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Hurricane Sam Forecast Discussion Number 31
2021-09-30 10:56:25| National Hurricane Center (Atlantic)
Issued at 500 AM AST Thu Sep 30 2021 113 WTNT43 KNHC 300856 TCDAT3 Hurricane Sam Discussion Number 31 NWS National Hurricane Center Miami FL AL182021 500 AM AST Thu Sep 30 2021 Sam is a very impressive hurricane in infrared satellite imagery this morning. The 25-nm-wide eye remains very distinct and is surrounded by a ring of convection with cloud tops around -65C. The outflow is also well established in all quadrants. An Air Force Reserve Hurricane Hunter aircraft that performed three eyewall penetrations overnight has reported peak 700-mb flight-level winds of 139 kt in the northeastern, southeastern, and eastern eyewall, and peak SFMR surface winds of 119 kt. These data still support an initial wind speed of 125 kt. Sam's minimum pressure has fallen a few millibars since the NOAA aircraft mission last evening. The latest estimated pressure from dropsonde data gathered by the Air Force is 937 mb. NOAA buoy 41044 has recently reported a peak one-minute wind of 68 kt with a gust to 86 kt in the northeastern eyewall of Sam. The buoy has also reported peak seas of 40 ft. Sam could still strengthen a little today as it continues to move over a warm ocean eddy and remains in low vertical wind shear conditions. After that time, difficult-to-predict eyewall cycles however, could cause some fluctuations in intensity. After 48 hours, gradually decreasing SSTs and ocean heat content are likely to cause a more definitive weakening trend, with a faster rate of weakening likely after day 3. Sam is forecast to complete its extratropical transition by day 5, and it is predicted by the global model guidance to become a large and powerful extratropical low over the north Atlantic. Sam is moving northwestward or 320/10 kt. The track forecast reasoning is again unchanged from the previous advisory. Sam will move northwestward, and then northward around the western portion of a subtropical ridge during the next 48 hours. After that time, Sam is predicted to turn northeastward between the ridge and a large mid-latitude trough over the northeastern United States. As that trough lifts northeastward by day 3, Sam is forecast to continue on a northeastward heading, but it is not likely to accelerate as much as a typical recurving tropical cyclone over the north-central Atlantic. The dynamical model guidance remains in excellent agreement through 72 hours, but there is increasing spread after that time. The latest runs of the GFS and ECMWF models have come into somewhat better agreement at days 3 through 5, and the NHC track forecast is near the consensus of those typically reliable models. Although the core of Sam is forecast 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, the Bermuda Weather Service has issued a Tropical Storm Watch 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 possible on Bermuda beginning Friday night or early Saturday, and a Tropical Storm Watch has been issued for that island. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 21.5N 59.1W 125 KT 145 MPH 12H 30/1800Z 23.0N 60.2W 130 KT 150 MPH 24H 01/0600Z 25.6N 61.4W 125 KT 145 MPH 36H 01/1800Z 28.4N 61.8W 120 KT 140 MPH 48H 02/0600Z 31.1N 61.3W 115 KT 130 MPH 60H 02/1800Z 33.5N 59.9W 105 KT 120 MPH 72H 03/0600Z 35.5N 58.0W 95 KT 110 MPH 96H 04/0600Z 38.7N 52.6W 80 KT 90 MPH 120H 05/0600Z 44.3N 45.0W 65 KT 75 MPH...POST-TROP/EXTRATROP $$ Forecaster Brown
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