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Hurricane Sam Forecast Discussion Number 21
2021-09-27 22:51:13| National Hurricane Center (Atlantic)
Issued at 500 PM AST Mon Sep 27 2021 000 WTNT43 KNHC 272051 TCDAT3 Hurricane Sam Discussion Number 21 NWS National Hurricane Center Miami FL AL182021 500 PM AST Mon Sep 27 2021 Sam's weakening trend ended earlier today, with the intensity likely bottoming out between the issuance of the previous advisory and now. Over the past few hours, microwave data and satellite imagery indicate that the inner core and eyewall of the hurricane are making a comeback. There is now a clear eye in visible imagery, convective cloud tops colder than -65 degrees C wrap completely around the center, and the convective mass is becoming more circular once again. An Air Force Reserve Hurricane Hunter aircraft currently investigating Sam indicates the central pressure has dropped to 957 mb, with peak 700-mb flight-level winds of 106 kt, SFMR values of 100 kt, and dropsonde surface winds of 102 kt. Given possible minor undersampling and the recent dropping of the hurricane's pressure, the initial advisory intensity is set at 105 kt. Sam continues to move northwest, or 315/8 kt. The hurricane is expected to continue this northwestward motion around a subtropical ridge over the next few days. By late Thursday or early Friday a turn toward the north is anticipated as Sam rounds the western periphery of the ridge. The cyclone should begin to gradually accelerate during that time in the increasing flow ahead of a mid- to upper-level trough emerging off the U.S. east coast later this week. Model guidance remains in very good agreement on this scenario and the latest NHC forecast track is little changed from the previous one, and lies near tightly clustered consensus track guidance. The southwesterly winds that brought in some dry air and disrupted Sam's impressive satellite appearance last night appear to have abated. As long as environmental winds surrounding the hurricane remain light, then the dry air surrounding the cyclone should not have as much of an impact on the hurricane's strength over the next few days. Based on the improving inner-core structure appearance of Sam, re-strengthening is forecast over the next 12 h. Thereafter, mainly minor fluctuations in intensity are indicated through 72 h. Beyond that time, increasing vertical wind shear and decreasing sea-surface temperatures should cause Sam to weaken. The latest NHC intensity forecast was nudged slightly upward through 36 h and is on the high end of the guidance. Thereafter, no changes were made, and that portion of the forecast is close to the various multimodel consensus solutions. 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 27/2100Z 16.3N 52.7W 105 KT 120 MPH 12H 28/0600Z 17.0N 53.5W 115 KT 130 MPH 24H 28/1800Z 17.8N 54.6W 115 KT 130 MPH 36H 29/0600Z 18.8N 55.8W 115 KT 130 MPH 48H 29/1800Z 19.8N 57.3W 110 KT 125 MPH 60H 30/0600Z 21.2N 59.0W 110 KT 125 MPH 72H 30/1800Z 22.9N 60.5W 110 KT 125 MPH 96H 01/1800Z 27.8N 62.0W 105 KT 120 MPH 120H 02/1800Z 34.7N 59.6W 100 KT 115 MPH $$ Forecaster Latto
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