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| #989888 (Received by flhurricane at: 3:51 PM 19.Nov.2019) TCDAT5 Tropical Storm Sebastien Discussion Number 2 NWS National Hurricane Center Miami FL AL202019 500 PM AST Tue Nov 19 2019 The low-level center of Sebastien has been exposed over the past several hours, as northwesterly shear of 20-30 kt has been pushing any deep convection off to the southeastern portion of the cyclone. The initial advisory intensity is being held at 40 kt and is based on an earlier scatterometer pass that sampled maximum winds of 39 kt. The initial motion is 305/10 kt. Sebastien is forecast to be steered to the northwest around a deep-layer ridge over the central Atlantic through tonight. By Wednesday, the cyclone should turn to the north, and then accelerate northeastward Wednesday night as the cyclone gets caught up in the flow between the retreating ridge and ahead of an approaching cold front. The model guidance remains in good agreement on this scenario, and the official forecast track keeps the cyclone over open waters for the duration of its existence. The shear is not expected to decrease much over the next couple days while the system remains a tropical cyclone. There may be an opportunity for some strengthening Wednesday and Wednesday night as Sebastien turns to the north and northeast and resides under some favorable upper-level diffluence. By 48 hours, Sebastien is expected to transition to an extratropical cyclone just ahead of an approaching cold front. Therefore, additional strengthening indicated beyond 36 hours should be due to baroclinic processes. The cyclone should then become absorbed by the cold front by late this week. The official forecast was adjusted slightly higher from the previous one at 36 and 48 hours to reflect the anticipated intensification of the system as it begins its extratropical transition. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 20.6N 59.7W 40 KT 45 MPH 12H 20/0600Z 21.3N 60.7W 40 KT 45 MPH 24H 20/1800Z 22.6N 61.9W 45 KT 50 MPH 36H 21/0600Z 24.2N 61.0W 50 KT 60 MPH 48H 21/1800Z 26.0N 58.9W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 22/1800Z...DISSIPATED $$ Forecaster Latto |
