Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 252141
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
541 PM EDT Mon Mar 25 2024

.SYNOPSIS...
High pressure will prevail through early Tuesday. A storm
system will impact the area Wednesday into Thursday, followed
by high pressure into next weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Aloft, a ridge of high pressure will prevail across the Southeast,
but its axis will slowly pivot offshore overnight well in advance of
a large trough traversing the Central United States. At the sfc, a
high pressure wedge extending across the Mid-Atlantic and Southeast
states will hold across the local area this evening with a weak
coastal trough lingering off the Georgia/Florida coast, before
starting to retreat north late well in advance of cold front
tracking across the Mississippi River Valley. Temps will remain
slightly more mild than the previous night with weak warm air
advection in place, sfc winds turning more easterly and isentropic
lift/low-lvl moisture favoring some clouds drifting onshore.
However, the mid-lvls will remain quite dry through the night,
favoring rain-free conditions for all areas. Overnight lows should
range in the upper 40s across inland areas of southeast South
Carolina and middle 50s across inland areas of southeast Georgia.
Closer to the coast, low temps are expected to remain in the upper
50s with potentially breezy conditions (15-20 mph winds) into late
night.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
Tuesday: Broad H5 ridging along the Eastern Seaboard will quickly
breakdown as a large longwave trough over the central and western
portion of the country nudges east. A potent southern stream
shortwave will help drive a cold front to the east as it begins to
eject out of the Southern Plains in response to its phasing with a
northern stream shortwave digging into the Northern Plains. At the
surface, the high pressure wedge will shift offshore into the
Atlantic during the day. The increasing southerly flow will likely
maintain some degree of stratocumulus moving inland from off the
Atlantic as an extensive high cirrus deck persists over the
Southeast States. This will yield mostly cloudy to cloudy conditions
into Tuesday night as the cold front to the west draws closer. Highs
look to warm into the lower-mid 70s over Southeast South Carolina
with mid-upper 70s over Southeast Georgia, except cooler at the
beaches. Lows Tuesday night will range from the lower-mid 60s with
a few upper 50s in the Francis Marion National Forest.

A few showers, mostly aided by pre-frontal warm air advection-
isentropic ascent, could propagate into far interior portions of
Southeast Georgia during the afternoon. Similarly, a rouge shower or
two could move inland from off the Atlantic, but models are
typically overdone with generating precipitation over the Atlantic
in onshore flow. Overall, expect most locations will remain rain-
free through sunset. Rain chances will begin to slowly ramp up late
Tuesday night as the region becomes more fully embedded in the zone
of pre-frontal warm air advection/isentropic ascent and a broad band
of rain associated with the approaching cold front draws ever
closer. It is around this time that the cold front begins to slow
down as it become increasingly parallel to the the flow aloft. How
quickly the front slow down will have an influence on how much
coverage of rain will spread east. For now, mentionable pops were
limited to areas along/west of I-95 with the highest pops (up to
60%) were limited to the Reidsville-Sylvania-Metter-Allendale
corridor. Further adjustments will likely be needed.

Wednesday and Thursday: This looks to be the wettest period of the
next seven days. The aforementioned cold front looks to stall over
Southeast South Carolina and Southeast Georgia on Wednesday as a
frontal wave begins to develop along the front and moves northeast
along it. This is in response to the large, mostly phased mid-level
trough nudging east out of the Plains. Deep-layered forcing looks to
peak Wednesday night into Thursday morning as a piece of negatively-
tilted vorticity propagates through the area as it rounds the base
of mean upper trough. PWATs >1.50 inches, a moist, southerly flow
fed by a 30-40 kt 850 hPa low-level jet and even some hint at some
surface based instability will support a broad region of light to
moderate rainfall. While a general wet trend is noted, there remains
some model uncertainties with respect to timing as much will depend
on where the front stalls and the corresponding frontal low tracks.
For now, pops were limited to 50-70% for Wednesday (highest west of
I-95) and 80% Wednesday night with 40-70% Thursday morning (highest
along the middle South Carolina coast). Similar to Tuesday, further
adjustments will likely be needed as models begin to better resolve
front`s timing. A general 1-2" rainfall looks reasonable for the
area. Flash flooding does not appear likely, but some minor flooding
of low-lying and poor drainage areas can not be completely ruled out.

Highs Wednesday will warm into the lower-mid 70s, then cool to the
upper 60s to near 70 by Thursday as low pressure pulls away and a
cooler and drier airmass settles into the region. Lows Thursday
morning will drop into the mid-upper 50s inland to around 60 at the
beaches.

Lake Winds: Winds will increase on Lake Moultrie Thursday in a post
frontal cold air advection regime. Expect northwest winds of 15-20
kt with gusts around 25 kt. The 25/13z NBM shows >80% chance for
wind gusts exceeding 25 kt on the open lake waters, mainly Thursday
afternoon. Waves will build 1-2 ft, mainly over the central and
southeast portions of the lake.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
Mostly quiet weather will dominate through the period with high
pressure centered across the Southeast United States late week
gradually sliding south and offshore through the weekend. Expect
moderating temperatures to unfold locally with afternoon highs
peaking into the lower 80s away from the coast by Sunday,
followed by another warm day in the lower 80s on Monday.

&&

.AVIATION /22Z MONDAY THROUGH SATURDAY/...
KCHS/KJZI: VFR conditions should prevail through 18Z Tuesday.
However, there could be a brief period of MVFR cigs around daybreak
Tuesday. Otherwise, northeast winds gusting up to 15-20 kt are
possible this afternoon, then again Tuesday afternoon.

KSAV: MVFR cigs should prevail for the next couple hours while low
clouds shift onshore. Tempo VFR conditions should return heading
into late afternoon (20-23Z), followed by prevailing VFR conditions
tonight and through 18Z Tuesday. Otherwise, northeast winds
gusting up to 15-20 kt are possible this afternoon, then again
Tuesday afternoon.

Extended Aviation Outlook: MVFR ceilings with possible periods of
IFR expected late Tuesday night through Thursday. Occasional vsby
restrictions possible in showers/tstms, especially Wednesday and
Wednesday night.

&&

.MARINE...
A quick update was made to issue a Small Craft Advisory for the
Charleston Harbor. Observations show persistent gusts around 25
kt over the last hour or so, with models showing this trend
continuing to around midnight.

Tonight: A wedge of high pressure extending across the Southeast
will favor an enhanced pressure gradient across the region through
the first half of the night, before gradually retreating north late.
Small Craft Advisories will be ongoing through the night across all
local waters outside the Charleston Harbor, although east-northeast
winds gusting up to 20-25 kt will gradually veer and weaken to 15-20
kt. Seas will also slowly subside, but will remain between 4-6 ft
across nearshore waters and 6-8 ft across offshore Georgia waters
after midnight.

Tuesday: East to southeast winds will prevail as high pressure shifts
offshore of the U.S. East Coast. Speeds will generally remain less
than 15 kt. Seas will start off the day in the 4-6 ft range with 20
NM, but should drop below the Small Craft Advisory 6-foot sea
criteria later in the day. Over the Georgia offshore water, seas
will hold into the 5-7 ft range into Tuesday night. Small Craft
Advisories remain in effect.

Wednesday through Saturday: A broad, southerly wind regime will
prevail Wednesday as high pressure shifts farther offshore a cold
front approaches from the west. The front and an associated area of
low pressure will exit the coast Thursday. Gusty, northwest winds
will occur behind the front Thursday into early Friday within a
period of post-frontal cold air advection. Winds will reach 15-25
with gusts to 30 kt, so Small Craft Advisories will likely be
needed. Seas will peak 4-6 ft over the nearshore waters within 20 NM
and 5-7 ft over the Georgia offshore waters 20-60 NM.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 3 AM EDT Tuesday for AMZ330.
     Small Craft Advisory until 6 PM EDT Tuesday for AMZ350.
     Small Craft Advisory until noon EDT Tuesday for AMZ352-354.
     Small Craft Advisory until 6 AM EDT Friday for AMZ374.

&&

$$

NEAR TERM...BRM/DPB
SHORT TERM...ST
LONG TERM...ST
AVIATION...DPB/ST
MARINE...DPB/ST


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