327
FXUS62 KCHS 091955
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
355 PM EDT Sun Mar 9 2025

.SYNOPSIS...
Low pressure will approach the area from the Florida Panhandle
tonight, before it moves into the nearby Atlantic Monday. The
low will then strengthen as it pulls away Monday night, with
high pressure to then prevail through much of the rest of the
week.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
09/18z surface analysis showed ongoing cyclogenesis across the
north-central Gulf coast along a stationary boundary that
stretches from southeastern Louisiana east across far northern
Florida. The developing frontal low will slowly deepen as it
moves east tonight and becomes increasingly influenced by the
power southern stream shortwave that is propagating east out of
Texas and into the lower Mississippi Valley. The combination of
strong DCVA ahead of the upper trough coupled with modest
isentropic ascent within an increasingly moist atmosphere
characterized by PWATs >1.50" will support a steady increase in
rain this evening and overnight as the surface low begins emerge
off the Georgia coast closer to daybreak. A corridor of locally
heavy rainfall could develop somewhere in the vicinity of a
Metter-Springfield-Charleston Metro line later this evening as
a pocket of enhanced elevated instability becomes aligned with a
west-east oriented area of 925-500 hPa frontogenesis. While
flash flooding is not anticipated given the recent dry spell and
ongoing drought conditions, some pockets of minor flooding of
low-lying and poor drainage areas can not be completely ruled
out, especially if some sustained convective elements can form.

Pops near 100% were highlighted for all areas tonight. Lows
will drop into the mid 40s inland with lower 50s at the beaches,
although temperatures may rise along parts of the Georgia coast
overnight late as the warm sector could briefly move onshore as
the surface low begins to emerge off the coast. Breezy
conditions will persist along the beaches and barrier island.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
Monday into Monday night will be the most active weather period of
the next several days, with the main issuing revolving around the
amount of rain, whether or not there will be any t-storms, as well
as winds across Lake Moultrie.

The day begins with low pressure very close to our southern zones,
as it lifts near or just offshore the coast through the afternoon,
and eventually begins to pull away at night, while deepening several
millibars. Good forcing ahead of the mid and upper level low
approaching from the west will coincide with a swath of deep
tropical-like moisture in the early morning, where PWat is near or
greater than 1.5 inches. But for the late morning and early
afternoon, moisture decreases and a somewhat pronounced dry slow
moves in, as the surface low reaches the nearby Atlantic. There will
still be some showers around, but not as widespread of steady as
early on. By late afternoon and into the evening, the strongest QG
forcing occurs with the intense low aloft to sweep through. This
will cause an increase again in coverage and possible intensity to
the showers. Eventually the rains will come to an end Monday night,
as the low, both surface and aloft pulls away, allowing for much
drier and sinking air to arrive.

The rainfall forecast presently has about 1.5 to 2 inches across the
entire region with this event, with some locally higher amounts,
especially in locations that are able to get any t-storm activity.
The grounds look to be able to take these rains, with the exception
of city and urban locations, where there could be some minor
flooding concerns. These rains will certainly help in easing the
Moderate Drought that is ongoing. Given the forcing aloft and at
least elevated instability, we do show slight chance t-storms into
the evening, but coverage might need to be increased if convection
is able to become surface based.

Cold advection will develop as the night wears on, leading to a risk
for a Lake Wind Advisory over Lake Moultrie. Since it is third
period, we held off for now, but it does appear likely given the 40-
50 kt of geostrophic winds at 1000 mb, and decent isallobaric
pressure climbs.

Highs Monday will range from the middle and upper 50s along the
northwest, where they are deeper into the cooler air mass, while
lower and middle 60s are common closer to the coast. Some
fluctuations could occur, depending on the exact track of the
surface low. Monday night will turn out quite a bit colder with a
continued influx of colder air on brisk north-northwest winds. Lows
will fall to the lower and middle 40s, except some upper 30s from
Allendale to Bulloch County. Frost is not expected due to the pre-
existing wet grounds and too much wind.

Tuesday through Wednesday: Flat ridging will prevail aloft, while
high pressure initially in the northern Gulf on Tuesday will pass
into the Atlantic off Florida during Wednesday. This will favor
mostly clear/clear skies and warmer conditions. Combining a mix of
the MOS and NBM guidance with the low level thickness forecast, we
arrive at highs in the lower and middle 70s Tuesday, middle and
upper 70s Wednesday. Local sea breeze influences will hold shoreline
communities down quite a bit cooler, especially Wednesday, when the
sea breeze gets an earlier start.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
A short wave passes through later Thursday, but it has very limited
moisture to work with. So we keep a rainfree forecast going. A more
amplified ridge aloft will control the weather Friday into
Saturday, before a deeper upper trough draws closer into Sunday.
Latest indications are for the best forcing of that feature to stay
to our west and northwest, but it will still drive a cold front
across the area late Saturday night and Sunday. That`s the next
chance of rains, and there could be some t-storm potential as well,
pending the timing. the severe risk is non-zero, since there could
be some surface based instability, and shear looks present.
Temperatures will be well above normal during the entire extended
period.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
09/18z TAF Discussion:
KCHS/KJZI/KSAV: Rain with embedded tstms will begin to fill
back in late this afternoon and continue into the overnight
hours as low pressure approaches the area. IFR conditions will
develop this evening and linger into Monday morning as the low
passes just east of the terminals. There is a risk for a tstm or
two just about any time through 18z Monday, but probabilities
are too low at any particular time period to include a mention
at this time.

Extended Aviation Outlook: Flight restrictions are expected with
rainfall and possible thunderstorms Monday afternoon into the
evening, as a low pressure system impacts the region. Depending on
the strength of the surface winds Monday night behind the low,
there could be some marginal LLWS.

&&

.MARINE...
Tonight: A pinched pressure gradient ahead of developing low
pressure and high pressure to the north will keep steady
northeast in place through tonight. Speeds will average 15-20 kt
with frequent gusts to 25 kt, a bit lower in the Charleston
Harbor, with seas 3-5 ft. Small Craft Advisories are in effect
for all legs except Charleston Harbor through tonight.

Monday and Monday night: Low pressure will pass through the waters
Monday, then deepen as it passes to the northeast at night. There
will be some modest improvement initially as the low passes through,
but this is only temporary, as cold advection, large isallobaric
pressure rises occur with 40-50+ kt of geostrophic winds at night.
While all waters will likely be beneath Small Craft Advisory
conditions for much of Monday, it looks to be too short-lived on the
Charleston County Atlantic waters, and for the Georgia waters beyond
20 nm offshore. Thus we kept it going before it transitions to a
Gale Watch for both of those waters late Monday and Monday evening.
The watch will likely be upgraded to a Gale Warning as the event
draws closer. The other nearshore Atlantic waters have sub-advisory
conditions long enough where we let the advisory come down. But it
will need to be re-issued Monday night, or maybe even require a Gale
Warning as well. Charleston Harbor seems to be beneath any advisory
levels until Monday night, when we hoist an advisory for that area.

Mariners are also advised for the possibility of t-storms, a few of
which might become strong, given the proximity to the surface low,
and the approach of low pressure aloft.

Tuesday and beyond: Lingering Gales and/or Small Craft Advisories
early Tuesday will be followed by improving marine conditions
thereafter, as high pressure becomes the main weather feature
through late week. Something to watch for will be the potential for
sea fog late week given a much warmer weather with higher dew
points.

&&

.TIDES/COASTAL FLOODING...
Although we aren`t forecasting tides reaching 7.0 feet MLLW with the
early morning high tide in downtown Charleston (~6 AM), given that
there will be moderate rains ongoing, this could lead to some minor
flooding problems. Additional rains will occur again with the
evening high tide (~6:30 PM) but tide levels will be lower. Still,
there is some concern that minor flooding could again occur.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory from 11 PM Monday to 7 AM EDT Tuesday for
     AMZ330.
     Small Craft Advisory until 11 PM EDT Monday for AMZ350.
     Gale Watch from Monday evening through Tuesday morning for
     AMZ350.
     Small Craft Advisory until 8 AM EDT Monday for AMZ352-354.
     Small Craft Advisory until 5 PM EDT Monday for AMZ374.
     Gale Watch from Monday afternoon through Tuesday morning for
     AMZ374.

&&

$$

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