114 WTNT43 KNHC 280234 TCDAT3 Hurricane Sam Discussion Number 22 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Mon Sep 27 2021 The NOAA Hurricane Hunters have been flying through Sam tonight, with radar and wind observations showing broken concentric eyewalls at times. The maximum 700-mb flight-level winds on the mission were 114 kt, with peak SFMR values to 103 kt and a central pressure around 956 mb, so the initial wind speed will remain 105 kt. While the overall intensity doesn't seem to have changed much in the past several hours, the aircraft data show that the hurricane has grown in size, and that is reflected in the initial wind radii. The future intensity of Sam is hard to pinpoint. Eyewall cycles will likely play a key role in determining its second peak intensity since the hurricane should remain in low- or moderate-shear and warm-water environments for the next few days. Those cycles are basically impossible to forecast, but there is some suggestion on satellite that a larger eye is trying to form, which could allow for some increase in strength in the near term. It seems likely that Sam will be a category 3 or 4 hurricane given the conducive environment for most of the forecast period, so the new NHC forecast is similar to the previous one, with the most significant change showing a larger hurricane consistent with the latest guidance. Sam continues to move northwestward, or 315/8 kt. The hurricane should move in that direction for the next few days around the southwestern portion of the subtropical high. A turn to the north is likely as Sam meets the western edge of the high, with a gradual acceleration to the northeast anticipated by the weekend due to an incoming mid-latitude trough from the west. While there are some speed differences in the models, especially near and after recurvature, the NHC forecast leans toward the faster GFS solution over the slower ECMWF, given the magnitude of the strong southwesterly flow that Sam should encounter at long range, plus a nod to the superior performance of late for the American model. The new forecast is adjusted a bit to the right and faster beyond day 3, with no significant changes made before that time. 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 28/0300Z 16.8N 53.2W 105 KT 120 MPH 12H 28/1200Z 17.5N 54.0W 115 KT 130 MPH 24H 29/0000Z 18.4N 55.1W 115 KT 130 MPH 36H 29/1200Z 19.3N 56.4W 110 KT 125 MPH 48H 30/0000Z 20.6N 58.0W 110 KT 125 MPH 60H 30/1200Z 22.2N 59.7W 110 KT 125 MPH 72H 01/0000Z 24.3N 61.0W 110 KT 125 MPH 96H 02/0000Z 29.6N 61.5W 105 KT 120 MPH 120H 03/0000Z 37.5N 56.9W 100 KT 115 MPH $$ Forecaster Blake