000 WTNT45 KNHC 220259 TCDAT5 Hurricane Tammy Discussion Number 15 NWS National Hurricane Center Miami FL AL202023 1100 PM AST Sat Oct 21 2023 Air Force and NOAA Hurricane Hunter aircraft data, along with radar imagery from Guadeloupe, indicate that the center of Tammy passed over the island of Barbuda a couple of hours ago. The hurricane continues to produce intense convection in a small CDO feature with some ill-defined bands north and northeast of the center. The eyewall has generally not been closed on the radar images. There are still some areas of strong convection affecting portions of the Lesser Antilles to the south of Tammy. Flight-level and SFMR winds along with Doppler velocity data suggested that the maximum winds had decreased slightly, but there was an unofficial report of a sustained wind of 78 kt from Barbuda. Based on a blend of the aircraft data and the Barbuda observation, the intensity is held at 75 kt for this advisory. Center fixes indicate that the hurricane continues on its north-northwestward trek with an estimated motion of 330/9 kt. Over the next couple of days, Tammy should turn northward while it moves along the western side of a large subtropical high. Then, the system should turn, at least temporarily, northeastward on the southeastern periphery of a mid-tropospheric trough over the western Atlantic. Beginning around 3 days, the track forecast becomes challenging, since the global models indicate that the western Atlantic trough will bypass Tammy after 72 hours while it continues eastward. A ridge could then build in to the northwest of the system and cause it to turn to the left. As noted earlier, there is a very large spread in the track guidance in the latter part of the forecast period. There is low confidence in the 4- and 5-day NHC forecast positions. Tammy should remain over very warm waters with moderate vertical wind shear for the next couple of days. So, some slight strengthening is still forecast. By 60 or 72 hours, increasing shear is likely to induce a weakening trend. The official intensity forecast is similar to the latest Decay-SHIPS guidance. KEY MESSAGES: 1. Hurricane and tropical storm conditions are expected in portions of the northern Leeward Islands through early Sunday. 2. The heaviest rains from Tammy will continue over the Leeward Islands through Sunday. This rainfall may produce isolated flash and urban flooding, along with isolated mudslides in areas of higher terrain. 3. A storm surge could produce coastal flooding in areas of onshore winds as the center of Tammy moves near or over the Leeward Islands. Near the coast, the surge will be accompanied by large and dangerous waves. FORECAST POSITIONS AND MAX WINDS INIT 22/0300Z 17.8N 61.9W 75 KT 85 MPH 12H 22/1200Z 18.8N 62.5W 75 KT 85 MPH 24H 23/0000Z 20.2N 63.4W 80 KT 90 MPH 36H 23/1200Z 21.5N 63.9W 80 KT 90 MPH 48H 24/0000Z 22.6N 63.8W 80 KT 90 MPH 60H 24/1200Z 23.1N 63.6W 80 KT 90 MPH 72H 25/0000Z 23.5N 63.0W 75 KT 85 MPH 96H 26/0000Z 25.0N 62.5W 70 KT 80 MPH 120H 27/0000Z 27.5N 63.5W 60 KT 70 MPH $$ Forecaster Pasch