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| #1064500 (Received by flhurricane at: 10:59 AM 04.Oct.2021) TCDAT3 Hurricane Sam Discussion Number 48 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Mon Oct 04 2021 Sam remains a formidable high-latitude hurricane this morning. An earlier 1102 UTC GMI microwave overpass continued to depict concentric eyewalls with a fairly well-defined inner-eye that was open over the southwestern quadrant. Although there has been a generally warming of the cloud tops in infrared satellite imagery, the eye remains apparent and the system is still fairly symmetric. A blend of the subjective Dvorak T- and CI-numbers from both SAB and TAFB yield an intensity of 85 kt, which is used as the initial intensity for this advisory. Now that Sam has moved north of the north wall of the Gulf Stream it will be moving over progressively colder waters and into much higher vertical wind shear conditions by this evening, which should cause some gradual weakening. However, the interaction of the cyclone with a strong upper-level trough is likely to lead to the rapid transition of Sam into a powerful extratropical cyclone later this evening. The post-tropical cyclone is expected to maintain hurricane-force winds until late Tuesday. Gradual weakening should occur after that time as the baroclinic forcing decreases, and the system is forecast to spin down slowly over the north Atlantic later this week. Sam continues to accelerate northeastward and is now moving at 045/28 kt. A faster northeastward motion is forecast through tonight as Sam moves within the strong southwesterly flow ahead of a mid-latitude trough. The post-tropical cyclone is forecast to slow down on Tuesday as it interacts with the trough/cut-off low. After that time, the system is forecast to turn east-northeastward, and then rotate cyclonically around the eastern flank of yet another cut-off low later in the week. The dynamical model guidance remains in generally good agreement on the overall track forecast scenario, albeit for some forward speed variations. The latest official forecast is similar to the previous advisory and lies near the TVCA multi-model consensus in deference to the aforementioned along-track differences. Key Messages: 1. Swells generated by Sam will impact the northern Leeward Islands, the Greater Antilles, portions of the Bahamas, and southeastern Newfoundland for the next day or so. 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 04/1500Z 42.6N 45.7W 85 KT 100 MPH 12H 05/0000Z 46.5N 41.9W 75 KT 85 MPH 24H 05/1200Z 50.0N 40.5W 70 KT 80 MPH...POST-TROP/EXTRATROP 36H 06/0000Z 50.3N 38.3W 60 KT 70 MPH...POST-TROP/EXTRATROP 48H 06/1200Z 51.4N 33.2W 55 KT 65 MPH...POST-TROP/EXTRATROP 60H 07/0000Z 54.3N 27.7W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 07/1200Z 58.3N 24.8W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 08/1200Z 61.0N 29.7W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 09/1200Z 60.0N 29.8W 40 KT 45 MPH...POST-TROP/EXTRATROP $$ Forecaster Brown |
