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Chill falls over the Atlantic as the official season heads towards close
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| #1063059 (Received by flhurricane at: 4:47 AM 24.Sep.2021) TCDAT3 Hurricane Sam Discussion Number 7 NWS National Hurricane Center Miami FL AL182021 500 AM AST Fri Sep 24 2021 Sam continues to rapidly intensify. Satellite images show a small, but well-developed inner core and pronounced curved bands that wrap most of the way around the center. There are some dry slots, however, between the core and bands. The latest satellite intensity estimates range from 55 to 77 kt, and based on that data and the continued improvement in the cyclone`s structure, the initial wind speed is increased to 65 kt. This makes Sam a hurricane, the seventh one of the 2021 Atlantic hurricane season. It should be noted that Sam has a compact wind field, with hurricane-force and tropical-storm-force winds estimated to only extend outward up to 15 and 50 n mi from the center, respectively. Sam is moving westward at about 13 kt, and this general motion should continue for another 12 to 24 hours as it moves in the flow on the south side of a mid-level ridge. After that time, a decrease in forward speed and a turn to the west-northwest are expected, and the official forecast has Sam moving at a slow pace of only 6-8 kt during the 48-96-hour time period. By the middle of next week, the ridge is forecast to slide eastward as a trough moves over the western Atlantic. In response, the hurricane will likely turn northwestward as it approaches the northern Leeward Islands. The models have changed little this cycle with the GFS still on the northern side of the guidance envelope and the ECMWF on the southern side. The NHC track forecast is essentially an update of the previous one and lies between the GFS and ECMWF models, and near the consensus aids. The large scale environmental conditions all appear favorable for continued rapid intensification during the next day or so as the hurricane is expected to remain over warm 29 deg C waters and in very low wind shear conditions. All of the SHIPS rapid intensification indices are well above the climatological means, and the NHC intensity forecast calls for Sam to become a major hurricane by early Saturday. Beyond a couple of days, the environment is likely to become a little less ideal, and most of the models show Sam leveling off in strength, and so does the official forecast. This intensity prediction lies near a blend of the FSSE, HCCA, and IVCN consensus models. Regardless of the details, Sam is expected to be a significant hurricane during the next several days. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 11.5N 42.2W 65 KT 75 MPH 12H 24/1800Z 11.7N 44.0W 80 KT 90 MPH 24H 25/0600Z 12.1N 45.9W 95 KT 110 MPH 36H 25/1800Z 12.5N 47.4W 105 KT 120 MPH 48H 26/0600Z 12.9N 48.7W 115 KT 130 MPH 60H 26/1800Z 13.4N 50.0W 115 KT 130 MPH 72H 27/0600Z 14.1N 51.1W 110 KT 125 MPH 96H 28/0600Z 15.9N 53.8W 110 KT 125 MPH 120H 29/0600Z 17.8N 56.6W 110 KT 125 MPH $$ Forecaster Cangialosi |
