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| #1200029 (Received by flhurricane at: 11:00 AM 25.Sep.2024) TCDAT4 Hurricane Helene Discussion Number 9 NWS National Hurricane Center Miami FL AL092024 1000 AM CDT Wed Sep 25 2024 Air Force Reserve and NOAA Hurricane Hunter aircraft have been investigating Helene, with each plane recently measuring peak flight-level winds of 81 kt and 78 kt, respectively. Helene has therefore become a hurricane with an estimated intensity of 70 kt. Dropsonde data also indicate that the pressure has fallen to about 979 mb. Radar data from Mexico and Cuba, as well as reconnaissance reports, indicate that Helene has formed a partial, elliptical eyewall that is open on the east side. Helene has turned north-northwestward (330/9 kt) and is expected to turn northward and north-northeastward later today and tonight, bringing the center to the coast of the Florida Big Bend Thursday evening. After landfall, Helene is expected to interact with a deep-layer trough over the Lower Mississippi Valley and swing back to the northwest and stall near the Tennessee Valley late Friday into the weekend. The track guidance remains tightly clustered, and the NHC forecast is generally just an update of the previous prediction. It is still too soon at this point to be overly focused on an exact landfall location and time, since NHC track forecasts can be off by an average of 60 nm at the 36-hour forecast time. Helene is expected to move through/over an environment of relatively low shear, strong upper-level divergence, and sea surface temperatures of 29-31 degrees Celsius, all of which should foster additional strengthening. Rapid Intensification (RI) indices indicate a high chance of RI during the next 24 hours, and as a result the NHC intensity forecast shows Helene becoming a major hurricane by Thursday morning. There is still some uncertainty on exactly how strong Helene will get, and upward adjustments to the forecast intensity could be required in subsequent advisories if Helene rapidly intensifies more than forecast. Regardless, Helene is forecast to be a large major hurricane when it reaches the Big Bend coast of Florida. As a result, 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, including strong gusts over higher terrain of the southern Appalachians. KEY MESSAGES: 1. Hurricane and tropical storm conditions are expected over northeastern portions of the Yucatan Peninsula of Mexico today where Tropical Storm and Hurricane Warnings are in effect. Tropical storm conditions are occurring over portions of western Cuba within the Tropical Storm Warning area, and hurricane conditions are possible today within the Hurricane Watch area. 2. Due to the large size of Helene, 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 of greater than 10 ft is expected along the Florida Big Bend coast. Residents in those areas should follow advice given by local officials and evacuate if told to do so. 3. Devastating hurricane-force winds are expected across portions of northern Florida and southern Georgia where the core of Helene moves inland. Preparations to protect life and property should be completed by early Thursday since tropical storm conditions are expected to begin within these areas on Thursday. Because of Helene’s expected fast forward speed, damaging and life-threatening wind gusts, are expected to penetrate well inland over portions of the southeastern United States, including in the higher terrain of the southern Appalachians. 4. Helene will bring heavy rain to portions of the western Caribbean with potentially significant flooding across western Cuba and the northeastern Yucatan Peninsula into early Thursday. Considerable and potentially life-threatening flash and urban flooding is expected across portions of northwestern and northern Florida, the Southeast, southern Appalachians, and the Upper 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 25/1500Z 21.6N 86.3W 70 KT 80 MPH 12H 26/0000Z 23.0N 86.4W 85 KT 100 MPH 24H 26/1200Z 25.7N 85.7W 105 KT 120 MPH 36H 27/0000Z 29.7N 84.6W 110 KT 125 MPH 48H 27/1200Z 34.3N 85.0W 50 KT 60 MPH...INLAND 60H 28/0000Z 36.6N 86.9W 30 KT 35 MPH...POST-TROP/INLAND 72H 28/1200Z 36.9N 87.8W 20 KT 25 MPH...POST-TROP/INLAND 96H 29/1200Z 36.6N 87.9W 15 KT 15 MPH...POST-TROP/INLAND 120H 30/1200Z...DISSIPATED $$ Forecaster Berg |
