189
FXUS62 KCHS 231522
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1022 AM EST Mon Dec 23 2024

.SYNOPSIS...
High pressure will prevail today as a weak area of low pressure
develops offshore through tonight, before lifting away from the
area Tuesday. High pressure is expected to return by mid week.

&&

.NEAR TERM /THROUGH TONIGHT/...
The coastal trough is slowly starting to sharpen off the Florida
and Georgia coast, while the inland wedge and its associated
cold air damming holds firm. Isentropic ascent will continue to
strengthen across the area, and a short wave approaches from the
west-southwest late. These conditions will initially lead to a
further increase and thickening of the stratocumulus cloud
cover, before some showers will advance onshore. We show
isolated to scattered showers arriving onshore from Beaufort
County south to Long and McIntosh County, and it`ll likely stay
rainfree elsewhere through the day. Temperatures are a huge
challenge, with low level warm advection counteracted by the
cloud cover and a persistent northeasterly breeze around 10 or
15 mph (even around 20 mph close to the ocean). Throughout the
day temperatures will generally only be the the 40s, even far
inland where there will initially be sunshine.

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 /15Z MONDAY THROUGH FRIDAY/...
The 12z TAF period begins with VFR ceilings in place at KCHS,
KJZI, and KSAV. VFR should prevail for most of the period, with
the chance of MVFR conditions increasing beginning this evening.
The ongoing VFR stratocumulus deck is expected to gradually
lower today, with MVFR ceilings begin at KSAV around 23z and
KCHS/KJZI around 01-02z. The potential for showers also
increases this afternoon at KSAV, and then by the early evening
at KCHS and KJZI. Current thinking is that showers will be
pretty light, so ceilings will control the flight category as
the potential for sub-VFR conditions increases. Showers will end
before sunrise on Tuesday, but model guidance suggests that
ceilings could lower into the IFR range near the end of the TAF
period.

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

&&

.MARINE...
Today 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. 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...BSH/ETM
MARINE...