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| #1199847 (Received by flhurricane at: 11:03 AM 24.Sep.2024) TCDAT4 Tropical Storm Helene Discussion Number 5 NWS National Hurricane Center Miami FL AL092024 1100 AM EDT Tue Sep 24 2024 An Air Force Reserve Hurricane Hunter measured peak 925-mb flight-level winds of 52 kt to the northeast of the center, which supports an initial intensity of 40 kt. Dropsonde data also indicated that the central pressure is down to 1000 mb. Very recently, data from the plane and one-minute visible satellite imagery indicate that the center has become better defined. Based on these data, the system is being designated as Tropical Storm Helene at this time. Helene is moving a little faster toward the northwest (310/10 kt) as it moves around a mid-level area of high pressure located over Florida and the Southeastern U.S. The high is expected to slide eastward through Wednesday as a deep-layer trough digs southward over the Lower Mississippi Valley. This pattern evolution should cause the system to turn northward and north-northeastward late Wednesday into Thursday. The track guidance is very tightly clustered, which would normally imply high forecast confidence. However, depending on exactly where the center forms could end up shifting the entire guidance suite in future cycles, so it is imperative to not focus on specific landfall locations this far in the future. Satellite trends suggest that the shear over the system is beginning to decrease, and model guidance continues to show relatively low to moderate shear for the next 48 hours or so. In addition, oceanic heat content values are very high, and the system will be moving through an environment of upper-level divergence. Therefore, significant strengthening is anticipated, and the NHC intensity forecast shows the system reaching a peak intensity around 100 kt in 48 hours while over the eastern Gulf of Mexico. There could be some increase in shear around the time the system reaches the coast, given the system's large size, it might only weaken slowly. As a result, there is still a risk that the system could reach the coast as a major hurricane. Helene's forecast radii are at the 90th percentile of hurricane size at similar latitudes. Due to the forecast large size of this system, storm surge, wind, and rainfall impacts will extend well away from the center, 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 reaches the far northwestern Caribbean Sea early Wednesday. Tropical storm conditions are expected over portions of western Cuba and the northeastern coast of the Yucatan Peninsula with hurricane conditions possible. 2. Helene is expected to rapidly intensify over the eastern Gulf of Mexico and be a major hurricane when it approaches the northeastern Gulf Coast on Thursday. The risk of impacts from life-threatening storm surge and damaging hurricane-force winds continues to increase along the coast of the Florida Panhandle and the Florida west coast. Hurricane and Storm Surge Watches are in effect for much of that area and residents in those areas should follow advice given by local officials. 3. Helene will bring heavy rain to portions of the western Caribbean, which will cause considerable flooding and mudslides across western Cuba. Heavy rainfall will likely result in locally considerable flash and urban flooding across portions of Florida, with isolated flash and urban flooding possible across the Southeast, Southern Appalachians, and the Tennessee Valley Wednesday through Friday. Minor to isolated moderate river flooding will be possible. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 19.5N 84.3W 40 KT 45 MPH 12H 25/0000Z 20.3N 85.2W 50 KT 60 MPH 24H 25/1200Z 21.5N 86.3W 65 KT 75 MPH 36H 26/0000Z 23.2N 86.3W 80 KT 90 MPH 48H 26/1200Z 25.9N 85.4W 100 KT 115 MPH 60H 27/0000Z 29.7N 84.3W 100 KT 115 MPH 72H 27/1200Z 33.9N 83.9W 45 KT 50 MPH...INLAND 96H 28/1200Z 39.7N 86.8W 20 KT 25 MPH...POST-TROPICAL 120H 29/1200Z...DISSIPATED $$ Forecaster Berg |
