold front is to the storm at this time, it is possible that extratropical transition could occur earlier than currently forecast. FORECAST POSITIONS AND MAX WINDS INIT 21/2100Z 23.8N 59.7W 50 KT 60 MPH 12H 22/0600Z 25.5N 57.1W 60 KT 70 MPH 24H 22/1800Z 27.9N 53.2W 65 KT 75 MPH 36H 23/0600Z 30.6N 48.4W 55 KT 65 MPH 48H 23/1800Z 33.5N 43.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 72H 24/1800Z 39.0N 31.0W 35 KT 40 MPH...POST-TROP/EXTRATROP 96H 25/1800Z...DISSIPATED $$ Forecaster Beven
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