Hurricane Helene Forecast Discussion

000
WTNT44 KNHC 260852
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