190
FXUS62 KCHS 261737
AFDCHS

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

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

&&

.NEAR TERM /THROUGH TONIGHT/...
Today and Tonight: Aloft, a ridge will be centered far off the
Southeast Coast while a large cutoff low remains over the middle
Mississippi River Valley. At the sfc, high pressure extending south
along the East Coast and into the Southeast will retreat offshore in
advance of Hurricane Helene, which is forecast to quickly track
north across the eastern Gulf of Mexico during the day, then make
landfall as a Major Hurricane across the Florida Panhandle around
nightfall. 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, which is forecast to track north across Central Georgia.
For this reason, a Tropical Storm Warning remains in effect for
the entire forecast area today and 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 day and night, bringing multiple
hazards to the immediate area. For full details, please refer to
weather.gov/chs and the Hurricane Local Statement (HLS).

Tornadoes: Perhaps the greatest concern in regards to impacts
associated with Helene will be the risk for tropical tornadoes
across the entire area, especially given the local area remains
situated in the favorable northeast quadrant of Helene for quite
some time today and tonight. Guidance continues to support
conditions favorable for tornadoes, with long and looping hodographs,
ample buoyancy along a northward shifting CAPE axis (SBCAPE
1500-2000 J/kg) extending from the nearby Atlantic and onshore,
sfc temps in the upper 70s to lower 80s, and 0-3 km SRH values
near 300-400 m2/s2. Additionally, the latest HREF run gives a
fairly strong indication of updraft helicity tracks supportive
of tornadic waterspouts that shift onshore, coinciding with
rainbands associated with Helene this afternoon and 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. 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 and move very quickly (potentially
in excess of 45 to 55 mph).

Winds: A Tropical Storm Warning is in effect for the entire forecast
area. Based on the 11 AM NHC advisory, the Earliest Reasonable
Arrival Time of Tropical Storm Forecast Winds (39+ mph) in our
area is mid to late afternoon across Georgia, and early evening
across South Carolina. The Most Likely Arrival of these winds is
early evening across Georgia and during Thursday night 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 in
association with Helene will impact the area, initially across
the Georgia counties, then spread north across South Carolina
counties. These showers along with a few thunderstorms will
then persist 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.

Rainfall totals are mainly in the 1.5 to 4 inch range, but with
locally higher amounts possible in heavy rainbands. The risk
currently does not support the issuance of a Flood Watch,
especially with dry soils in many places per NASA Sport
guidance. A Slight to Moderate risk of Excessive Rainfall is in
effect from WPC. Area rivers and creeks will rise in response
to the rainfall, with some flooding possible, even after the
rains come to an end.

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: Max temperatures this afternoon will be held down
due to the elevated rain chances, and will be held in the 70s
when the rains are ongoing. It`ll be a very warm and humid
night, and we could be close to record high minimums for
September 26.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY/...
Strong upper low lingers near the Tennessee Valley through the end
of the week. Attendant surface low/remnants of Helene remain quasi-
stacked despite trending more baroclinic, but steadily fills. A few
lingering showers Friday morning will quickly diminish by midday
Friday, with strong subsidence and mainly dry conditions persisting
across SE SC/GA thereafter. Breezy S to SW winds Friday morning
trend moderate by Friday afternoon, with humid conditions persisting
into the weekend and temps remaining very close to normal.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
A mid and upper low initially over the Ohio Valley will eventually
open into a wave as it reaches the Mid-Atlantic coast Tuesday.
Forcing for ascent remains north of the area, with subsidence aloft
persisting locally, keeping a mainly dry forecast in place. At the
surface, weak ridging persists early in the week, with a cold front
(which appears mainly dry) crossing mid to late week. Temps
generally near to a few degrees below normal through the period.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
KCHS/KJZI/KSAV: Numerous to widespread showers and a few TSRA
associated with Helene will impact all terminals this afternoon
into tonight, before rapid drying takes place on Friday. Flight
restrictions will occur through the majority of the 18Z TAF
cycle, generally down in the MVFR range for both ceilings and
visibilities. However, IFR is possible at times, especially
during heavier rains.

SE winds will steadily increase this afternoon into tonight,
peaking as high as 30 or 35 kt sustained with gusts of at least
40 or 45 kt. Higher wind gusts can also occur in any of the
heavier showers/rainbands associated with Helene. Winds will
veer around to the S or SSW Friday, with winds still as high as
20-30 kt.

We included a few hours of LLWS at KSAV overnight, but since the
potential for LLWS looks too marginal at KCHS and KJZI, we opted
to hold off including at this time. We will re-evaluate this in
subsequent forecasts.

One side note is that conditions remain favorable for isolated
tornadoes across southern South Carolina and southeast Georgia
into 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...
Today and Tonight: Dangerous marine conditions will occur as Helene
strengthens across the eastern Gulf of Mexico during the day, then
moves onshore across the Florida Panhandle around nightfall and
continues north across Central Georgia during the night. Helene is a
large tropical cyclone with wind fields that are expected to expand
wide as the cyclone gains latitude, favoring impacts well to the
east. Tropical Storm Warnings will remain in effect for all local
waters as a result. Winds as high as 45-50 kt will start impacting
the Georgia waters this evening, then spread into the South Carolina
waters overnight. Seas will build as high as 10-15 feet within 20 nm
from shore, and up to 17 feet across the outer Georgia waters
tonight.

Mariners are also alerted to the risk for waterspouts this afternoon
and through much of the night. Some of these will be tornadic
waterspouts, moving much quicker and producing much stronger winds
than most waterspouts in our area.

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 today through Friday, as winds and seas 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 both days, and
breakers will reach at least 5 feet in height starting this
afternoon and persisting through at least Friday afternoon or
evening. Areas of significant beach erosion are also likely during
this time.

&&

.TIDES/COASTAL FLOODING...
We have issued a Coastal Flood Advisory for coastal Charleston
and coastal Colleton from 2-6 pm today, with tides expected to
reach around 7.3 ft MLLW. This will occur during ongoing rains,
which could further exacerbate the flooding situation. For the
moment there is not expected to be any coastal flooding
elsewhere along the coast with the afternoon 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.

NHC continues to forecast a storm surge inundation of 1 to 3 ft
along the coast of southeast Georgia and Southern South
Carolina from Helene. Keep in mind that if the higher surge
and/or heavy rains occur near or during the time of the high
tides, the coastal flooding could be worse.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Tropical Storm Warning for GAZ087-088-099>101-114>119-137>141.
     High Risk for Rip Currents until 8 PM EDT this evening for GAZ117-
     119-139-141.
     High Surf Advisory until 5 PM EDT Friday for GAZ117-119-139-
     141.
SC...Tropical Storm Warning for SCZ040-042>045-047>052.
     High Risk for Rip Currents until 8 PM EDT this evening for
     SCZ048>051.
     High Surf Advisory from 5 PM this afternoon to 8 PM EDT Friday
     for SCZ048>051.
     Coastal Flood Advisory until 6 PM EDT this evening for SCZ049-
     050.
MARINE...Tropical Storm Warning for AMZ330-350-352-354-374.
&&

$$

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