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Hurricane Sam Forecast Discussion Number 27
2021-09-29 10:38:37| National Hurricane Center (Atlantic)
Issued at 500 AM AST Wed Sep 29 2021 000 WTNT43 KNHC 290838 TCDAT3 Hurricane Sam Discussion Number 27 NWS National Hurricane Center Miami FL AL182021 500 AM AST Wed Sep 29 2021 The eye of Sam has become cloud filled in infrared satellite imagery overnight, but the ring of deep convection surrounding the eye has cooled. There appears to be a southwest-to-northeast tilt to Sam's circulation as some modest southwesterly shear seems to be impinging on the hurricane. An Air Force Reserve Hurricane Hunter aircraft that has been investigating Sam overnight shows that the hurricane continues to fluctuate in intensity. The aircraft has reported a peak flight-level wind of 124 kt and SFMR winds of 108 kt. Therefore the initial wind speed has been set at 115 kt, which leans toward the higher flight-level-to-surface wind reduction. The aircraft also reported a minimum pressure of 950 mb, which is up a few millibars from the flight yesterday afternoon. The fluctuations in intensity that Sam has experienced over the past couple of days are fairly typical for hurricanes of this strength. As mentioned in the previous advisory, Sam will be moving into a lower vertical wind shear environment, and the expected faster forward speed decreases the potential for upwelling beneath the storm. As a result, some re-intensification is forecast, but difficult-to-predict eyewall replacement cycles could cause additional fluctuations in intensity over the next day or two. After 60 to 72 hours, a more pronounced weakening trend is likely to begin as the storm moves over cooler SSTs and the vertical shear increases. The global models indicate that Sam will begin its transition into a powerful extratropical cyclone late in the period, but that transition will likely not be completed until after 120 h. Sam is moving northwestward at about 8 kt. The track guidance remains in excellent agreement through 96 hours as Sam should move northwestward and then northward around the western portion of the subtropical ridge over the eastern Atlantic during the next few days. By Saturday, Sam is forecast to turn northeastward between the aforementioned ridge and a deep-layer trough over the northeastern U.S. and Nova Scotia. By late in the period, there is an increase in spread in the guidance which is related to how Sam interacts with the deep-layer trough. For now, the NHC track forecast at days 4 and 5 lies between the TVCA multi-model consensus and the HFIP corrected consensus aid. Overall, little change to the previous NHC track forecast was required for this advisory. Key Messages: 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend. FORECAST POSITIONS AND MAX WINDS INIT 29/0900Z 18.9N 56.2W 115 KT 130 MPH 12H 29/1800Z 19.7N 57.3W 120 KT 140 MPH 24H 30/0600Z 21.0N 58.9W 120 KT 140 MPH 36H 30/1800Z 22.8N 60.4W 120 KT 140 MPH 48H 01/0600Z 25.2N 61.5W 115 KT 130 MPH 60H 01/1800Z 27.9N 62.0W 115 KT 130 MPH 72H 02/0600Z 30.8N 61.5W 110 KT 125 MPH 96H 03/0600Z 35.8N 57.5W 95 KT 110 MPH 120H 04/0600Z 40.8N 52.0W 80 KT 90 MPH $$ Forecaster Brown
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