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Tropical Storm Sebastien Forecast Discussion Number 22

2019-11-24 21:59:33| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sun Nov 24 2019 000 WTNT45 KNHC 242059 TCDAT5 Tropical Storm Sebastien Discussion Number 22 NWS National Hurricane Center Miami FL AL202019 500 PM AST Sun Nov 24 2019 Sebastien is gradually losing tropical characteristics. Satellite images show a large shield of relatively cold cloud tops on the north side of the system and in a band that extends from east of the center south-southwestward. This cloud pattern resembles an extratropical cyclone. However, satellite images and microwave data also show a small area of deep convection lingering over the center, which is why the system is still designated a tropical cyclone for now. That being said, it is still not clear that the surface circulation of the storm is well defined. Hopefully, ASCAT data later this evening can help assess the structure of the circulation. The initial intensity is held at 55 kt, which is in agreement with the most recent SATCON estimate from CIMSS at the University of Wisconsin. Sebastien is expected to slowly weaken as it remains in very strong wind shear conditions while moving over progressively colder waters. FSU phase-analysis diagrams from the global models all show Sebastien becoming fully extratropical overnight, and the NHC forecast follows that scenario. The extratropical low is expected to merge with another cyclone in 24 to 36 hours. It is also possible, as alluded to above, that Sebastien could open into a trough before then. Sebastien is racing toward the east-northeast ahead of a cold front, and the initial motion estimate is 060/34 kt. This continued heading at an even faster rate is expected within the fast mid-latitude flow until the cyclone dissipates. Regardless of whether it is a tropical cyclone, extratropical cyclone, or a remnant trough, Sebastien or its remnants will likely bring gusty winds and heavy rains to portions of the Azores through Monday morning. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 39.2N 32.5W 55 KT 65 MPH 12H 25/0600Z 42.2N 25.7W 55 KT 65 MPH...POST-TROP/EXTRATROP 24H 25/1800Z 46.3N 17.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 36H 26/0600Z...MERGED WITH ANOTHER EXTRATROPICAL LOW $$ Forecaster Cangialosi

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