Tropical Storm Helene Discussion Number   5
NWS National Hurricane Center Miami FL       AL092024
1100 AM EDT Tue Sep 24 2024

An Air Force Reserve Hurricane Hunter measured peak 925-mb
flight-level winds of 52 kt to the northeast of the center, which
supports an initial intensity of 40 kt.  Dropsonde data also
indicated that the central pressure is down to 1000 mb. Very 
recently, data from the plane and one-minute visible satellite 
imagery indicate that the center has become better defined. Based 
on these data, the system is being designated as Tropical Storm 
Helene at this time.

Helene is moving a little faster toward the northwest (310/10 kt) 
as it moves around a mid-level area of high pressure located over 
Florida and the Southeastern U.S.  The high is expected to slide 
eastward through Wednesday as a deep-layer trough digs southward 
over the Lower Mississippi Valley.  This pattern evolution should 
cause the system to turn northward and north-northeastward late 
Wednesday into Thursday.  The track guidance is very tightly 
clustered, which would normally imply high forecast confidence.  
However, depending on exactly where the center forms could end up 
shifting the entire guidance suite in future cycles, so it is 
imperative to not focus on specific landfall locations this far in 
the future.

Satellite trends suggest that the shear over the system is
beginning to decrease, and model guidance continues to show
relatively low to moderate shear for the next 48 hours or so.  In
addition, oceanic heat content values are very high, and the system
will be moving through an environment of upper-level divergence.
Therefore, significant strengthening is anticipated, and the NHC
intensity forecast shows the system reaching a peak intensity
around 100 kt in 48 hours while over the eastern Gulf of Mexico.
There could be some increase in shear around the time the system
reaches the coast, given the system's large size, it might only
weaken slowly.  As a result, there is still a risk that the system
could reach the coast as a major hurricane.

Helene's forecast radii are at the 90th percentile of hurricane
size at similar latitudes.  Due to the forecast large size of this
system, storm surge, wind, and rainfall impacts will extend well
away from the center, 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 reaches the far northwestern Caribbean Sea early Wednesday. 
Tropical storm conditions are expected over portions of western 
Cuba and the northeastern coast of the Yucatan Peninsula with 
hurricane conditions possible.

2. Helene is expected to rapidly intensify over the eastern
Gulf of Mexico and be a major hurricane when it approaches the
northeastern Gulf Coast on Thursday.  The risk of impacts from
life-threatening storm surge and damaging hurricane-force winds
continues to increase along the coast of the Florida Panhandle and
the Florida west coast.  Hurricane and Storm Surge Watches are in
effect for much of that area and residents in those areas should
follow advice given by local officials.

3. Helene will bring heavy rain to portions of the western 
Caribbean, which will cause considerable flooding and mudslides 
across western Cuba.  Heavy rainfall will likely result in locally 
considerable flash and urban flooding across portions of Florida, 
with isolated flash and urban flooding possible across the 
Southeast, Southern Appalachians, and the Tennessee Valley 
Wednesday through Friday. Minor to isolated moderate river flooding 
will be possible.


FORECAST POSITIONS AND MAX WINDS

INIT  24/1500Z 19.5N  84.3W   40 KT  45 MPH
12H  25/0000Z 20.3N  85.2W   50 KT  60 MPH
24H  25/1200Z 21.5N  86.3W   65 KT  75 MPH
36H  26/0000Z 23.2N  86.3W   80 KT  90 MPH
48H  26/1200Z 25.9N  85.4W  100 KT 115 MPH
60H  27/0000Z 29.7N  84.3W  100 KT 115 MPH
72H  27/1200Z 33.9N  83.9W   45 KT  50 MPH...INLAND
96H  28/1200Z 39.7N  86.8W   20 KT  25 MPH...POST-TROPICAL
120H  29/1200Z...DISSIPATED

$$
Forecaster Berg