Hurricane Helene Forecast Discussion

020 
WTNT44 KNHC 261454
TCDAT4

Hurricane Helene Discussion Number  13
NWS National Hurricane Center Miami FL       AL092024
1000 AM CDT Thu Sep 26 2024

Helene is gaining strength as it approaches the Florida Big Bend. A 
large eye has become apparent in satellite images, and that feature 
has been gradually clearing out over the past few hours. Convective 
banding is wrapping around the eye, and recent microwave images and 
reports from the NOAA and Air Force Hurricane Hunter aircraft 
indicate than an eyewall is almost completely closed.  The minimum 
pressure has been dropping steadily to near 960 mb, and based on the 
aircraft flight-level wind data, the initial intensity is estimated 
to have increased to 90 kt.  There have been several reports of 
tropical-storm-force winds occurring in portions of South Florida 
and the Florida Keys during the past few hours.

The large hurricane is picking up speed and has turned slightly to
the north-northeast.  The latest initial motion is 025/12 kt.  A
notably faster north-northeastward motion is expected through
landfall, which is expected to occur in the Florida Big Bend region
this evening.  After Helene moves inland, it is forecast to track
over Georgia and then slow down or stall over the Tennessee Valley
when it merges with a mid- to upper-level low pressure system.  Only
minor changes were made to the previous NHC track forecast.

Significant intensification is expected until Helene reaches the
coast since the overall atmospheric and oceanic conditions will
remain conducive.  The amount of strengthening will likely depend on
how much the inner core can contract and consolidate.  The HWRF and
HMON models show significant strengthening to category 4 status,
while most of the other models are a little lower.  The main message
is that Helene will likely make landfall as a large major hurricane
in the Florida Big Bend this evening, and all preparations in the
hurricane warning area should be rushed to completion.

It should also be re-emphasized that Helene is a very large
hurricane.  In fact, comparing the system with previous hurricanes
in the Gulf of Mexico over the past couple of decades, Helene is at
the upper bound in terms of size. As a result, storm surge, wind,
and rainfall impacts will extend far away from the center and well
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.  Accordingly, 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 this evening.  Preparations to protect life and
property should be rushed to completion.  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.


FORECAST POSITIONS AND MAX WINDS

INIT  26/1500Z 25.5N  85.5W   90 KT 105 MPH
 12H  27/0000Z 28.8N  84.3W  105 KT 120 MPH
 24H  27/1200Z 33.9N  84.5W   55 KT  65 MPH...INLAND
 36H  28/0000Z 36.4N  86.6W   30 KT  35 MPH...POST-TROP/EXTRATROP
 48H  28/1200Z 36.5N  87.5W   25 KT  30 MPH...POST-TROP/EXTRATROP
 60H  29/0000Z 36.3N  87.6W   20 KT  25 MPH...POST-TROP/EXTRATROP
 72H  29/1200Z 36.2N  87.2W   20 KT  25 MPH...POST-TROP/EXTRATROP
 96H  30/1200Z...DISSIPATED

$$
Forecaster Cangialosi