Tropical Storm Helene Forecast Discussion

000
WTNT44 KNHC 250256
TCDAT4

Tropical Storm Helene Discussion Number   7
NWS National Hurricane Center Miami FL       AL092024
1000 PM CDT Tue Sep 24 2024

Helene continues to become better organized with increased
convective banding features and an overall more symmetrical-looking
cloud pattern.  The convection is very deep with cloud tops to -80
deg C or colder.  Doppler velocity data from a NOAA Hurricane
Hunter aircraft indicate no significant vertical tilt of the
vortex in the low- to mid-troposphere.  This suggests that the
vertical wind shear is low.  Earlier flight-level wind data from the
aircraft indicate the the maximum winds are near 50 kt, although
this may be conservative.

Although the storm has been wobbling over the past few hours,
center fixes from the NOAA Hurricane Hunter aircraft indicate a
generally west-northwestward motion at around 300/9 kt.  Helene
should turn northwestward soon as a high pressure area over Florida
shifts eastward, with the tropical cyclone center passing near the 
northeastern Yucatan Peninsula tomorrow morning.  Meanwhile, a 
mid-tropospheric trough is digging over the Lower Mississippi 
Valley.  This evolution of the steering flow should cause Helene to 
turn northward with an increase in forward speed during the next day 
or so.  An accelerating northward to north-northeastward motion over 
the eastern Gulf of Mexico should take the system to the 
northeastern Gulf of Mexico coast in about 48 hours.  After 
landfall, Helene should curve cyclonically around a mid-level low 
over the south-central United States.  The official forecast is very 
similar to the previous one and is on top of the latest corrected 
consensus track guidance.

Over the next couple of days, Helene will be moving through an 
environment of low vertical wind shear, ample environmental 
moisture, and over waters of high oceanic heat content. Thus, 
significant strengthening is anticipated before landfall on the 
northeast Gulf coast.  The NHC intensity forecast explicitly shows 
steady to rapid intensification (RI) of 25 kt for the next 24 hours 
and 30 kt for the 24- to 48-hour forecast interval.  This is in 
general agreement with the SHIPS RI indices.

Helene is predicted to grow to a very large size in the NHC
forecast.  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. Tropical Storm Helene will bring heavy rain to portions of the
western Caribbean with potentially significant flooding across
western Cuba and the northeastern Yucatan Peninsula.  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  25/0300Z 19.9N  85.5W   50 KT  60 MPH
 12H  25/1200Z 21.1N  86.3W   65 KT  75 MPH
 24H  26/0000Z 22.7N  86.4W   75 KT  85 MPH
 36H  26/1200Z 25.4N  85.5W   90 KT 105 MPH
 48H  27/0000Z 29.5N  84.2W  105 KT 120 MPH
 60H  27/1200Z 34.0N  84.5W   30 KT  35 MPH
 72H  28/0000Z 36.5N  86.5W   20 KT  25 MPH...POST-TROP/INLAND
 96H  29/0000Z 37.0N  89.0W   20 KT  25 MPH...POST-TROP/INLAND
120H  30/0000Z...DISSIPATED

$$
Forecaster Pasch