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| #990148 (Received by flhurricane at: 9:54 PM 21.Nov.2019) TCDAT5 Tropical Storm Sebastien Discussion Number 11 NWS National Hurricane Center Miami FL AL202019 1100 PM AST Thu Nov 21 2019 Sebastien continues to produce a central dense overcast and a large band on the southeastern side of the circulation. Recent scatterometer data indicate that the maximum winds are at least 45 kt and, since there has been little overall change in the satellite presentation within the past several hours, the initial wind speed will stay 50 kt. Two notable changes have been made on this forecast. As foreshadowed in the previous forecast, the models are no longer rapidly accelerating the storm to the northeast ahead of a mid-latitude trough. Instead, there is better agreement tonight on a steadier motion to the northeast due to the trough in the short term. Sebastien will then continue moving northeastward embedded in the mid-latitude southwesterly flow over the weekend. The new forecast is substantially slower than the last one, but is still on the faster side of the guidance due to continuity concerns. The second significant change is that Sebastien is no longer expected to become a hurricane. The cyclone will soon be moving over sub-26C waters with strong shear, and it seems unlikely to intensify much in these conditions. Model intensity guidance continues to decrease, and the NHC forecast follows that trend. Weakening should start by the weekend over even cooler waters in a high-shear environment, and these factors are forecast to cause Sebastien to transition into a non-convective low or extratropical cyclone in 36-48 h. Careful watchers of the forecast will note that NHC has predicted extratropical transition at 48 h since Sebastien was born over 48 h ago, and this timing has been pushed back primarily due to the slower-than-expected motion. Thus, this timing can`t be considered a particularly confident prediction until the track forecast becomes more consistent. FORECAST POSITIONS AND MAX WINDS INIT 22/0300Z 24.7N 57.8W 50 KT 60 MPH 12H 22/1200Z 26.0N 55.5W 55 KT 65 MPH 24H 23/0000Z 28.0N 51.7W 55 KT 65 MPH 36H 23/1200Z 30.3N 47.7W 45 KT 50 MPH 48H 24/0000Z 32.7N 43.4W 40 KT 45 MPH...POST-TROP/EXTRATROP 72H 25/0000Z 37.5N 34.0W 35 KT 40 MPH...POST-TROP/EXTRATROP 96H 26/0000Z...DISSIPATED $$ Forecaster Blake |
