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| #1199978 (Received by flhurricane at: 4:57 AM 25.Sep.2024) TCDAT4 Tropical Storm Helene Discussion Number 8 NWS National Hurricane Center Miami FL AL092024 400 AM CDT Wed Sep 25 2024 Helen has strengthened some since the last advisory. Reports from an Air Force Reserve Hurricane Hunter aircraft indicate that the central pressure has fallen to 985 mb, and that the maximum 850 mb flight-level were 58 kt in the northeastern quadrant. A dropsonde in the northwest quadrant reported a splash wind of 59 kt, but the boundary-layer average suggested sustained winds closer to 45 kt. Based on these data and increasing satellite intensity estimates, the initial intensity is increased to 55 kt. Despite the increased intensity, the aircraft data, along with radar data from Mexico and Cuba, show that Helen has not yet formed a well-defined inner core. The initial motion is 325/8 kt. Helen should turn northward during the next 12 h or so on the western side of a mid-level ridge, and then it should accelerate northward to north-northeastward as it become embedded in the deep-layer flow between the ridge and a mid-latitude trough/developing cut-off low over the Mississippi valley. This motion should bring the center near the northeastern coast of the Yucatan Peninsula this morning, then across the eastern Gulf of Mexico tonight and Thursday to a landfall along the northeast coast of the Gulf of Mexico late Thursday or Thursday night. After landfall, Helene should curve cyclonically around the cut-off low until it dissipates. The new forecast track is little changed from the previous track. Over the next couple of days, Helene will be moving through an environment of low vertical wind shear, ample environmental moisture, and over waters of high oceanic heat content. All guidance forecasts steady to rapid intensification, including the RI indices associated with the SHIPS model. The latest forecast calls for a 105 kt intensity by 36 h, and this could be conservative as some of the guidance is stronger. After landfall, Helen is forecast to weaken and become post-tropical as it gets tangled up in the baroclinic system over the southeastern United States. Helene's wind field is predicted to grow to a very large size in the NHC forecast. Therefore storm surge, wind, and rainfall impacts will likely extend well away from the center and outside the forecast cone, particularly on the east side. In addition, the fast forward speed while Helene crosses the coast will likely result in farther inland penetration of strong winds over parts of the southeastern United States after landfall. KEY MESSAGES: 1. Helene is forecast to intensify and be near hurricane strength when it passes near the northeastern coast of the Yucatan Peninsula during the next several hours, where a Hurricane Warning is in effect. 2. Helene is expected to rapidly intensify and grow in size over the eastern Gulf of Mexico. There is a danger of life-threatening storm surge along the entire west coast of the Florida Peninsula and Florida Big Bend. The highest inundation levels are expected along the coast of the Florida Big Bend. Residents in those areas should follow advice given by local officials and evacuate if told to do so. 3. Damaging hurricane-force winds are expected along portions of coast of the Florida Big Bend, where a Hurricane Warning is now in effect. Preparations to protect life and property should be completed by early Thursday since tropical storm conditions are expected to begin within this area on Thursday. 4. Tropical Storm Helene will bring heavy rain to portions of the western Caribbean with potentially significant flooding across western Cuba and the northeastern Yucatan Peninsula. Considerable and potentially life-threatening flash and urban flooding is expected across portions of Florida, the Southeast, the Southern Appalachians, and the Tennessee Valley beginning today through Friday. This includes the risk of landslides across the southern Appalachians. Widespread minor to moderate river flooding is likely, and isolated major river flooding is possible. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 20.7N 86.2W 55 KT 65 MPH 12H 25/1800Z 21.9N 86.5W 70 KT 80 MPH 24H 26/0600Z 24.1N 86.2W 90 KT 105 MPH 36H 26/1800Z 27.4N 85.0W 105 KT 120 MPH 48H 27/0600Z 32.0N 84.2W 65 KT 75 MPH...INLAND 60H 27/1800Z 35.9N 85.4W 30 KT 35 MPH...POST-TROP/INLAND 72H 28/0600Z 37.0N 87.8W 20 KT 25 MPH...POST-TROP/INLAND 96H 29/0600Z 36.5N 88.0W 20 KT 25 MPH...POST-TROP/INLAND 120H 30/0600Z...DISSIPATED $$ Forecaster Beven |
