950
FXUS62 KCAE 252339
AFDCAE

Area Forecast Discussion
National Weather Service Columbia SC
739 PM EDT Wed Sep 25 2024

.SYNOPSIS...
Rain chances increase tonight as tropical moisture is advected
into the forecast area. Hurricane Helene is forecast strengthen
into a major hurricane in the eastern Gulf of Mexico as it
approaches the Florida Panhandle region. Widespread rain and
severe weather will become increasingly possible across the area
by Thursday night. Uncertainty remains in the exact track and
impacts of the system. However, confidence in impacts is
increasing with tropical storm force winds the main threat. A
few tornadoes will be possible Thursday night along with flash
flooding. The storm is expected to move west and north of the
area Friday with diminishing winds.

&&

.NEAR TERM /UNTIL 7 AM THURSDAY MORNING/...
Key Message(s):

- Rain chances increase tonight

Moisture advection increases over the region tonight allowing
showers to persist through the overnight period. In addition, a
cold front across northern Ga will move slowly eastward,
increasing rain chances across the western Midlands and CSRA
overnight. Weak instability will remain over the area through
the near-term. With 30 to 40 kts of effective shear over the
region there will be a few organized cells but with limited
instability the threat of severe weather is low. Late tonight
and early Thursday morning, additional moisture and rainfall
will begin moving into the area from the south, bringing more
widespread rainfall to the region by daybreak. Low temperatures
will be in the lower 70s.

&&

.SHORT TERM /7 AM THURSDAY MORNING THROUGH FRIDAY/...
Key Message(s):

- Impacts from Helene begin to affect the region Thursday.
- Tropical storm force wind gusts are the main threat.
- Nocturnal tornadoes possible.
- Potential for flash flooding especially southwest of I-26.
- Most significant impacts expected Thursday evening through
early Friday morning.

Confidence continues to increase that the region will be
impacted by Hurricane Helene. Helene is forecast to become a
major hurricane in the eastern Gulf of Mexico by early Thursday
morning. The storm is forecast to move toward the Florida
Panhandle during Thursday afternoon then move inland across
western Georgia as a deep upper low moves into the southern
Mississippi Valley. Moisture flux and isentropic lift appears
to develop early Thursday in east-central Georgia, so pops will
be increasing through the morning. Locally heavy rain/bands will
be possible especially later in the day. Categorical pops in
the CSRA in the morning with showers spreading north.

The most significant impacts are expected Thursday evening
through the overnight as the pressure gradient tightens
significantly. Severe weather possible especially Thursday
night. Flash flooding possible especially in the CSRA. Of
concern is the latest 12z deterministic GFS/ECMWF which also
suggest an increasing threat of significant impacts.

Wind:
NBM wind gusts tropical storm force probabilities are quite high
(>70%) over the central Midlands and CSRA Thursday night. MOS
guidance also appears unusably strong with winds. As the cyclone
becomes a hybrid, wind field will likely expand. A strong low-
level jet (850mb) is noted in the deterministic guidance to the
east of the cyclone. Model soundings show the potential for
strong gusty winds with 50+ knots just off the surface.

Tornadoes:
A few nocturnal tornadoes are possible. The model soundings and
HREF data indicate strong shear with impressive looking
hodographs/SR helicity. HREF CAPE around 500 J/kg maybe higher
in the east. In general, SR helicity values max out around
500-700 m2/s2 with the different model solutions.

Heavy rain:
Moisture from Helene will increase through the day on Thursday,
bringing PWATs to around 2.5 inches or greater. This will lead
to the heavy rain threat, especially overnight as the center
nears the forecast area. The heaviest rain threat is currently
over the CSRA into the western Midlands, with an even greater
threat in the Upstate. As with many tropical systems, a sharp
rainfall gradient is likely to set up, which looks to be along
the I-26 corridor as of the latest forecast. The location of the
gradient is hard to pin down at this point, so it could change.
Total rainfall amounts range roughly 2-5", with locally higher
amounts possible. If the track is a little further east, the
heavy rain threat will increase. So, heavy rain threat still
uncertain.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Key Message(s):

- Helene becomes absorbed by upper level low on Friday night and
  Saturday
- Cutoff low hangs around through early next week

Helene is expected to get absorbed by the upper low. Then the low
meanders around the area for a few days. As a result, daily chances
of precipitation continue for much of the period. Temperatures are
expected to be seasonal through the time period.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Mainly vfr conditions outside of any afternoon convection through
early tonight.  Increasing likelihood of mvfr/ifr conditions
developing late tonight through Thursday.

Atlantic moisture will continue to push inland through this evening,
with isolated to scattered showers and thunderstorms developing
across the area.  Even with the moisture increase, ceilings and
visibilities remaining mainly vfr at all sites into early tonight.
Deteriorating conditions then expected to develop late tonight as
deeper moisture and rainfall begins pushing into the area from the
south.  Ceilings begin dropping into mvfr between 09-13z, then into
ifr after 15z with increasing rainfall.

Winds this afternoon remaining gusty up to 20 knots, then drop town
to between 5-10 knots after sunset.  There will be a lot level jet
around 30 knots at 2kft developing late tonight, so if winds near
the surface drop too much, llws may become an issue.  Wind gusts
may pick back up again later Thursday morning.

EXTENDED AVIATION OUTLOOK... Restrictions are possible into Friday
as Hurricane Helene passes west of the forecast area. Low ceilings,
heavy rainfall, and strong winds are possible.

&&

.CAE WATCHES/WARNINGS/ADVISORIES...
SC...Tropical Storm Warning for SCZ016-018-020>022-025>031-035-037-
     038-041-115-116-135>137.
     Flood Watch from Thursday morning through Friday afternoon for
     SCZ018-025-030-035.
GA...Tropical Storm Warning for GAZ040-063>065-077.
     Flood Watch from Thursday morning through Friday afternoon for
     GAZ040-063>065-077.

&&

$$