585
FXUS62 KCHS 232300
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
600 PM EST Mon Dec 23 2024

.SYNOPSIS...
An area of low pressure will track northeast off the Carolina
coast through Tuesday. High pressure will settle over the area
on Christmas Day. A broad area of low pressure will approach
from the west late this week, drifting across the region this
weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
23/22z surface analysis placed a wedge of high pressure over the
Southeast U.S. with a pronounce coastal trough extending from
off the far northeast Florida coast to offshore of the lower
South Carolina coast. The trough is forecast to sharpen up
overnight as a west-east oriented band of vorticity extending
east from a well-defined vorticity maximum propagating across
eastern Mississippi begins to interact with the trough. The
trough could cut off into weak surface low southeast of
Charleston by daybreak Tuesday, but how organized this low
becomes is unclear. KCLX radar data show a pronounced southwest-
northeast oriented band of light rain and drizzle along the
Georgia coast and extending into the Charleston Metro Area. This
area of enhanced returns is aligned with a modest area of 850
hPa frontogenesis. The rain is expected to intensify a bit later
this evening and overnight as low-level convergence builds
along the lower South Carolina coast in response to a sharpening
of the coastal trough just offshore. There will also likely be
a further enhancement of the 850 hPa frontogenetic zone as the
mid-level vorticity maximum over eastern Mississippi propagates
farther east across the Deep South. The best rain chances look
to remain along the coast and extending into the Charleston Tri-
County area later this evening before diminishing late as the
best forcing shifts into Northeast South Carolina.

Pops from 20-40% inland to the 50-70% at the coast still looks
reasonable, although some slight timing and areal adjustments
were needed for the next several hours to take current radar
trends into account. There was an earlier concern that as some
of the rain that pushes inland could mix with some sleet pellets
at the onset due to diabatic cooling, but RAP forecast soundings
show wet-bulb profiles remaining well above freezing through
the night. So, the risk for sleet seems unlikely at this time.
Lows will range from the mid 30s well inland to the mid 40s at
the beaches.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
Tuesday: Short range models indicate a gradually deepening
coastal low will track NE off the Carolina coast during the
morning. The low is timed to lift north of the NC line by noon.
As the low tracks north, dry air will steadily advance across SE
GA and the SC Lowcountry. Isolated to scattered showers should
come to an end by mid-day, with cloud cover decreasing through
the rest of the day. High temperatures are forecast to range
from the low 60s across SE GA to the low to mid 50s across
Charleston Tri-county. High pressure will build across the
forecast area Tuesday night. Forecast soundings and simulated
satellite indicate mostly clear sky, primarily thin high clouds
across GA. Low temperatures are forecast to range from the mid
30s inland to the upper 30s along the coast.

Christmas Day: The mid-level pattern will feature a shortwave
ridge centered across the region in the morning as a trough
amplifies across the Arklatex. At the sfc, high pressure
centered over New England will remained ridged across region
through the day. Conditions should feature dry weather with
light NE winds. Highs should peak around 60 degrees.

Thursday: Low pressure over the Great Plains is forecast to
organize, lifting a warm front across the Deep South. Cloud
cover is expected to gradually increase from the west. Thursday
should begin with temperatures around 40 degrees warming to
around 60 degrees in the afternoon. Isolated showers are
possible across the outer Atlantic waters, with dry conditions
across SE GA/SC.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
Friday: Long term guidance generally indicates that a secondary
ridge will build across the region, with the ridge axis passing
over the region during the afternoon and evening. The sfc ridge
will remain across the forecast area, axis near the east facing
slopes of the Southern Appalachians. Weak isentropic lift may
yield increasing cloud cover across the CWA, isolated to
scattered showers over the coastal areas. High temperatures are
forecast to range between 60 to 65 degrees.

Saturday and Sunday: A cold front will approach from the west,
resulting in strengthening return flow across the forecast area.
Weak synoptic scale forcing and building moisture may support
isolated to scattered showers. Temperatures will remain around
10 degrees above normal. Low temperatures are forecast to range
around 50 degrees on Saturday to the mid 50s on Sunday morning.
High temperatures are forecast to generally range between 70 to
75.

Monday: A cold front may push over the forecast area, then
stalling along the coast. Conditions should gradually dry from
the west through the day. Temperatures are forecast to begin the
day around 50 degrees with highs around 70 degrees.

&&

.AVIATION /22Z MONDAY THROUGH SATURDAY/...
24/00Z TAF Discussion:
KCHS/KJZI: Slight rain and drizzle will quickly spread into both
terminals this evening as weak low pressure tries to develop
offshore. Light rain will pick up in intensity later overnight
with MVFR vsbys in RA and BR peaking 04-07z. Observations along
the Georgia coast into far southern South Carolina suggest vsbys
could drop as low as 2-3SM, which will most likely impact KJZI.
TEMPO groups were used at both terminals to try and capture the
heaviest conditions. MVFR cigs will linger overnight, returning
to VFR by early afternoon as clouds begin to scour out as low
pressure exits the area.

KSAV: Light rain and drizzle will impact the terminal this
evening as low pressure begins to organize offshore. MVFR cigs
will linger for much of the night before scouring out after
daybreak as low pressure begins to exit the area.

Extended Aviation Outlook: VFR.

&&

.MARINE...
Tonight: A 1020-1021 mb low will form over the Gulf Stream off
the northeast Florida coast early on, then lifts north-northeast
to a position about 175 nm east of Savannah by morning. About a
3-4 millibar gradient will exist between an inland wedge and
the developing low, allowing for solid Small Craft Advisories
for all Atlantic waters. Winds start the night N-NE as high as
20-25 kt, with gusts near 30 kt. There will even be a few gusts
near Gale Force as a 35-40 kt low level jet moves through early
on. But any such winds are too isolated and/or infrequent to
raise a Gale Warning. Winds back a bit more to the N and NNW
late as the low gains latitude, and speeds do drop off a little
behind the low level jet. Seas will average 5 to 7 feet, except
for up to 8 feet on the outer Georgia waters. Winds in
Charleston Harbor will start off NE around 15 kt, then back more
N at 10-15 kt late.

While light to moderate rains will be common throughout, there
will also be a risk for a few t-storms. This is mainly on the
South Carolina waters and parts of the Georgia waters out beyond
20 nm from shore. There might be just enough instability
(either elevated or surface based) for isolated t-storms to
occur.

Tuesday: A coastal low will track northeast away from the
marine zones through the period. Gusty northwest winds will
remain across the coastal waters during the morning, veering
from the northeast during the night. Seas are forecast to begin
the day between 4 to 7 ft, decreasing to 3 to 5 ft during the
evening. Small Craft Advisories will remain across the
Charleston County nearshore waters and outer GA waters through
most of the day, then ending in the evening.

Christmas: High pressure will build across the region,
supporting northeast winds between 15 to 20 kts. Seas are
forecast to range from 3 to 5 ft.

Wednesday Night through Saturday: A sfc ridge will remain
across the coastal waters during the period, speeds generally
between 15 to 20 kts. Wednesday night into Thursday, winds may
surge to around 25 kts. As a result, seas are forecast to build
3 to 5 ft across the nearshore, with 6 to 7 ft across the outer
GA waters. A Small Craft Advisory may become necessary for the
outer GA waters on Thursday. Otherwise, conditions appear to
remain below Small Craft Advisory criteria.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 8 PM EST Tuesday for AMZ350.
     Small Craft Advisory until 1 PM EST Tuesday for AMZ352.
     Small Craft Advisory until 7 AM EST Tuesday for AMZ354.
     Small Craft Advisory until 2 AM EST Wednesday for AMZ374.

&&

$$