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| #1200206 (Received by flhurricane at: 4:57 AM 26.Sep.2024) TCDAT4 Hurricane Helene Discussion Number 12 NWS National Hurricane Center Miami FL AL092024 400 AM CDT Thu Sep 26 2024 Helene is sending some mixed structural signals this morning. On one side, an Air Force Reserve Hurricane Hunter aircraft reports that the central pressure has fallen to near 966 mb, and satellite imagery shows the eye is trying to clear out. On another side, the aircraft reported concentric eyewalls of 16 and 32 n mi diameter, with the maximum flight-level winds, 80 kt at 700 mb, occurring about 50 n mi southeast of the center likely in relation to the outer eyewall. Based on the central pressure fall and the available data, the initial intensity is increased to 80 kt. The hurricane is starting its expected turn toward the north-northeast, with the initial motion now 015/10. The hurricane should accelerate north-northeastward during the next 24 h or so in the flow between a ridge over the Atlantic and a deep-layer baroclinic trough/cut-off low over the Tennessee Valley. This motion should bring the center of Helene to the coast of the Florida Big Bend region Thursday evening or early Friday morning. After landfall, Helene should turn northward and northwestward around the northeast and north side of the Tennessee Valley system. After 48 h, what is left of Helene should make a cyclonic loop as it merges with the baroclinic system. The latest track guidance has nudged a little to the northeast, especially near 36 h, and the portion of the new forecast track over Georgia and Tennessee has been moved a bit to the east. Helene remains in an atmospheric and oceanic environment over the eastern Gulf of Mexico that is very conducive for strengthening, with the system forecast to traverse the Loop Current while it remains in a moist and low-shear environment. The main restraining factor on intensification is the current concentric eyewall status. All of the intensity guidance continues to forecast strengthening until landfall. Based on this, the new intensity forecast calls for Helen to become a major hurricane in about 12 h, and additional intensification is expected in the ensuing 6 h period or so before landfall. After landfall, Helene should weaken and undergo extratropical transition as it merges with the baroclinic system over the Tennessee Valley, and this is expected to be complete by about 36 h. The remnant extratropical low is forecast to dissipate after 72 h based on global model forecasts. 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 extend well away from the center and outside the forecast cone, particularly on the east side. In addition, the fast forward speed when Helene moves inland will result in a far inland penetration of strong winds over parts of the southeastern United States, including strong gusts over higher terrain of the southern Appalachians. Indeed, the GFS and ECMWF models suggest the possibility that the inner wind core may persist until the center reaches northern Georgia. A higher-than-normal gust factor is indicated in the official forecast while Helene is inland. KEY MESSAGES: 1. A catastrophic and deadly storm surge is likely along portions of the Florida Big Bend coast, where inundation could reach as high as 20 feet above ground level, along with destructive waves. There is also a danger of life-threatening storm surge along the remainder of the west coast of the Florida Peninsula. Residents in those areas should follow advice given by local officials and evacuate if told to do so. 2. Potentially catastrophic hurricane-force winds are expected within the eyewall of Helene when it makes landfall in the Florida Big Bend region later today. Preparations to protect life and property should be completed this morning before tropical storm conditions arrive. Damaging and life-threatening hurricane-force winds, especially in gusts, will penetrate well inland over portions of northern Florida and southern Georgia later today and tonight where Hurricane Warnings are in effect. Strong wind gusts are also likely farther north across portions of northern Georgia and the Carolinas, particularly over the higher terrain of the southern Appalachians. 3. Catastrophic and life-threatening flash and urban flooding, including numerous significant landslides, is expected across portions of the southern Appalachians through Friday. Considerable to locally catastrophic flash and urban flooding is likely for northwestern and northern Florida and the Southeast through Friday. Widespread significant river flooding and isolated major river flooding are likely. 4. Tropical storm conditions are expected over portions of western Cuba within the Tropical Storm Warning area. Hurricane Helene will bring heavy rain to portions of the western Caribbean with potentially significant flooding across western Cuba and the northeastern Yucatan Peninsula through this morning. FORECAST POSITIONS AND MAX WINDS INIT 26/0900Z 24.2N 86.2W 80 KT 90 MPH 12H 26/1800Z 26.8N 85.2W 100 KT 115 MPH 24H 27/0600Z 31.6N 84.0W 75 KT 85 MPH...INLAND 36H 27/1800Z 36.1N 84.9W 30 KT 35 MPH...POST-TROP/EXTRATROP 48H 28/0600Z 37.0N 87.8W 25 KT 30 MPH...POST-TROP/EXTRATROP 60H 28/1800Z 36.3N 87.8W 20 KT 25 MPH...POST-TROP/EXTRATROP 72H 29/0600Z 36.3N 86.7W 20 KT 25 MPH...POST-TROP/EXTRATROP 96H 30/0600Z...DISSIPATED $$ Forecaster Beven |
