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Tropical Storm Sebastien Forecast Discussion Number 20
2019-11-24 09:47:24| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sun Nov 24 2019 000 WTNT45 KNHC 240847 TCDAT5 Tropical Storm Sebastien Discussion Number 20 NWS National Hurricane Center Miami FL AL202019 500 AM AST Sun Nov 24 2019 Sebastien is on the slow boat to becoming fully extratropical. Just within the past few hours, the cyclone has taken on an appearance more resembling a subtropical cyclone, with all of its deep convection located in a band to the north of the center and it being co-located with an upper-level shortwave trough. Based on Sebastien's evolving structure, TAFB provided a subtropical classification of ST3.5. Combining this estimate with the most recent ADT and SATCON estimates, the initial intensity remains 55 kt. Since Sebastien's extratropical transition has been so drawn out, it's difficult to pinpoint exactly when the process will be complete. Phase-space diagrams from the GFS and ECMWF models indicate that the cyclone will be deep cold core within the next 6-12 hours. However, these models keep the low-level vortex separate from a second upstream upper-level trough for the next 30 hours, and they also keep deep convection going (in simulated satellite imagery) near but to the north of the center for that same amount of time. Because of deep-layer shear around 40 kt and sea surface temperatures near 20C, the official forecast continues to show extratropical transition complete by 24 hours, but that forecast should be considered with a margin of error of plus or minus 12 hours. Regardless of its status, Sebastien is forecast to only gradually lose strength over the next 48 hours and is likely to bring gusty winds and heavy rains to portions of the Azores later today and tonight. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information. The northeastward acceleration continues, with Sebastian moving 050/29 kt. Additional acceleration is expected during the next 24 hours, with the new NHC track forecast a little faster than the previous iteration and generally a blend of the TVCN and HCCA consensus aids. No forecast points are provided starting at 72 hours since by that time Sebastien is expected to have merged with another weather system southwest of Ireland and the United Kingdom. This larger complex system could bring gusty winds and heavy rains to portions of western Europe within the next few days. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 36.0N 40.0W 55 KT 65 MPH 12H 24/1800Z 38.5N 34.8W 55 KT 65 MPH 24H 25/0600Z 42.1N 26.2W 50 KT 60 MPH...POST-TROP/EXTRATROP 36H 25/1800Z 46.2N 17.5W 45 KT 50 MPH...POST-TROP/EXTRATROP 48H 26/0600Z 50.0N 11.4W 45 KT 50 MPH...POST-TROP/EXTRATROP 72H 27/0600Z...MERGED WITH ANOTHER WEATHER SYSTEM $$ Forecaster Berg
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