079
FXUS62 KCHS 180954
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
554 AM EDT Sun Aug 18 2024

.SYNOPSIS...
A cold front will move through the area tonight. High pressure
will then gradually build in from the north through the
remainder of the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
A small cluster of showers moving across east-central Georgia
is holding together despite little/no instability. The 18/08z
RAP seems to initialize this activity the best and keeps in tact
as it brushes the far interior counties and into parts of the
Charleston Tri-County over the next few hours. Pops were updated
to include a mention of isolated showers this morning in these
areas.

Incoming model data suggest convection may be a bit slower to
develop this afternoon than what was originally expected,
possibly due to a slower approach of the upstream shortwave.
Hesitant to make any major timing/coverage changes with the
sunrise update until models can better settle down a bit on how
convection will unfold later today. Hopefully, the 12z CAMs will
be in better agreement for the late morning update.

Through This Evening: The broad longwave trough over the
eastern CONUS will steadily sharpen through tonight as a series
of shortwave propagates through its base. Shortwave energy
moving across the Tennessee Valley this morning will push across
the Carolinas late this afternoon/evening along the nose of a
northwest-southeast oriented H5 jet streak. This will help
drive a cold front into the area overnight. Ahead of the front,
a warm and increasingly moist airmass will support the
development of scattered showers/tstms with coverage peaking
late afternoon into this evening as forcing with the passing
shortwave and associated jet streak pivot through.

As is typical this time of year, the mesoscale pattern will
help drive how convection will evolve. The 18/00z CAMs are
painting a number of possible convective scenarios as they all
appear to be handing the placement of outflow boundaries that
are meandering over the area early this morning differently.
While the hourly specifics are still unknown, there is broad
consensus that the best chances for showers/tstms will extend
across much of Southeast Georgia into south coastal South
Carolina, roughly in the 21-03z time frame, possibly a tad
earlier across interior Southeast Georgia. Pops will were capped
at 30-50% this afternoon/evening with the greatest coverage
clustered across the coastal counties.

Modified soundings yield modest instability with MLCAPE peaking
2200-2700 J/kg during the late afternoon. Convection should
exhibit some degree of upscale growth as it becomes embedded
within a belt of 30 kt of 0-6km bulk shear. This coupled with
some mesoscale boundary interactions should yield several
pronounced tstm clusters with some degree of organization. Given
the strength of instability and shear, expect isolated to
perhaps scattered severe tstms to occur with the greatest risk
occurring from roughly 4PM-11PM with the greatest chances
occurring across the coastal counties and areas roughly
along/south of the I-16 corridor aligned with the axes of
highest H8 theta-e and mixed-layer instability. The primary
hazard looks to be damaging winds with potential maximum wind
gusts approaching 75 mph per latest WINDEX calculations.
Frequent cloud-to-ground/water lightning will also occur and
there will be a risk for locally heavy rainfall. Thankfully,
convection looks to remain fairly progressive, but given how wet
soils remain in the wake of Tropical Storm Debby, it will not
take much to get some minor flooding of low-lying and poor
drainage areas.

Highs will warm into the lower-mid 90s away from the coast this
afternoon. Low-level moisture advection ahead of the front will
help drive dewpoints into the lower-mid 70s this afternoon with
a corridor of upper 70s possible across parts of Charleston into
eastern Berkeley Counties. Heat indices look to peak in the
105-108 range--just a tad below Heat Advisory criteria.

Overnight: The ill-defined cold front will meander through the
area early Monday, pushing offshore just after daybreak. The
bulk of the convection should shift offshore by midnight with
rain chances ending quickly from west-east. As high clouds
thin, some patches of low stratus could develop across the
interior late. Lows will range from the lower 70s well inland to
the mid-upper 70s at the beaches.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
Aloft, a large trough extending from extreme eastern Canada
down to the Florida Keys will sit nearly stationary, with bouts
of vorticity traversing the base through the period. A weak cold
front is expected to reach the Atlantic coast Monday with a
slight stall before it pushes offshore Tuesday as high pressure
begins to build in from the north. The forecast looks mostly dry
behind the front, with the exception of an isolated
showers/thunderstorms near/along the coast with the help of
upper lift and moisture offshore. Modest subsidence will limit
the overall coverage of convection, especially inland. Another
limitation for convection is the relatively weak sea breeze,
with little land/sea temperature differential especially
Wednesday.

Low to mid 90s are forecast for most locations Monday.
Overnight temperatures will drop into the upper 60s/lower 70s
inland and mid 70s along the immediate coast. High temperatures
will be somewhat cooler the following days, reaching the upper
80s/lower 90s Tuesday and mid/upper 80s Wednesday.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
The upper trough will lift northeast Wednesday night and the
main feature will be the strengthening surface high pressure
building in from the north, forming a wedge pattern. Meanwhile,
a broad upper low looks to detach from the departing trough and
form over the northern Gulf of Mexico. High pressure over the
Atlantic will expand southwestward tilting the axis of deep
moisture across the Southeastern U.S. Scattered showers and
thunderstorms could develop mainly along the coast during the
afternoon each day with the return of moisture and upper forcing
back in play. High temperatures will gradually lower Thursday
and Friday, but begin to warm into the upper 80s by next
weekend.

&&

.AVIATION /10Z SUNDAY THROUGH THURSDAY/...
18/12z TAF Discussion:
KCHS/KJZI/KSAV: No major changes were made from the 06z TAFs.
VFR for much of the period although it is possible a brief
shower could occur at KCHS by mid-morning if showers moving
across east-central Georgia hold together. No major impacts are
expected.

Scattered showers/tstms are expected to develop late this
afternoon into this evening for the coastal corridor. Confidence
remains diminished as a number of the near term convective
models are showing a number of possible scenarios for both
coverage and timing for convection. There seems to be a broad
consensus that the periods from roughly 22-02z at KCHS/KJZI and
23-03z at KSAV will have the best chances for impacts at the
various terminals. For now, VCTS was maintained for the 12z TAFs
for the time periods noted. The introduction of TEMPO or
prevailing groups for TSRA may need to be considered in later
TAF cycles as confidence in timing/coverage increases.

Extended Aviation Outlook: VFR will prevail through the week.

&&

.MARINE...
Today: South to southwest winds will increase this afternoon
ahead of an approaching cold front. Winds look reach as high as
15-20 kt over the South Carolina waters and closer to 15 kt over
the Georgia waters. Seas will build through the day, reaching
3-5 ft for the waters within 20 NM and 5-6 ft over the Georgia
waters 20-60 NM. A Small Craft Advisory continues for the
Georgia waters 20-60 NM offshore through tonight. Convection
will begin to move into the coastal waters by late
afternoon/early evening. These storms could produce winds in
excess of 34 kt and frequent cloud-to-water lightning. Special
Marine Warnings may be needed for some areas.

Tonight: Assuming convection pushing offshore does not
completely disrupt the pre-frontal wind field, southwest winds
will increase to 20 kt with gusts to 25 kt over the South
Santee-Edisto Beach nearshore leg overnight with seas building
4-6 ft. A Small Craft Advisory has been issued beginning this
evening and continuing into daybreak Monday given these
conditions. Elsewhere southwest winds 15-20 kt will occur with
seas 3-5 ft in the Edisto to Altamaha Sound out 20 NM area with
4-6 ft over the Georgia 20-60 NM leg where a Small Craft
Advisory remains in effect until daybreak Monday. Mariners
should remain alert for possible strong to severe tstms into the
evening hours. Special Marine Warnings may be needed for some
areas.

Monday through Friday: Conditions will improve throughout the
day Monday as a weak front along the coast gradually departs
further into the Atlantic. Southwest winds will drop to 10-15 kt
and seas will drop to 2-4 ft. By Tuesday, winds will shift out
of the northeast through the remainder of the week as inland
high pressure builds from the north. A surge in both winds and
seas is possible beginning late Wednesday night into Friday with
a modest pressure gradient in place over the waters. Small
Craft Advisories could be needed for most of the marine zones
for seas 6 ft or greater and gusts around 25 kt.

Rip Currents:
Sunday: A high risk for rip currents continues for all beaches
today given the continued long period swells from Tropical Storm
Ernesto impacting the Southeast U.S. coast.

Monday: Long period swell around 11 seconds will gradually
subside, but continue to bring impacts to the coast. A moderate
risk of rip currents is in place for all beaches.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...High Risk for Rip Currents through this evening for GAZ117-119-139-
     141.
SC...High Risk for Rip Currents through this evening for SCZ048>051.
MARINE...Small Craft Advisory from 6 PM this evening to 8 AM EDT Monday
     for AMZ350.
     Small Craft Advisory until 8 AM EDT Monday for AMZ374.

&&

$$