Hurricane Helene Forecast Discussion

000
WTNT44 KNHC 252056
TCDAT4

Hurricane Helene Discussion Number  10
NWS National Hurricane Center Miami FL       AL092024
400 PM CDT Wed Sep 25 2024

A ragged eye is becoming apparent in visible satellite imagery and
has also been observed in recent radar data from Cancun and western
Cuba.  Based on the improved structure, the initial intensity is
set at 75 kt, which is at the high end of the satellite intensity
estimates.  This is also supported by the earlier Air Force Reserve
reconnaissance mission, which measured a peak 700-mb flight-level
wind of 81 kt.

Helene has turned northward (355/9 kt) and is expected to
accelerate toward the north or north-northeast during the next
24-36 hours as it moves across the Gulf of Mexico and crosses the
Florida Big Bend coast Thursday evening.  After landfall, the
system is forecast to interact with a deep-layer trough to the west
and turn northwestward with a decrease in speed over the Tennessee
Valley.

The environment appears primed for significant strengthening while
Helene moves across the Gulf of Mexico.  The forecast track more or
less runs along the axis of the Loop Current during the next 24
hours, where ocean temperatures are about 30 degrees Celsius.  In
addition, Helene will be moving through an environment of low
vertical shear and strong upper-level divergence.  Some of the
Rapid Intensification (RI) indices, particular DTOPS, respond to
this environment by indicating at least a 90 percent chance of a
35-kt increase in intensity over the next 24 hours.  The NHC
intensity forecast now shows an intensity of 115 kt (Category 4) at
24 hours, which is mirrored by several of the regional hurricane
models and the SHIPS guidance.  It should be noted that additional
strengthening is possible beyond 24 hours before Helene makes
landfall Thursday evening.

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.  A higher-than-normal gust factor has been
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 late Thursday.  Preparations to protect life and
property should be completed by early Thursday 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 late
Thursday and Thursday night 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 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 minor to
moderate river flooding is likely, and isolated major river flooding
is possible.

4. Hurricane and tropical storm conditions are expected over
northeastern portions of the Yucatan Peninsula of Mexico this
evening 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 this evening within the Hurricane Watch
area. 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.


FORECAST POSITIONS AND MAX WINDS

INIT  25/2100Z 22.5N  86.6W   75 KT  85 MPH
 12H  26/0600Z 24.1N  86.2W   95 KT 110 MPH
 24H  26/1800Z 27.4N  85.2W  115 KT 130 MPH
 36H  27/0600Z 32.1N  84.6W   70 KT  80 MPH...INLAND
 48H  27/1800Z 36.1N  86.0W   35 KT  40 MPH...POST-TROP/EXTRATROP
 60H  28/0600Z 37.1N  88.1W   25 KT  30 MPH...POST-TROP/EXTRATROP
 72H  28/1800Z 36.8N  88.7W   20 KT  25 MPH...POST-TROP/EXTRATROP
 96H  29/1800Z 37.0N  88.0W   15 KT  15 MPH...POST-TROP/EXTRATROP
120H  30/1800Z...DISSIPATED

$$
Forecaster Berg