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646
FXUS62 KCHS 242349
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
749 PM EDT Tue Sep 24 2024

.SYNOPSIS...
High pressure will linger across the region through Wednesday.
A tropical cyclone is forecast to track northward across the
Gulf of Mexico, then move inland late week across portions of
Florida, Georgia, and possibly South Carolina. Drier weather
then arrives for the weekend.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Quiet weather out there this evening and that will remain the
case through the overnight as mid level ridging and surface high
pressure remaining along the southeast coast. That said, there
are a few showers moving through the outer waters within the
larger scale southeast flow. HRRR guidance does drive a few
showers into coastal areas overnight. Not prepared to add any
precip chances to the going forecast at this juncture, but will
keep an eye on trends. Low temperatures are forecast to range
from near 70 inland to the mid to upper 70s along the coast.
-- End Changed Discussion --

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
Wednesday: The upper level ridge that was over the Pacific Northwest
on Tuesday will translate east towards the Four Corners region with
a trough axis centered across the Great Lakes. Meanwhile, energy
that was near MN/ IA earlier this week will be diving south towards
AR with wave breaking starting to occur. As this happens, the energy
will break off into a closed upper level low and become quasi-
stationary over AR. Towards SC/ GA, a mid-level ridge will nudge
east, but still keep the area dry as a subsidence inversion remains
around 800/ 900 mb. PWATs are also forecast to be around or below
1.50" with substantial mid level dry air in place. At this time
though, Helene will likely be a hurricane and turn to the north as
it begins to feel the influence of the mid level low (or weakness).
Widespread cirrus will filter into the region and thicken. Expect
high temperatures in the mid to upper 80s with low temperatures in
the lower to mid 70s.

Thursday and Friday: All attention then turns to Tropical Storm
Helene on Thursday as it begins to accelerate to the north/
northeast or towards the Florida coastline. Rapid intensification of
Helene is expected Thursday due to warm Gulf of Mexico water and
little in the way of upper level shear. The National Hurricane
Center is forecasting Helene to make landfall as a major hurricane
along the Big Bend region of Florida Thursday evening. During the
day Thursday, cloud bases will continue to lower as showers
overspread the region from the southwest. Helene will bring multiple
hazards to GA and SC. For full details, please refer the the HLS.

Wind: A tropical storm watch is now in effect for all GA zones and
GA marine zones. Peak wind gusts will likely occur Thursday night
into early Friday morning as Helene moves north across central GA.
Wind gusts across SC will likely remain mostly below Tropical Storm
Force at this time. Therefore we opted only for a Tropical Storm
Watch across GA.

Tornadoes: As Helene passes west of the region Thursday night,
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 the area in a slight risk
for Thursday into Friday.

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.

Flooding Rainfall: As mentioned above, multiple rounds of showers or
rainbands in association with Helene will begin to impact the area
Thursday morning (esp towards GA). By Thursday afternoon, rainfall
will have spread over the entire region. Showers and a isolated
rumble of thunder will then persist through Thursday night into
early Friday morning. Helene will continue to head north and be
across central/ northern GA by sunrise Friday morning. At this
time, rainfall will start to come to an end across coastal GA
and SC. Rainfall totals of 3 - 5" are possible for GA with 2 -
4" across SC.

Helene will be across northern GA and SC Friday morning with
dry air wrapping around the southern flank of the storm. As this
occurs, rainfall will come to an abrupt end Friday after
sunrise with breaks in the clouds appearing. Southwest winds
will be gusty at times with temperatures warming into the upper
80s.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Friday night into Saturday, long term guidance indicates that the
forecast area will remain between a closed mid-level low over the
Mississippi River Valley and a 592 DM ridge over the Bahamas. Dry air
in the wake of the remnants of Helene is forecast to drift
northward, with a stream of moisture flowing NE between the two mid-
level features across the forecast area. The moisture should yield
some gradually increase in cloud cover with isolated showers across
extreme SE GA. High temperatures should favor values in the mid to
upper 80s.

Sunday 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 /00Z WEDNESDAY THROUGH SUNDAY/...
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00Z TAFs: VFR. Winds will remain steady from the SSE although
tend to weaken during the overnight hours. Some gustiness will
develop during th day Wednesday with gusts to around 20 knots
possible.

Extended Aviation Outlook:

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

Thursday: Hurricane Helene will be across the northeast Gulf of
Mexico Thursday morning heading to the north/ northeast. Initially,
all terminals will be VFR in the morning hours with high clouds
thickening and bases slowly lowering. By the afternoon hours, MVFR
conditions are likely at KSAV and KCHS. Transient IFR cigs will then
be possible by the evening 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: 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.
-- End Changed Discussion --

&&

.MARINE...
Tonight, a ridge of high pressure will southeast winds around 10 kts
across the marine zones. Seas are forecast to range between 3-4 ft.

Wednesday: Broad high pressure offshore will result in southeast
winds around 15 kt. Seas will average 2-3 ft for the nearshore
waters out 20 nm and 3-4 ft for the outer GA waters from 20-60 nm.
Conditions will begin to deteriorate Wednesday night ahead of an
approaching potential tropical cyclone in the southern Gulf of
Mexico.

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 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:
Wednesday: Onshore winds and swells with 10 to 11 seconds will
develop along the Ga Coast. Local calculation and rip current MOS
support a moderate risk for the GA 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. 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 Watch for GAZ087-088-099>101-114>119-137>141.
SC...None.
MARINE...Tropical Storm Watch for AMZ354-374.
&&

$$

NEAR TERM...Adam
SHORT TERM...Haines
LONG TERM...NED
AVIATION...Adam/Haines
MARINE...NED