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

HURRICANE HELENE FORECAST/ADVISORY NUMBER 12
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092024
0900 UTC THU SEP 26 2024

HURRICANE CENTER LOCATED NEAR 24.2N 86.2W AT 26/0900Z
POSITION ACCURATE WITHIN 20 NM

PRESENT MOVEMENT TOWARD THE NORTH-NORTHEAST OR 15 DEGREES AT 10 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 966 MB
EYE DIAMETER 25 NM
MAX SUSTAINED WINDS 80 KT WITH GUSTS TO 100 KT.
64 KT....... 50NE 50SE 30SW 0NW.
50 KT....... 80NE 80SE 40SW 40NW.
34 KT.......250NE 300SE 100SW 120NW.
12 FT SEAS..240NE 270SE 150SW 180NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

REPEAT...CENTER LOCATED NEAR 24.2N 86.2W AT 26/0900Z
AT 26/0600Z CENTER WAS LOCATED NEAR 23.6N 86.5W

FORECAST VALID 26/1800Z 26.8N 85.2W
MAX WIND 100 KT...GUSTS 120 KT.
64 KT... 50NE 50SE 30SW 20NW.
50 KT... 80NE 90SE 50SW 40NW.
34 KT...270NE 270SE 120SW 120NW.

FORECAST VALID 27/0600Z 31.6N 84.0W...INLAND
MAX WIND 75 KT...GUSTS 95 KT.
64 KT... 30NE 40SE 20SW 0NW.
50 KT... 60NE 80SE 50SW 40NW.
34 KT...270NE 270SE 140SW 70NW.

FORECAST VALID 27/1800Z 36.1N 84.9W...POST-TROP/EXTRATROP
MAX WIND 30 KT...GUSTS 50 KT.

FORECAST VALID 28/0600Z 37.0N 87.8W...POST-TROP/EXTRATROP
MAX WIND 25 KT...GUSTS 35 KT.

FORECAST VALID 28/1800Z 36.3N 87.8W...POST-TROP/EXTRATROP
MAX WIND 20 KT...GUSTS 30 KT.

FORECAST VALID 29/0600Z 36.3N 86.7W...POST-TROP/EXTRATROP
MAX WIND 20 KT...GUSTS 30 KT.

EXTENDED OUTLOOK. NOTE...ERRORS FOR TRACK HAVE AVERAGED NEAR 125 NM
ON DAY 4 AND 175 NM ON DAY 5...AND FOR INTENSITY NEAR 15 KT EACH DAY

OUTLOOK VALID 30/0600Z...DISSIPATED

REQUEST FOR 3 HOURLY SHIP REPORTS WITHIN 300 MILES OF 24.2N 86.2W

INTERMEDIATE PUBLIC ADVISORY...WTNT34 KNHC/MIATCPAT4...AT 26/1200Z

NEXT ADVISORY AT 26/1500Z

$$
FORECASTER BEVEN