Tornado and Flash Flooding in Georgia, September 26, 2024

Area Forecast Discussion
National Weather Service Columbia SC
355 AM EDT Thu Sep 26 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 likely 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 /THROUGH TONIGHT/...
Key Messages through 7am Thursday:
- Rain chances increase from southwest to northeast.
- A thunderstorm or two cannot be ruled out though daybreak, mainly
in the CSRA.

It's a balmy night across the forecast area as tropical moisture
continues to stream northward out of the Gulf of Mexico ahead of an
approaching cold front. Much of the region has been dry so far
tonight with the majority of the rain passing west of the CWA.
However, this heavier rainfall is moving slowly eastward and is
starting to fall across the CSRA with a few showers further east in
the southwestern Midlands. Expect rain chances to rise from
southwest to northwest through the overnight period. Will need to
monitor for the potential of a few embedded thunderstorms tonight,
primarily in the CSRA. The latest SPC Mesoanalysis shows MLCAPE of
at least 500 J/kg across the entire FA with an area in excess of
1000 J/kg over much of the Midlands. This, combined with 0-6km wind
shear in excess of 40 knots, could promote thunderstorm development
despite the time of day. Lows tonight will be in the upper 60s to
lower 70s.

Key Messages Today into Tonight:
- Overall messaging in the near term is unchanged.
- Conditions deteriorate through the daytime hours as Hurricane
Helene moves north, passing to our west tonight.
- Tropical Storm Warning in effect for the entire CWA. Potential
gusty winds in excess of 40 mph, especially tonight.
- Tornado threat increases, peaking this evening into tonight.
- Flood Watch in effect this morning through Friday afternoon
  for the CSRA and southwestern Midlands. Areas of heavy
  rainfall could result in flash flooding, especially south and
  west of I-26.

As of the 2am NHC advisory, Hurricane Helene has maximum sustained
winds of 85 mph and is located in the southeastern Gulf of Mexico.
The current forecast track brings the hurricane ashore this evening
in the Florida Panhandle, rapidly moving northward through Western
Georgia tonight. Rain chances will continue to rise ahead of the
cyclone as tropical moisture and isentropic lift increase across the
region. The most significant impacts are expected this evening
continuing through the overnight hours. Potential hazards from
Helene include strong wind gusts, isolated tornadoes, heavy
rainfall, and localized flooding.

Wind: A Tropical Storm Warning is in effect for the entire CWA. The
hurricane has an unusually large wind field for a tropical
cyclone in the lower latitudes and will be transitioning to an
extratropical cyclone after landfall resulting in a tricky wind
forecast for the Midlands and CSRA. What we can say with higher
confidence is that the strongest winds are likely across the
CSRA and Southwestern Midlands which will be closer to the
cyclone center. NBM wind gust probabilities greater than of 40
mph are in excess of 60 percent across the FA and 90 percent
south and west of I-26. Winds of this strength have the
potential to produce localized wind damage including isolated to
scattered power outages.

Tornadoes: With Helene forecast to pass to our west, this places the
FA in the right-front quadrant of the cyclone which is a favorable
location for tropical tornadoes. The Day 1 SPC Outlook places the
majority of the CWA in a Slight (2/5) risk for severe weather and a
5 percent probability of a tornado. An Enhanced (3/5) risk, with a
10 percent tornado probability, is located to the southeast over the
Charleston CWA. The combination of strong 0-6 km wind shear and
increasing helicity will support tornado development in any
thunderstorms that develop. The greatest risk will come tonight in
rainbands associated with Helene. It is important to note that many,
but not all, tropical tornadoes develop quickly and are short lived
resulting in limited lead time. This threat is compounded by the
timing of greatest risk which is during the overnight hours.

Heavy Rainfall and Flash Flooding: A Flood Watch in effect this
morning through Friday afternoon for the CSRA and Southwestern
Midlands. The latest WPC Excessive Rainfall Outlook has the western
half of the FA in a Moderate (3/4) risk and a Slight (2/4) risk in
the east. Rainfall amounts are likely to be highest closest to the
track. NBM rainfall probabilities in excess of 2 inches line up well
with the existing Flood Watch, therefore no changes are being made
with this forecast package. Forecast rainfall amounts vary greatly
from east to west with the potential for 2-5" inches of rainfall
with locally higher amounts in the watch area.

&&

.SHORT TERM /FRIDAY through Saturday Night/...
Key Message(s):

- Very strong and gusty winds along with heavy rain continuing
  Friday morning
- Conditions improving with winds subsiding through the day
  Friday
- Remnants of Helene becomes absorbed by upper level low on
  Friday night and Saturday

Helene will continue marching northward through GA Friday
morning and then turn northwestward during the late morning and
afternoon hours. By Friday evening Helene is expected to be
along the central TN/KY border. At this point the system will be
caught up with a cutoff upper level low and become quasi-
stationary Friday night and Saturday before beginning to return
eastward Saturday night.

Wind: The time of concern will be through midday Friday. Winds
at sunrise Friday are expected to continue in the 25 to 30 mph
range with gusts 40 to 45 mph with the highest gusts across the
CSRA and central Midlands. Through mid morning winds will begin
slowly subsiding and by 11 am sustained winds will be 20 to 25
mph with gusts 30 to 35 mph. The slowing trend will continue
through the midday and afternoon hours as Helene pushes NW of
the forecast area with winds by mid afternoon around 15 mph with
gusts around 25 mph. With sunset Friday evening winds will
further diminish with around 10 mph and gusts to 15 mph Friday
night.

Rainfall: Heavy rainfall will also continue Friday morning
mainly across the across the central Midlands through the Pee
Dee. With Helene moving rapidly northwest of the area Friday
morning expect a short window for heavy rainfall with most of
the heavy rain moving north of the forecast area by midday. The
next concern with the rainfall will be rising rivers due to the
rainfall across the forecast area as well as heavier amounts
across the Upstate which will be moving through area rivers and
lakes.

Tornadoes: The tornado threat will also continue Friday morning
mainly for the Pee Dee area and a slightly lower degree in the
northern and central Midlands. Most of the central and northern
Midlands are in a marginal risk of severe weather from SPC which
has a 2 percent chance of a tornado and for the Pee Dee region
a slight risk which is a 5 percent chance of a tornado. Expect
the tornado threat to rapidly push northward and exit the
forecast area by midday.

Saturday and Saturday night: Helene will be stalled well
northwest of the area through the day Saturday then begin moving
eastward Saturday night. This will keep the area under partly
cloudy skies with drier air over the forecast area. Although the
weather concern Saturday and Saturday night will be minimal the
threat of flooding as water from the Upstate moves through the
hydrologic system will need to be monitored. High temperatures
Saturday afternoon will be in the low to mid 80s with overnight
lows in the mid to upper 60s.

&&

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

- Cutoff low hangs around through early next week

The remnants of Helene are likely to be completely phased with an
upper low centered over the Mid-Atlantic. This upper low along with
an attendant surface low are forecast to drift eastward through the
long-term period. The latest guidance is trending toward a more
drier few days with just a slight chance for afternoon showers in
the northern portions of the forecast area through Tuesday. The
trend in temperatures remains to be around or slightly cooler than
normal.

&&

.AVIATION /07Z THURSDAY THROUGH MONDAY/...
Conditions Deteriorate at the Terminals Today....

An area of rain and embedded thunderstorms is starting to move into
the CSRA early this morning with a few lighter returns elsewhere.
Rain chances will increase from southwest to northwest this morning
while ceilings continue to fall. MVFR ceilings are currently being
reported at CUB and the other terminals are likely to follow suit
before daybreak. Low ceilings and periodic visibility restrictions
will then persist for the remainder of the TAF period, especially in
areas of heavier rainfall. There is a thunderstorm risk as well but
confidence in timing is not high enough to warrant inclusion in the
TAFs at this time. East to southeast winds will generally be between
5 and 10 knots for much of the day, increasing this evening as
Hurricane Helene approaches. Restrictions are expected to be
widespread and persistent tonight with low ceilings, isolated to
scattered thunderstorms, heavy rainfall, and strong winds both
aloft and at the surface.

EXTENDED AVIATION OUTLOOK... Restrictions are possible into Friday
morning as 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 8 AM EDT this morning through Friday
     afternoon for SCZ018-025-030-035.
GA...Tropical Storm Warning for GAZ040-063>065-077.
     Flood Watch from 8 AM EDT this morning through Friday
     afternoon for GAZ040-063>065-077.

&&