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| #1158186 (Received by flhurricane at: 5:02 PM 20.Oct.2023) TCDAT5 Hurricane Tammy Discussion Number 10 NWS National Hurricane Center Miami FL AL202023 500 PM AST Fri Oct 20 2023 Moderate shear that is likely undercutting the higher-level outflow layer has disrupted the inner-core development seen earlier today. The eye that became apparent in radar imagery from Barbados this morning has degraded since that time, but there is still a large curved band that wraps around the eastern and southern portions of the circulation. The Air Force aircraft that investigated Tammy through early afternoon measured SFMR winds of around 65 kt during its final pass through the center, and it reported that the minimum pressure had fallen to around 991 mb. Although there has been the recent degradation of the inner core, the initial intensity remains 65 kt, and is based on the earlier aircraft data and more recent subjective satellite estimates from TAFB and SAB. The next reconnaissance mission into Tammy is scheduled for this evening. Very recent radar imagery from Barbados suggest that Tammy may finally be making its anticipated northwestward turn, however the long-term motion is still west-northwestward about 6 kt. A more pronounced northwestward motion should begin very soon as a trough moving off the east coast of the United States erodes the western portion of a subtropical ridge over the central Atlantic. The expected northwestward motion should bring the center of Tammy near or over portions of the Leeward Islands later tonight and Saturday. While the numerical model guidance is in relatively good agreement during the first 48-72 hours of the forecast period, the spread has greatly increased after that time. The GFS shows a stronger and more vertically deep cyclone recurving over west-central Atlantic after day 3, while the latest UKMET and ECMWF models depict a weaker cyclone that is left behind by an eastward-moving trough over the western Atlantic. The NHC forecast continues to predict recurvature, although the latter portion has been adjusted slower once again. The environment ahead of Tammy is not predicted to change much during the next day or so. Warm waters and moderate shear are expected to allow for some modest strengthening during the next couple of days, and the NHC intensity forecast during that time is similar to the previous advisory. After Tammy moves north of the Leeward Islands, increasing vertical wind shear is likely to initiate weakening as Tammy moves northward. The NHC intensity forecast is close to the IVCN intensity aid. Due to the differences in how fast Tammy recurves, there is more uncertainty than normal on when Tammy will begin extratropical transition. The current NHC forecast maintains Tammy as a hurricane through day 5, but if it trends toward the faster side of the guidance it could be post-tropical by that time. KEY MESSAGES: 1. Hurricane conditions are expected in portions of the Leeward Islands by late tonight through Saturday. Tropical storm conditions are expected to begin in portions of the Lesser Antilles within the warning area this evening or overnight. 2. Heavy rains from Tammy will begin to affect the Leeward and northern Windward Islands tonight and Saturday spreading into the British and U.S. Virgin Islands and Puerto Rico as early as 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 20/2100Z 14.3N 59.2W 65 KT 75 MPH 12H 21/0600Z 15.3N 60.1W 65 KT 75 MPH 24H 21/1800Z 16.7N 61.2W 70 KT 80 MPH 36H 22/0600Z 18.1N 62.1W 75 KT 85 MPH 48H 22/1800Z 19.5N 63.0W 80 KT 90 MPH 60H 23/0600Z 20.8N 63.4W 80 KT 90 MPH 72H 23/1800Z 22.0N 63.4W 75 KT 85 MPH 96H 24/1800Z 24.5N 61.7W 70 KT 80 MPH 120H 25/1800Z 28.4N 57.7W 65 KT 75 MPH $$ Forecaster Brown |
