Tropical Storm Helene Forecast Discussion

504 
WTNT44 KNHC 242058
TCDAT4

Tropical Storm Helene Discussion Number   6
NWS National Hurricane Center Miami FL       AL092024
500 PM EDT Tue Sep 24 2024

Deep convection is gradually filling in within Helene's
circulation, and the well-defined center that formed earlier this
morning is now obscured by cloudiness and showers.  Data from NOAA
buoy indicate that the central pressure has fallen to 995 mb, and
the initial intensity is therefore estimated to be 45 kt.  NOAA and
the Air Force Reserve Hurricane Hunters will be investigating Helene
this evening to provide more information about the storm's intensity
and structure.

With the center formation this morning, Helene has taken a
short-term jog to the west-northwest (300/10 kt).  The storm is
expected to turn northwestward by tonight and then northward on
Wednesday as high pressure over Florida shifts eastward, and a
deep-layer trough digs southward over the Lower Mississippi Valley.
The NHC track forecast has been shifted westward during the first
24 hours to account for the recent motion, and Helene's center
could get very close to the northeastern coast of the Yucatan
Peninsula Wednesday morning.  After that time, however, the NHC
track forecast is relatively unchanged from the previous
prediction, except for being a little bit slower based on the
latest guidance.  Helene is expected to accelerate while it moves
northward across the eastern Gulf of Mexico and approaches the
Florida Gulf coast.

Warm sea surface temperatures, decreasing shear, and strong
upper-level divergence are likely to foster Helene's strengthening
while it moves across the northwestern Caribbean Sea and eastern
Gulf of Mexico.  The statistical-dynamical SHIPS/LGEM models, as
well as the regional hurricane models, continue to show Helene
reaching major hurricane intensity while over the eastern Gulf of
Mexico, and that continues to be shown in the NHC forecast.  Helene
could maintain that level of intensity until it reaches the Gulf
coast of Florida.

Of equal importance to the forecast intensity is Helene's forecast
size.  Helene's forecast radii are at the 90th percentile of
major hurricane size at similar latitudes, and therefore 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 it crosses
the coast will likely result in farther inland penetration of strong
winds over parts of the southeastern United States after landfall.


KEY MESSAGES:

1. Helene is forecast to intensify and be near hurricane strength
when it passes near the northeastern coast of the Yucatan
Peninsula on Wednesday, where a Hurricane Warning is in effect.

2. Helene is expected to rapidly intensify and grow in size over
the eastern Gulf of Mexico.  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 levels are expected along
the coast of the Florida Big Bend.  Residents in those areas should
follow advice given by local officials and evacuate if told to do
so.

3. Damaging hurricane-force winds are expected along portions of
coast of the Florida Big Bend, where a Hurricane Warning is now in
effect.  Preparations to protect life and property should be
complete by early Thursday since tropical storm conditions are
expected to begin within this area on Thursday.

4. Helene will bring heavy rain to portions of the western Caribbean
with potentially significant flooding and mudslides across western
Cuba.  Considerable flash and urban flooding is expected across
portions of Florida, the Southeast, southern Appalachians, and the
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  24/2100Z 19.7N  84.7W   45 KT  50 MPH
 12H  25/0600Z 20.4N  86.0W   55 KT  65 MPH
 24H  25/1800Z 21.7N  86.6W   65 KT  75 MPH
 36H  26/0600Z 23.7N  86.1W   80 KT  90 MPH
 48H  26/1800Z 27.0N  84.9W  100 KT 115 MPH
 60H  27/0600Z 31.5N  84.0W   60 KT  70 MPH...INLAND
 72H  27/1800Z 35.5N  84.8W   30 KT  35 MPH...INLAND
 96H  28/1800Z 38.2N  89.0W   20 KT  25 MPH...POST-TROP/INLAND
120H  29/1800Z...DISSIPATED

$$
Forecaster Berg