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Hurricane Sam Forecast Discussion Number 23
2021-09-28 10:44:51| National Hurricane Center (Atlantic)
Issued at 500 AM AST Tue Sep 28 2021 053 WTNT43 KNHC 280844 TCDAT3 Hurricane Sam Discussion Number 23 NWS National Hurricane Center Miami FL AL182021 500 AM AST Tue Sep 28 2021 The Air Force Reserve Hurricane Hunters that investigated Sam early this morning found evidence that the hurricane has strengthened. Peak 700-mb flight-level winds of 126 kt were reported in the northeastern quadrant, which after the standard adjustment would support an intensity of 110-115 kt. Satellite data also indicate that Sam's structure has improved overnight. Sam's inner core appears to have consolidated into a single, primary eyewall in recent GMI and AMSR2 microwave imagery, and the latest GOES-16 infrared imagery shows a colder ring of convective cloud tops developing around the eye of Sam. This has brought the latest UW-CIMSS ADT and SATCON estimates up to around 115 kt. Based on the flight-level wind data and ongoing satellite trends, the initial intensity is raised to 115 kt for this advisory. The minimum central pressure of 953 mb is based on a center dropsonde of 954 mb with 12-kt surface winds. In the short-term, internal dynamic processes that are difficult to predict could cause some fluctuations in Sam's intensity. If Sam's eyewall is able to contract today, some additional strengthening could occur, and this possibility is reflected in the latest NHC intensity forecast. Thereafter, the warm sea-surface temperatures and weak to moderate vertical wind shear along Sam's forecast track suggest it should remain a major hurricane for the next several days. Thus, the NHC forecast only shows gradual weakening through 72-96 h, in agreement with the consensus aids IVCN and HCCA. By days 4-5, increasing southerly wind shear along with decreasing SSTs should increase Sam's rate of weakening as it recurves deeper into the mid-latitudes. Aircraft and microwave data indicate that Sam is slightly tilted in the vertical, with the low-level center displaced just a bit to the south of the mid-level eye. Based on recent aircraft fixes, Sam's initial motion is northwestward, or 310/8 kt. This motion is expected to continue for the next few days as Sam moves around the southwestern periphery of the subtropical ridge. A mid- to upper-level trough is forecast to dig southward over the western Atlantic later this week, which should steer Sam toward the north by Friday. Then, Sam is expected to accelerate north-northeastward within the deep-layer southerly flow ahead of the trough this weekend. The official NHC track forecast is very similar to the previous one, with just a slight adjustment to the left based on the latest track guidance consensus aids. The along-track spread in the guidance noticeably increases as Sam recurves over the western Atlantic, with the ECMWF much slower than the GFS. Once again, the NHC forecast trends closer to the faster GFS solution at longer ranges, given its better overall performance this season. 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 28/0900Z 17.2N 53.9W 115 KT 130 MPH 12H 28/1800Z 17.9N 54.7W 120 KT 140 MPH 24H 29/0600Z 18.8N 55.9W 115 KT 130 MPH 36H 29/1800Z 19.9N 57.4W 115 KT 130 MPH 48H 30/0600Z 21.3N 59.0W 110 KT 125 MPH 60H 30/1800Z 23.2N 60.5W 110 KT 125 MPH 72H 01/0600Z 25.6N 61.5W 110 KT 125 MPH 96H 02/0600Z 31.4N 60.7W 105 KT 120 MPH 120H 03/0600Z 39.0N 56.0W 95 KT 110 MPH $$ Forecaster Reinhart
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