290
FXUS62 KCHS 231828
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
128 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 /THROUGH TONIGHT/...
For the early afternoon update, the surface pattern remains
status quo, with the trough offshore and the CAD wedge inland.
Now that skies have become mostly cloudy or overcast,
temperatures will struggle to rise more than another degree or
so. Combined with the winds it`ll feel several degrees colder.
Showers offshore have had a difficult time penetrating the dry
air mass over land, but we do maintain isolated to scattered
PoPs as before from Beaufort County south to McIntosh and Long
County.

Tonight: The southern stream shortwave aloft will shift into
the area through the early morning hours, then start to exit to
the northeast by sunrise Tuesday. As the shortwave passes, the
offshore surface wave is expected to become a closed low,
staying just off the SC coast. A nicely defined slot of low-
level convergence is expected to spread northward into southeast
SC beginning in the late evening and early morning hours,
producing an increasing coverage of showers. We have increased
the rain chances into the 60-70 percent range for the Colleton
County and the Tri-County region, with a relatively sharp
gradient expected on the western edge of the area of showers.
Also of note, guidance indicates Showalter Indices falling to
around zero within the low-level forcing area just offshore,
along with surface-based instability beyond the nearshore
waters. We have maintained the mention of thunder for the
coastal waters, and don`t think the potential for land areas is
particularly high. Despite the relatively high rain chances,
rainfall amounts aren`t expected to be very significant. For
southeast SC, max amounts should be no more than 0.10-0.20",
with less than 0.10" along the Savannah River and into southeast
GA. Overnight lows are forecast to fall into the mid to upper
30s inland, and the low 40s along the coastal corridor.

&&

.SHORT TERM /TUESDAY THROUGH THURSDAY/...
Models are consistent in indicating an area of low pressure
will be located off the SC coast at the start of the
period/Tuesday morning. The low should gain latitude through the
day, allowing high pressure to reassert itself over the area.
While some shower activity could linger over South Carolina
zones in the morning, the latter half of the day will be dry as
the low moves away and deeper moisture exits. There will likely
be a decent temperature gradient with highs ranging from the low
50s in the northern zones near the Santee River to around
60/low 60s over far southeast Georgia. Lows Tuesday night fall
to the mid/upper 30s, except low 40s near the coast.

A ridge of high pressure will extend south into region for
Wednesday/Christmas Day and Thursday, with another coastal
trough likely developing offshore. Aloft, mid level ridge will
briefly give way to shortwave energy moving in Wednesday night
into Thursday. No notable weather concerns during this
timeframe. Some guidance hints at isolated showers Wednesday
night into Thursday, but probabilities for measurable rainfall
are too low to explicitly mention in the forecast at this time,
so a dry forecast holds. Temperatures will be seasonable with
highs largely in the 60-65 F range both days. Lows Christmas
night will range from the upper 30s across the interior to the
low to upper 40s closer to the coast.

&&

.LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/...
High pressure will persist inland late week, while a coastal
trough lingers offshore. Forecast becomes more uncertain over
the weekend due to notable differences in model solutions.
Consensus would indicate mid level ridge over the Southeast will
give way to a larger scale trough moving in from the west,
though strength and timing of that feature is uncertain. This
system could bring a cold front to the area. At this time, PoPs
are no higher than 30-40%. Seasonable temperatures Friday should
trend on the warmer side of normal over the weekend.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
KCHS/KJZI: BKN to OVC ceilings around 3500-4000 feet will
continue into early tonight, before eventually dropping into the
MVFR range as low pressure develops offshore. This will also
result in the onset of light rain, which looks to become
steadiest between 04Z and 11Z Tuesday. IFR ceilings are likely
late tonight into Tuesday morning with plenty of moisture
trapped beneath a very prominent inversion. For now we have
ceilings at 1000 feet, with subsequent forecasts to show IFR if
necessary as we get closer to when it could occur.

KSAV: Low end MVFR ceilings will drop into the MVFR this
evening, and persist as such into Tuesday morning in response to
low pressure offshore. the better chance for steady rain looks
to stay north and east of the terminal, so we prefer to show
VCSH from 22Z to 06Z. There is a low end chance of IFR
overnight, but probabilities are too low to show such at this
time. VFR appears to return later Tuesday morning as the surface
low pulls far away, and drier air returns.

Extended Aviation Outlook: Flight restrictions in low clouds
could linger into Tuesday. Otherwise, VFR.

&&

.MARINE...
This afternoon through tonight: Northeast flow will persist
today, then turn more northerly overnight as a surface low
develops. The presence of the developing low will help to pinch
the gradient across the local waters, producing strong northeast
flow into the evening. Winds of 20-25 knots with frequent gusts
up to around 30 knots will still occur. There will even be some
gusts on the Charleston County Atlantic waters that will be near
35 kt at time. But they look to be too limited in area to go
with a Gale Warning.

Winds will diminish a bit late tonight, but should still hold
in the 15-20 knot range. Seas are expected to average 4-7 feet
across the nearshore waters, and in the 5-8 ft range for the
outer waters. For Charleston Harbor, northeast winds will peak
in the 15-20 knot range with an occasional gust up to 25 knots
possible. Conditions will likely be close to Small Craft
Advisory criteria, but current thinking is it will stay just
below. For the rest of the waters outside of Charleston Harbor,
Small Craft Advisories remain in effect through the period.

Tuesday through Saturday: Marine conditions are expected to
improve through the day on Tuesday as weak low pressure off the
SC/GA coast moves away from the area. All Small Craft Advisories
are expected to be down by 00z Wednesday. The pattern through
the latter half of the week will largely be characterized by
high pressure inland and another developing coastal trough.
Northeast winds in between these two features will be gusty,
possibly approaching advisory criteria at times Thursday into
Friday. 6 foot seas could necessitate an advisory over the outer
Georgia waters late week.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 4 PM EST Tuesday for AMZ350.
     Small Craft Advisory until 10 AM EST Tuesday for AMZ352.
     Small Craft Advisory until 7 AM EST Tuesday for AMZ354.
     Small Craft Advisory until 7 PM EST Tuesday for AMZ374.

&&

$$

NEAR TERM...
SHORT TERM...ETM
LONG TERM...ETM
AVIATION...
MARINE...