753
FXUS62 KCHS 270014
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
814 PM EDT Thu Sep 26 2024

.SYNOPSIS...
Hurricane Helene is forecast to move inland across portions of
Florida and Georgia later this evening into Friday. Multiple
impacts are expected across our area through tonight. Drier
weather then arrives later Friday and persists into next week.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Early evening update: Status quo at the moment. Initial batch
of widespread precip continues to work up through the tri-county
area with a lull in precip further south down through southeast
Georgia. But...next round of showers/thunderstorms will be
spreading northward across the forecast area through the evening
hours. Tornado parameters will remain favorable for a good
portion of the night and the going tornado watch will more than
likely be extended through the overnight hours.

Previous discussion...
Tonight: the upper air pattern features a large cutoff low in
the vicinity of the Middle Mississippi Valley area, while a
strong anticyclone is across the western Atlantic, centered west
of Bermuda. The southerly flow between these two features will
steer Helene north or north-northeast through the night, as she
makes landfall as a Category 4 hurricane in the vicinity of
Apalachee Bay and the Florida Big Bend later this evening.
After landfall, a turn to the north over Georgia is expected
late tonight. Helene will likely still be a hurricane as it
moves through Georgia, even as it draws closer to Atlantic by
daybreak.

Given the speed of Helene, strong winds are expected further
inland than typical. Additionally, wind fields will expand as
Helene gains latitude, producing impacts well east of its
center. For this reason, a Tropical Storm Warning remains in
effect for the entire forecast area through the night, including
across nearby coastal waters. The latest forecast will keep
Southeast Georgia and Southeast South Carolina along Helene`s
eastern side throughout the night, bringing multiple hazards to
the immediate area. For full details, please refer to
weather.gov/chs and the Hurricane Local Statement (HLS).

Tornadoes: A Tornado Watch continues for all counties and the
adjacent coastal waters through 9 PM. Another watch will likely
be issued beyond that time, as conditions remain favorable for
scattered tornadoes to occur. Long, looping hodographs will
occur in advance of Helene, most especially as Helene moves
onshore during tonight. 0-1 km SRH is forecast to reach as much
as 250-400 m2/s2, with impressive STP and SCP values. There is
also evidence of dry air wrapping around Helene, which would
enhance any tornadic risk. Additionally, the latest HREF run
and recent runs of the WoFS (Warn on Forecast System) gives a
fairly strong indication of updraft helicity tracks supportive
of tornadic waterspouts that shift onshore, coinciding with
rainbands associated with Helene through the night. The latest
SPC Day 1 Convective Outlook does include an Enhanced Risk for
severe weather across the majority of the local forecast area,
with 10% tornado probabilities driving the bulk of the risk.
While any tornado is a dangerous situation, those that occur at
night, when most people are sleeping, is an even worse
situation. Have a tornado plan in place in case a warning is
issued. But keep in mind that tornadoes of this type can form
with little advance warning, move very quickly (potentially in
excess of 35 to 45 mph), and will be hard to see due to the
night time conditions and the potential that they will be rain-
wrapped.

Winds: A Tropical Storm Warning is in effect for the entire forecast
area. Tropical storm force winds, mainly in frequent gusts of 45 to 65
mph, are forecast to increase across Southeast Georgia and the
adjacent Atlantic Georgia waters this evening, then expand into
Southeast South Carolina and the adjacent coastal waters tonight.
Wind speeds should peak late tonight as the center of Helene moves
across central Georgia. Based on the 5 PM NHC advisory, the Most
Likely Arrival of these winds is prior to midnight in Georgia,
and after midnight across South Carolina. Gusty winds as high as
40 to 60 mph could produce downed and/or uprooted trees,
isolated to scattered power outages locally, with greatest
impacts anticipated during the night.

Flooding Rainfall: Multiple rounds of showers/rainbands and a
few t-storms in association with Helene will impact the
area through the night, before considerably drier air wraps
around the southern edge of departing Helene and overtakes the
entire forecast area by around daybreak Friday. Additional
rainfall of 1.5 to 4 inches will occur, but with locally higher
amounts possible in heavy rainbands. Minor flooding of low-
lying and poor drainage areas is likely in some locations with
isolated flash flooding possible. Urban flooding is also a
possibility. The risk for flooding may become locally enhanced
near and at the coast during the time of high tide. Radar
estimates and reports from gages show a swath of some 3-6 inch
amounts since early Wednesday from parts of Chatham and Jasper
County to parts of Colleton, Hampton, and Allendale County. Area
rivers and creeks will rise in response to the rainfall, with
some flooding possible, even after the rains come to an end.
Recent trends show that the drier air will encompass the
southern half of South Carolina and Georgia early on, before it
spreads north across the Charleston tri-county. However, more
widespread rains will spread back north as Helene makes landfall
and heads north during the late evening and overnight.

Storm Surge: The latest forecast has storm surge values in the 1 to
3 foot range. As such, coastal flood products might be required at a
later time. Additional information can be found in the Tides/Coastal
Flooding section below.

Temperatures: It`ll be a very warm and humid night, and we could
be close to record high minimums for September 26.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/...
Broad cutoff low will be in place across the south/southeast U.S.
Friday morning with the circulation of Helene (forecast to still be
a TS Friday morning) migrating up through northwestern Georgia and
becoming absorbed by the larger scale cutoff. Strong subsidence and
dry air wrapping along the southern/southeastern side of the broader
circulation should lead to rather quick ending of precip, early to
mid Friday morning, and there will likely be a fairly quick S-N
decrease in cloud cover with skies trending mainly sunny by
afternoon. But augmented by clearing and subsequent mixing, tropical
storm force gusts will linger through the morning hours with gusts
of 40 to 50 mph likely, diminishing to 20 to 30 mph through the
afternoon hours.

Otherwise, in the wake of Helene, warm temperatures and somewhat
humid conditions will persist on Friday with highs in the middle to
upper 80s and dewpoints in the lower to middle 70s, and peak heat
index values in the upper 80s to middle 90s. Not great for those
that may be without power.

Through the weekend: Remnant low of Helene will continue to get
absorbed into the larger scale cutoff low with the combined larger
scale circulation center remaining somewhat fixed over the
Kentucky/Tennessee region into Sunday. System is expected to then
weaken and elongate to the mid Atlantic into early next week. Across
our area, we stay on the warm (mid to upper 80s for highs) and
somewhat humid side through the weekend, although there may be a
subtle decrease in dewpoints/heat index values for Saturday and
Sunday.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As mentioned, vertically stacked system will continue to weaken and
elongate to the mid Atlantic coast early next week before finally
moving off the coast by midweek. Weak low pressure lingers across
the region through the early part of the week and could lead to a
few showers through early week largely tied to the diurnal heating
cycle. Surface high pressure will gain a foothold on the southeast
region for the mid to late week period. Daytime high temperatures
will continue to run a bit above normal...in the mid to upper 80s.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
KCHS/KJZI/KSAV: Numerous to widespread showers and a few TSRA
associated with Helene will impact all terminals tonight,
before rapid drying takes place on Friday. Flight restrictions
will occur through the tonight timeframe as rounds of showers
and some thunderstorms move through, generally down in the MVFR
range for both ceilings and visibilities. IFR is possible at
times, especially during heavier rains.

SE winds will steadily as we go through tonight, peaking as
high as 30 or 35 kt sustained with gusts of at least 40 to 50
kt, with the peak winds occurring largely between 06Z and 12Z.
Higher wind gusts are possible. Winds will veer around to the S
or SSW Friday, with winds still as high as 20-30 kt through the
day.

Finally, we included a few hours of LLWS at all three terminal
sites, although given the strong gusty surface winds, LLWS may
not truly come to fruition.

One side note is that conditions remain favorable for isolated
tornadoes across southern South Carolina and southeast Georgia
through tonight.

Extended Aviation Outlook: Prevailing VFR, though with persistent
elevated low level moisture and a strong subsidence inversion,
will need to monitor for morning radiation fog and afternoon
MVFR strato-cu potential through the long term.

&&

.MARINE...
Tonight: Dangerous conditions will impact the Atlantic waters
and Charleston Harbor as Major Hurricane Helene makes landfall
in the Florida Big Bend area. Winds will steadily increase as
Helene gains latitude and the wind fields expand northeastward
to cover our local waters. Winds speeds tonight will peak as
high as 50-55 kt in gusts, locally even near 60 kt. Tropical
Storm Warnings will remain in effect for all local waters as a
result. These winds will allow for seas to build as high as
10-15 feet on the waters out 20 nm, and up to 17 feet on the
Georgia waters beyond 20 nm from shore.

Mariners are also alerted to the risk for waterspouts through
much of the night, as convective bands move through the area in
association with helene. Some of these will be tornadic
waterspouts, moving much quicker and producing much stronger
winds than most waterspouts in our area.

Extended marine: Very large, medium period swell associated
with the large wind field of Helene peaks Friday morning,
gradually subsiding into the weekend. Near Tropical Storm Force
winds early Friday morning will subside to more moderate winds
by Friday afternoon as well. Light to moderate winds persist
Friday night into early next week as high pressure moves
overhead and more seasonable 2-3 ft seas prevail.

Rip Currents and High Surf: Conditions at the beach will be
dangerous tonight through Friday, as winds and seas continue to
build in response to Hurricane Helene tracking across the
northeast Gulf of Mexico and into the interior of the Southeast.
A High Risk of Rip Currents will be in effect for all
beaches through Friday. Breakers will reach at least 5 feet in
height through Friday, and as result, a High Surf Advisory
remains in effect. Areas of significant beach erosion are also
likely during this time.

&&

.TIDES/COASTAL FLOODING...
We have cancelled the Coastal Flood Advisory for the most recent
high tide.

The early Friday morning high tide will be very problematic, and
much will depend upon the strength and the direction of the
winds. There is a wide range in possible outcomes, from barely
any flooding to possible major flooding. With such a dichotomy
in potential occurrences, we will make no changes at this time,
and re-evaluate in later forecasts.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Tropical Storm Warning for GAZ087-088-099>101-114>119-137>141.
     High Risk for Rip Currents through Friday evening for GAZ117-119-
     139-141.
     High Surf Advisory until 8 PM EDT Friday for GAZ117-119-139-
     141.
SC...Tropical Storm Warning for SCZ040-042>045-047>052.
     High Risk for Rip Currents through Friday evening for SCZ048>051.
     High Surf Advisory until 8 PM EDT Friday for SCZ048>051.
MARINE...Tropical Storm Warning for AMZ330-350-352-354-374.
&&

$$

NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...
MARINE...