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Chill falls over the Atlantic as the official season heads towards close
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| #1064540 (Received by flhurricane at: 4:45 PM 04.Oct.2021) TCDAT3 Hurricane Sam Discussion Number 49 NWS National Hurricane Center Miami FL AL182021 900 PM GMT Mon Oct 04 2021 Sam`s cloud pattern has finally begun to degrade as it moves over the cooler waters of the north Atlantic. The cloud tops have warmed and the eye is no longer discernible in infrared imagery, but banding features still remain well defined in all quadrants. Dvorak T- and CI-numbers from both SAB and TAFB have continued to decline and a blend of the latest subjective and objective estimates yields an initial intensity estimate of 80 kt for this advisory. Scatterometer data that arrived just after the release of the previous advisory showed that Sam`s wind field has continued to expand and the latest radii estimates are based on those data. Sam should gradually weaken this evening as it moves over progressively colder waters and into an area of strong southwesterly upper-level winds. However, an approaching mid-latitude trough should result in the system`s quick transition into a powerful extratropical cyclone overnight. The post-tropical cyclone is forecast to maintain hurricane-force winds for until late Tuesday, but gradual weakening should occur after that time as the system occludes and baroclinic forcing decreases. It is not entirely clear as to whether Post-Tropical Sam will remain the dominant low (GFS solution) or if it will be absorbed by another extratropical low (ECMWF solution) later in the week. As a compromise, the new official forecast calls for absorbed by day 5 which is between the solutions from those typically reliable global models. Sam is moving briskly northeastward or 035/31 kt. The cyclone is currently caught in strong southwesterly flow ahead of the aforementioned mid-latitude trough and a rapid northeastward motion is expected overnight. After that time, the post-tropical cyclone is forecast to slow down as it merges with the cut-off low. By late Tuesday, the post-tropical cyclone should turn east- northeastward before it rotates cyclonically around the eastern portion of another trough/cut off low over the north Atlantic. The latest NHC forecast is a blend of the GFS and ECMWF models and their respective ensemble means. Key Messages: 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, portions of the Bahamas, and southeastern Newfoundland through early Tuesday. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 45.1N 42.5W 80 KT 90 MPH 12H 05/0600Z 49.0N 39.8W 70 KT 80 MPH...POST-TROP/EXTRATROP 24H 05/1800Z 50.9N 39.9W 65 KT 75 MPH...POST-TROP/EXTRATROP 36H 06/0600Z 50.5N 36.0W 55 KT 65 MPH...POST-TROP/EXTRATROP 48H 06/1800Z 52.5N 30.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 60H 07/0600Z 56.5N 25.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 72H 07/1800Z 60.0N 25.5W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 08/1800Z 60.0N 32.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 09/1800Z...DISSIPATED $$ Forecaster Brown |
