-- Discussion containing changed information from previous version are highlighted. --
669
FXUS62 KCHS 181043
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
643 AM EDT Sat May 18 2024

.SYNOPSIS...
A storm system will bring impacts to our area this weekend.
High pressure returns to our region next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
Showers and thunderstorms continue to evolve to our W/SW with
the first bit reaching our far southwestern counties after 9 am.
Notably, recent high-res guidance runs diminish the convection
across central Georgia while pushing into the forecast area,
likely in response to the weakening LLJ. Will see how that goes,
but forecast refinements are likely through the day.

Previous discussion...
Overnight composite analysis reveals a short-wave trough axis
extending from Kansas down through central Texas with a couple
of embedded circulations; one over Arkansas, a second in
eastern Texas and possibly a third near the Louisiana/Mississippi
coastal region. A corridor of stronger low-mid level flow/moist
advection ahead of the trough in tandem with short-wave forcing
has kicked off several clusters of thunderstorms overnight from
the Louisiana Gulf Coast up through central/southern Alabama
and about to expand into western Georgia while spottier shower
activity stretches through central Georgia. Meanwhile, a
secondary plume of low level moisture is in place across the
central Gulf into northern Florida where convection has
persisted overnight in the northern Florida Panhandle region.

Convective evolution and risk for strong/severe storms later
this morning is the main forecast concern.

An overall convoluted and active pattern is emerging across the
region. Short-wave trough will advance along the northern
Gulf/Deep South region through today. With associated forcing
and deep layer moisture transport, ongoing convection from the
Louisiana Gulf Coast into Alabama is likely to continue expanding
through Georgia this morning and into parts of southeast South
Carolina late morning and into the afternoon, possibly congealing
with another line of convection that may develop across the
Florida Panhandle over the next several hours. Additional
shower/thunderstorm development might also occur further north
across northern Georgia into South Carolina in response to
daytime heating and increased forcing for ascent. Upshot; we
have increased precip chances across the forecast area with the
highest chances and higher impact anticipated across southeast
Georgia later this morning and into the afternoon hours.

Severe weather potential: Strongest convection and greater
severe weather threat is likely to occur across southern Georgia
and the Florida Panhandle region where the greatest instability
will be located. A lower severe weather risk is anticipated
further north, although strong mid level and respectable 0-6km
shear values in excess of 40 knots into the afternoon will
support storm organization and a risk for severe storms with
the primary threat damaging winds and highest risk across
southeast Georgia where better instability will be found. Heavy
rainfall is also possible given PWAT values increasing to around
2 inches. Given the anticipated convection developing across the
Florida Panhandle, the heaviest rainfall potential appears to
be across southeast Georgia and a bit further south than
previous forecast. Rainfall amounts have been adjusted
accordingly.

Tonight: Upper level trough settles into the region. The bulk
of showers/storms is looking to diminish and move off the
Atlantic Coast later in the afternoon into this evening, but
there may be some additional sub-severe convection that
percolates through the region through the course of the night as
the upper trough settles in.
-- End Changed Discussion --

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
Sunday: A positively tilted mid-level trough will be over the
Southeast U.S. in the morning. It`ll slowly shift offshore into the
evening and overnight. Meanwhile, a ridge will build in from the
west. A cold front will be located just offshore in the morning, and
moving away as time progresses. Surface troughing is expected to
move over our region in the afternoon, then shift offshore into the
overnight. Deep moisture will remain in place across the region with
PWATs in the 1.5-2.0" range. These values are above the 90% mark per
SPC sounding climatology for CHS. They`re also about 2 standard
deviations above normal per NAEFS. With all of the moisture and lift
in place, models are in good agreement indicating scattered showers
in the morning increasing to numerous or widespread showers by the
afternoon. Temperatures should peak in the upper 70s or lower 80s,
limited by the aforementioned showers. This will also limit
instability. MLCAPE should peak around 1,500 J/kg, mainly across our
GA counties. Lapse rates won`t be that steep and 0-6 km bulk shear
may only peak in the 15-25 kt range, which is not that strong. So
while there should be some thunderstorms, the overall severe risk is
low. Locally heavy rainfall will be a bigger concern given the high
PWATs and slower storm motions. A relatively quick 0.5-1.0" is
possible, so we can`t rule out flooding in some areas. This could be
exasperated if soils are saturated from Saturday`s rainfall. Showers
will decrease in coverage and intensity from west to east during the
evening and overnight hours. Most locations should be dry by
daybreak Sunday, except maybe the immediate coast. Lows should range
from the upper 50s to the mid 60s.

Monday: The mid-levels will consist of a positively tilted trough
just off the Southeast U.S. coast in the morning, and a ridge just
to our west. The ridge will slowly build into our area as time
progresses. At the surface, High pressure centered well to our north
will build down the coast, with it`s southern periphery making its
way into our area. The High will bring drier conditions with skies
becoming mostly sunny/clear. Low-level thickness values support
temperatures in the middle 80s. But we went a little below this due
to the northeasterly surface winds and the expectation that surface
evaporation should limit temperatures. Highs should peak in the
lower 80s. Lows should range from the upper 50s far inland to the
mid 60s closer to the coast.

Tuesday: A positively tilted mid-level trough will be just off the
Southeast U.S. coast in the morning, while a ridge will prevail over
the Southeast U.S. High pressure centered just to our north in the
morning will shift off the Mid-Atlantic Coast as the day progresses.
Though, it`s southern periphery will continue to dominate our
weather. Subsidence will yield dry conditions with mostly sunny
skies. Temperatures should rise to near normal.

&&

.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
Mid-level ridging will be over our region through Wednesday,
followed by southwest flow on Thursday. A weak trough may pass to
our north on Friday. Broad surface High pressure will stretch from
just to our north into the Atlantic Tuesday night into Wednesday.
It`s expected to shift offshore later Thursday. A cold front could
approach from the northwest later Friday, possibly bringing showers
and thunderstorms. Temperatures will be trending higher, reaching
into the 90s by Thursday.

&&

.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
KCHS/KJZI/SAV: SCT-BKN MVFR cloud cover has materialized at the
KSAV terminal in the last few hours. Otherwise, VFR conditions
is looking to hold early this morning.

Meanwhile, terminal locations will see increasing chances for
showers and some thunderstorms later this morning and into the
afternoon hours, with the greater impact likely to occur at the
KSAV terminal. We have a period of flight restrictions at all
three terminals between roughly 14Z and 20Z with further timing
refinements highly likely. Bulk of showers/thunder will exit the
region Saturday evening.

Extended Aviation Outlook: A departing storm system could bring
periodic flight restrictions on Sunday. VFR returns on Monday.
-- End Changed Discussion --

&&

.MARINE...
A weak surface pressure pattern will maintain overall lighter
winds (10 to 15 kt) and seas 2 to 4 ft across the coastal
waters through tonight. That said, a period of showers and
thunderstorms is anticipated later this morning and through the
afternoon hours. Some strong storms will be possible
which may produce locally strong winds and higher seas.

Extended Marine: Westerly flow early Sunday will turn to the NE
and increase Sunday night. Wind gusts could briefly approach 25
kt late Sunday night into Monday morning, mainly over the
Charleston County nearshore waters. But no Small Craft
Advisories are anticipated. Winds should gradually ease later
Monday. High pressure will continue to bring NE or E winds
through Tuesday. Winds should then shift to the SE on Wednesday.
Seas will remain in the 2-4 ft range.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$