102
FXUS62 KCHS 251504
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1104 AM EDT Wed Sep 25 2024

.SYNOPSIS...
High pressure will linger across the region today. Tropical
Cyclone Helene is forecasted to track northward across the Gulf
of Mexico, then move inland across portions of Florida and
Georgia Thursday into Friday. Multiple impacts are expected
across our area Thursday and Thursday night. Drier weather then
arrives later Friday and persists into the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
No major changes made to the near term forecast with the mid-
morning update. Forecast is on track.

Today: Aloft, a ridge centered off the Southeast Coast will become
further displaced offshore while a large trough begins to cutoff
near the middle Mississippi River Valley late day. At the sfc, high
pressure placed along the East Coast will extend across Southeast
South Carolina and Southeast Georgia for much of the day well ahead
of a cold front progressing east across the Deep South. Some hires
guidance does suggest showers currently occurring offshore to brush
the coast this morning, and perhaps make some progress further
inland across Southeast Georgia. Given latest radar trends, have
added slight chance pops to the latest forecast to account for this
activity. Otherwise, most areas should remain dry into early-mid
afternoon hours. By late afternoon, deeper moisture will begin to
advect north across inland Georgia, which could lead to a few
showers and/or isolated thunderstorm in a marginally unstable
environment displaying SBCAPE between 500-1000 J/kg. Increasing
clouds throughout the day should limit high temps a few degrees
cooler than the previous day. In general, temps should peak in the
mid-upper 80s, warmest across the Georgia interior.

Tonight: The mid-upper low near the middle Mississippi River Valley
becomes cutoff, slowly nudging south with time while the first signs
of vort energy traverse the region. At the sfc, high pressure will
begin showing signs of being forced offshore late night as deep
tropical moisture advances north across the Southeast United States
well in advance of Tropical Cyclone Helene making northward progress
into the Gulf of Mexico. Given the setup, expect few/scattered
showers along with isolated thunderstorms to shift into Southeast
Georgia early evening, then become numerous to potentially
widespread showers with a few embedded thunderstorms a few hours
prior to daybreak as precip activity continues to advance into
Southeast South Carolina. Lows should remain mild overnight,
generally ranging in the low to mid 70s. Locations near the coast
could stay in the upper 70s.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY/...
Thursday: The NHC is forecasting Tropical Cyclone Helene to move
northward across the eastern Gulf of Mexico Thursday, reaching the
Big Bend coast of Florida late Thursday. The center is then
forecasted to pass to our west Thursday night into daybreak Friday.
Helene will bring multiple impacts to our area Thursday and Thursday
night. For full details, please refer the the HLS.

Winds: A Tropical Storm Watch is in effect for our entire area.

Tornadoes: As Helene passes west of our region, hodographs rapidly
elongate with an impressive amount of instability forecast for a
tropical cyclone. Both the GFS and NAM have MLCAPE values at 500
J/kg or greater with Helicity values around 300 m2/s2. Critical
angles are also near 90 degrees, which would make most of the
vorticity streamwise, or suitable for the development of tropical
tornadoes. As such, the SPC has raised our GA counties to a Enhanced
Risk for Thursday, with our SC counties remaining a Slight Risk. If
models stay consistent, we expect this Enhanced Risk to be expanded
further north.

Flooding Rainfall: Multiple rounds of showers/rainbands in
association with Helene will begin to impact the area Thursday
morning (for our GA counties). By Thursday afternoon, showers will
have overspread our entire areas. Showers and a isolated rumble of
thunder will then persist through Thursday night into early Friday
morning. Showers will come to an end across GA by Friday afternoon
and SC. Rainfall totals of 2 - 4" are possible for GA and 1.5 - 3"
across SC.

Storm Surge: Currently surge values are in the 1 - 3 ft range MHHW.
As such, coastal flood products might be required. More on this can
be found in the Tides/Coastal Flooding section.

Friday: Helene will be across northern GA and SC Friday morning with
dry air wrapping around the southern flank of the storm. This is due
to a cutoff low located near MS/AL which will cause Helene to
accelerate north. As this occurs, the main barotropic low will
absorb Helene with the system rapidly becoming baroclinic. The
absorption of Helene will feature a potent dry conveyor belt on the
southern flank of the storm with a weak front expected to pass
through Friday morning. The frontal boundary will be more of
moisture boundary as PWATs fall from 2.5"+ (in association with
Helene) to less than 1". It should also be noted that GFS forecast
soundings have a deep mixed layer Friday afternoon with gusts 20 to
30 mph possible at times in the afternoon. A Lake Wind Advisory
might be required for Lake Moultrie. Expect high temperatures in the
mid 80s. Lows Friday night will mostly be in the upper 60s.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
The closed mid-level low will shift east over the Ohio River Valley
on Sunday. Then opening into a wave across the east coast early next
week. Isolated to scattered showers and thunderstorms will remain
across the forecast area through the period, especially during the
afternoon and evening. High temperatures are forecast to range
between 80 to 85 degrees with lows between 65 to 70.

&&

.AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
A few showers could drift onshore and impact the SAV and/or JZI
terminal this morning, producing brief MVFR conditions. Otherwise,
expect VFR conditions to prevail at CHS/JZI/SAV terminals through at
least 00Z Thursday with occasionally gusty winds up to 15-20 kt at
CHS and SAV terminals between 18Z this afternoon to 00Z Thursday.
Thereafter, some showers are expected to arrive from the southwest
late tonight, impacting the SAV first by around 08Z and perhaps the
CHS/JZI terminals near 12Z Thursday. Prevailing showers and MVFR
cigs have been introduced at the SAV terminal at 08Z Thursday. Have
opted to keep VFR at CHS/JZI for the 12Z Wednesday TAF issuance.

Extended Aviation Outlook:

Wednesday night: All terminals VFR with winds out of the southeast.

Thursday: Tropical Cyclone Helene will be across the northeast
Gulf of Mexico Thursday morning heading to the north/ northeast.
Initially, all terminals will be VFR in the early morning hours
with high clouds thickening and bases slowly lowering. By mid-
morning hours, MVFR conditions are likely at KSAV and KCHS.
Transient IFR cigs will then be possible by the afternoon hours
as precipitation from Helene spreads over the entire region.
Visibilities will also be reduced to IFR or lower at times due
to the heavy rainfall. Restrictions in cigs, vsbys, heavy rain,
and gusty winds will then persist Thursday evening into early
Friday morning.

Friday: Tropical Cyclone Helene will quickly start to pull away
from the region with a return to VFR conditions. Southwest
winds will be gusty at times.

Saturday and Sunday: VFR conditions. No precipitation is expected on
Saturday with a slight chance of showers and thunderstorms on Sunday.

&&

.MARINE...
Today and Tonight: High pressure will prevail across local waters
for much of the day, leading to quiet marine conditions. In general,
southeast winds will range between 10-15 kt with seas of 1-3 ft
across nearshore waters and 3-4 ft across offshore Georgia waters
that gradually build into the evening. Overnight, the pressure
gradient will become noticeably stronger between high pressure
becoming more displaced offshore well in advance of Tropical Cyclone
Helene making northern progress into the Gulf of Mexico. This will
lead to gradually deteriorating marine conditions through the night,
with southeast winds gusting up to 15-20 kt across South Carolina
nearshore waters and 20-25 kt across Georgia waters, highest after
midnight. Seas will also build, ranging between 3-5 ft across
nearshore waters and 5-7 ft across offshore Georgia waters.

Thursday through Friday: Expect dangerous marine conditions as a
northward moving tropical system in the Gulf of Mexico strengthens.
Fine details are likely to change depending on the exact track and
strength of the system. At this juncture, guidance indicates ESE
gusts could range between 25-30 kt by Thursday morning. The tropical
cyclone is expected to track inland and make it closest approach
late Thursday night/early Friday, which is when tropical storm force
gusts or greater are likely across the local waters. A Tropical
Storm Watch has been issued for the SC and GA waters. Tornadic
waterspouts will also be possible during this time frame, in
addition to bands of heavy rain. Seas are expected to peak between 8-
13 ft within 20 nm and 14-17 ft over the outer GA waters from 20-60
nm late Thursday night. Conditions will begin to improve Friday
night.

Saturday through Sunday: As the tropical system rapidly weakens to
our north, seas will drop to 2-3 ft with westerly winds shifting out
of the north around 10-15 kt.

High Surf and Rip Currents:
Today: Onshore winds and swells with 10 to 11 seconds will
develop along the Southeast Georgia and Southeast South Carolina
coasts. Local calculation and rip current MOS support a Moderate
Risk for rip currents along the entire coast.

Breaking waves of 5 feet or greater are likely to occur at the
beaches Thursday into Thursday night. A High Surf Advisory will
likely be needed for all area beaches. Significant beach erosion is
likely to occur during this time. A high risk for rip currents has
been posted for the SC/GA coast on Thursday as well. Additionally,
an elevated risk for rip currents is possible Friday with
lingering gusty winds, large breakers, and long period swells.

&&

.TIDES/COASTAL FLOODING...
On Thursday, strengthening onshore winds could lead to an increasing
tidal departure, with some coastal flooding possible along the
Charleston and Colleton County coasts Thursday afternoon. The threat
for coastal flooding is then expected for the entire coast through
the end of the week due to impacts from a tropical cyclone. Storm
surge inundation of 1 to 3 ft is possible along the coast of
southeast Georgia and the South Carolina Lowcountry.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Tropical Storm Warning for GAZ099>101-114>119-137>141.
     Tropical Storm Watch for GAZ087-088.
SC...Tropical Storm Warning for SCZ045-047>052.
     Tropical Storm Watch for SCZ040-042>044.
MARINE...Tropical Storm Warning for AMZ330-350-352-354-374.
&&

$$

NEAR TERM...CPM/DPB
SHORT TERM...Haines/MS
LONG TERM...Haines/MS
AVIATION...DPB/Haines
MARINE...CPM/DPB/Haines