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Hurricane Sam Forecast Discussion Number 39
2021-10-02 10:44:03| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sat Oct 02 2021 000 WTNT43 KNHC 020843 TCDAT3 Hurricane Sam Discussion Number 39 NWS National Hurricane Center Miami FL AL182021 500 AM AST Sat Oct 02 2021 The Air Force Reserve Hurricane Hunters conducted their final mission into Sam a few hours ago and found that the hurricane is still of category 4 intensity, but the maximum winds have decreased some. The crew reported a peak 700-mb flight-level wind of 130 kt (equating to surface winds of 115-120 kt) and unflagged SFMR winds of 110-115 kt. Based on these wind data, Sam's intensity is now estimated to be 115 kt. This value is at the upper end of the latest satellite estimates, which range from 100-115 kt. The hurricane's central pressure has also risen to 945 mb. Sam is still moving toward the north-northeast with a motion of 020/15 kt. The hurricane is entering the area between a deep-layer ridge to the east and a large mid- to upper-level low over Atlantic Canada, and this flow should cause Sam to turn toward the northeast by tonight and then maintain that general heading for much of the forecast period. The storm is also expected to accelerate, reaching a peak forward speed of more than 25 kt in 60-72 hours. The track guidance is tightly clustered through day 3, but there is significantly more spread on days 4 and 5 due to uncertainty on exactly how Sam will interact with the aforementioned mid-/upper-level low. The NHC forecast has been placed near a blend of the GFS-ECMWF mean and the HCCA consensus aid, which necessitated a northward shift from the previous forecast only on days 4 and 5. Deep-layer southerly shear of 15-20 kt appears to be contributing to Sam's current weakening. Continued shear and cooler waters along Sam's path should lead to additional weakening in the coming days, although not at a rapid rate due to some baroclinic forcing. Global models suggest that Sam will begin extratropical transition in about 48 hours, with that process completing by 72 hours. The NHC intensity forecast closely follows the HCCA and IVCN consensus aids during Sam's tropical phase, but then transitions to a blend of the GFS and ECMWF global model guidance on days 3 through 5 during its extratropical phase. Sam is expected to continue producing hurricane-force winds through at least day 3, with more significant weakening occurring on days 4 and 5 once it is a vertically stacked occluded low. Key Messages: 1. Swells generated by Sam will impact the northern Leeward Islands the Greater Antilles, the Bahamas, and Bermuda during the next couple of days. Swells are expected to reach the United States east coast and Atlantic Canada today. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. 2. Although the Tropical Storm Warning for Bermuda has been discontinued, a few wind gusts to tropical storm force will still be possible on the island during the morning. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 32.8N 60.5W 115 KT 130 MPH 12H 02/1800Z 34.6N 59.3W 100 KT 115 MPH 24H 03/0600Z 36.6N 57.1W 95 KT 110 MPH 36H 03/1800Z 38.4N 54.2W 90 KT 105 MPH 48H 04/0600Z 40.7N 50.0W 85 KT 100 MPH 60H 04/1800Z 44.4N 44.8W 80 KT 90 MPH 72H 05/0600Z 48.9N 40.7W 75 KT 85 MPH...POST-TROP/EXTRATROP 96H 06/0600Z 53.4N 34.7W 55 KT 65 MPH...POST-TROP/EXTRATROP 120H 07/0600Z 58.7N 26.4W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Berg
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