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Hurricane Sam Forecast Discussion Number 33
2021-09-30 22:57:39| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Sep 30 2021 000 WTNT43 KNHC 302057 TCDAT3 Hurricane Sam Discussion Number 33 NWS National Hurricane Center Miami FL AL182021 500 PM AST Thu Sep 30 2021 Sam remains a formidable hurricane, with a well-organized cloud pattern on satellite imagery. The 25 n mi wide eye is surrounded by a fairly symmetric pattern of deep convective cloud tops, and the upper-level outflow pattern is very well defined. An Air Force Reserve Unit Hurricane Hunter aircraft investigated the system and found peak 700-mb flight level winds of 133 kt, and a highest unflagged SFMR-observed surface wind of 125 kt. Based on these observations, the advisory intensity is kept at 125 kt. Measurements from the aircraft also showed that the central pressure remains quite low, near 938 mb. Sam will continue to traverse waters of high oceanic heat content for another 12-18 hours, and the shear should remain fairly low for the next few days. Therefore, some intensification could still occur overnight. In any event, the system is expected to maintain major hurricane status for the next 36-48 hours. Over the weekend, a gradual weakening trend should commence due to cooler SSTs. However, Sam will likely remain a powerful cyclone with hurricane- force winds until the end of the forecast period. By 120 hours, simulated satellite imagery from the ECMWF and GFS models forecast the system's cloud pattern to resemble that of an extratropical low, and this is reflected in the official forecast, which shows extratropical status at that time. The official intensity forecast remains close to the HCCA corrected consensus model solution. Center fixes continue to show a gradual increase in forward speed, and the hurricane is moving northwestward, or about 320/12 kt. During the next 24-36 hours, Sam should turn toward the north as it moves around the western periphery of a large subtropical high over the eastern and central Atlantic. Then, the cyclone is expected to move generally northeastward in the flow between the anticyclone and a trough near Atlantic Canada. There is fairly close agreement in the track models through 60 hours or so. However, there are some significant differences in the model-predicted track of Sam at higher latitudes, probably due to variations in how the system interacts with the trough in the various guidance. The NHC track forecast follows the latest model consensus, TVCN. Although the core of Sam is predicted to pass east of Bermuda early Saturday, the tropical-storm-force wind field is forecast to expand and could pass very close to the island beginning late Friday night or early Saturday. Therefore, a Tropical Storm Warning has been issued for Bermuda. Key Messages: 1. Swells generated by Sam will impact the Northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next few days. Swells are expected to reach Bermuda and the Bahamas by Friday, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. 2. Tropical storm conditions are expected on Bermuda beginning Friday night or early Saturday, and a Tropical Storm Warning has been issued for that island. FORECAST POSITIONS AND MAX WINDS INIT 30/2100Z 23.6N 60.9W 125 KT 145 MPH 12H 01/0600Z 25.6N 61.5W 130 KT 150 MPH 24H 01/1800Z 28.5N 61.9W 125 KT 145 MPH 36H 02/0600Z 31.3N 61.4W 110 KT 125 MPH 48H 02/1800Z 33.8N 60.1W 100 KT 115 MPH 60H 03/0600Z 35.8N 58.3W 90 KT 105 MPH 72H 03/1800Z 37.4N 56.2W 85 KT 100 MPH 96H 04/1800Z 42.5N 50.0W 75 KT 85 MPH 120H 05/1800Z 47.5N 46.0W 65 KT 75 MPH...POST-TROP/EXTRATROP $$ Forecaster Pasch
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