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| #1199937 (Received by flhurricane at: 11:00 PM 24.Sep.2024) TCDAT4 Tropical Storm Helene Discussion Number 7 NWS National Hurricane Center Miami FL AL092024 1000 PM CDT Tue Sep 24 2024 Helene continues to become better organized with increased convective banding features and an overall more symmetrical-looking cloud pattern. The convection is very deep with cloud tops to -80 deg C or colder. Doppler velocity data from a NOAA Hurricane Hunter aircraft indicate no significant vertical tilt of the vortex in the low- to mid-troposphere. This suggests that the vertical wind shear is low. Earlier flight-level wind data from the aircraft indicate the the maximum winds are near 50 kt, although this may be conservative. Although the storm has been wobbling over the past few hours, center fixes from the NOAA Hurricane Hunter aircraft indicate a generally west-northwestward motion at around 300/9 kt. Helene should turn northwestward soon as a high pressure area over Florida shifts eastward, with the tropical cyclone center passing near the northeastern Yucatan Peninsula tomorrow morning. Meanwhile, a mid-tropospheric trough is digging over the Lower Mississippi Valley. This evolution of the steering flow should cause Helene to turn northward with an increase in forward speed during the next day or so. An accelerating northward to north-northeastward motion over the eastern Gulf of Mexico should take the system to the northeastern Gulf of Mexico coast in about 48 hours. After landfall, Helene should curve cyclonically around a mid-level low over the south-central United States. The official forecast is very similar to the previous one and is on top of the latest corrected consensus track guidance. 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. Thus, significant strengthening is anticipated before landfall on the northeast Gulf coast. The NHC intensity forecast explicitly shows steady to rapid intensification (RI) of 25 kt for the next 24 hours and 30 kt for the 24- to 48-hour forecast interval. This is in general agreement with the SHIPS RI indices. Helene 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 it 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 on Wednesday, 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 complete 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 flash and urban flooding is expected across portions of Florida, the Southeast, southern Appalachians, and the Tennessee Valley Wednesday 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 25/0300Z 19.9N 85.5W 50 KT 60 MPH 12H 25/1200Z 21.1N 86.3W 65 KT 75 MPH 24H 26/0000Z 22.7N 86.4W 75 KT 85 MPH 36H 26/1200Z 25.4N 85.5W 90 KT 105 MPH 48H 27/0000Z 29.5N 84.2W 105 KT 120 MPH 60H 27/1200Z 34.0N 84.5W 30 KT 35 MPH 72H 28/0000Z 36.5N 86.5W 20 KT 25 MPH...POST-TROP/INLAND 96H 29/0000Z 37.0N 89.0W 20 KT 25 MPH...POST-TROP/INLAND 120H 30/0000Z...DISSIPATED $$ Forecaster Pasch |
