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| #990349 (Received by flhurricane at: 3:42 PM 23.Nov.2019) TCDAT5 Tropical Storm Sebastien Discussion Number 18 NWS National Hurricane Center Miami FL AL202019 500 PM AST Sat Nov 23 2019 Sebastien`s cloud pattern is beginning to resemble that of an extratropical cyclone, with a shield of moderately cold cloud tops extending well to the north of the cyclone`s surface center. That said, Sebastien is still producing enough convection near its center to be considered a tropical cyclone for now. ASCAT-B data that arrived shortly after the last advisory was issued had support for 55 kt and that is the basis for the initial intensity. The ASCAT data also showed that the surface wind circulation of Sebastien is elongated but still closed. Sebastien is accelerating northeastward, with an initial motion of 045/26 kt. Most of the 12Z models show a significantly faster northeastward track for the tropical storm and the official forecast has been adjusted accordingly. However, the NHC forecast is slower than the model consensus and additional large changes may be required in the next advisory. Confidence in the track forecast remains low due to the continued lack of run-to-run consistency amongst the models. With regard to intensity, a majority of the guidance calls for little change in strength since it seems that baroclinic forcing will at least offset the negative influences of decreasing SSTs and high shear during the next day or two. The NHC intensity forecast is very near the consensus. Cyclone phase-space diagrams derived from the GFS and ECMWF indicate that extratropical transition could occur by Sunday afternoon, but the NHC forecast is slightly more conservative. The reality is that Sebastien could become extratropical or open into a trough at nearly any time between this evening and early next week. Sebastien or its remnants are expected to bring gusty winds and rain to the Azores beginning on Sunday. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 32.4N 45.4W 55 KT 65 MPH 12H 24/0600Z 35.0N 41.5W 55 KT 65 MPH 24H 24/1800Z 37.8N 35.3W 55 KT 65 MPH 36H 25/0600Z 40.9N 27.6W 55 KT 65 MPH...POST-TROP/EXTRATROP 48H 25/1800Z 44.8N 19.3W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 26/1800Z...DISSIPATED $$ Forecaster Zelinsky |
