25/09/2024
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
pe*******on 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 pe*****te 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
