Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 250818
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
418 AM EDT Sun Sep 25 2016

.SYNOPSIS...
High pressure will build into the area Sunday but will weaken by
late Monday ahead of an approaching cold front. The front will
cross the forecast area and push off the coast by mid week and
high pressure will build into the region by the end of the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Pre-dawn: Skies have been mostly clear overnight with some thin
cirrostratus across South Georgia associated with a weak upper jet
depicted nicely by latest water vapor satellite imagery. Weak low
level moisture convergence should result in an increase in stratocu
along coastal Southeast Georgia, but high condensation pressure
deficits suggest any showers will remain spotty at best. Across
Southeast South Carolina, there is already quite a bit of fog and
ground fog and we maintained patchy/area fog mentions. We could
see some locally dense fog inland from Charleston toward dawn per
latest SREF vsby progs and recent observational trends.

Today: While progressive surface high pressure builds north of the
region today, our low level synoptic flow turns onshore. Deeper
moisture sources are forecast to remain both north and south of
the region today while models indicate a slot of drier and more
stable air bisecting the forecast area. We think most of the area
will remain dry today with partly to mostly sunny skies as a
result of high clouds and scattered cumulus and stratocu. We were
compelled to leave just slight chance pops in to the south of
I-16 in Southeast Georgia, especially near the Altamaha River.
Another warm day is in store with highs in the upper 80s many
areas. A few spots could touch 90 once again since low level
thickness values are similar to yesterday.

Tonight: The combination of high pressure moving off the mid Atlc
coast and development of a weak wedge pattern inland suggests
genesis of a weak coastal surface trough while a weak mid level
disturbance is expected to lift north off the florida coast.
Models show an increase in isentropic ascent and moisture late and
we should see an uptick in coastal showers which could start
translating more inland toward daybreak. We maintained 30-20 pop
scheme from east to west after midnight. Lows in the upper 60s
well inland to lower 70s closer to the coast.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
Heights aloft will be falling through the period as a large cut-off
low slowly drops over the Great Lakes. Troughing overhead will spell
increasingly unstable conditions Monday and Tuesday (compared to the
weekend). At the surface, Monday will begin with onshore flow over
the area around the southern flank of high pressure wedging down the
eastern seaboard. This wedge of high pressure will weaken as the
parent high pushes further off the New England coast during the day
Monday, and our area will become entrenched in pre-frontal warm air
advection, with an inverted trough off the coast.

Thunder chances maintained over land Monday and Tuesday afternoons
thanks to ample moisture and at least modest instability.
Unimpressive lapse rates in the mid levels will keep the severe
threat low both days. Wednesday`s convective coverage is a bit more
uncertain as the axis of highest pre-frontal moisture shifts off the
coast during the daytime. Still, have maintained low-end chance POPs
Wednesday to account for possible convective enhancement immediately
along the approaching front.

Nighttime convection over the water is expected given the inverted
trough that will be in place. The potential for this convection to
bleed back over land justified keeping chance POPs in the forecast
for mainly coastal areas through the overnight hours Monday and
Tuesday.

Despite ample cloud cover and rainfall, temps through the period
will be a couple degrees above normal owing to persistent warm air
advection.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
The cold front will move through the area Wednesday night, with
global models showing the front over the coastal waters by sunrise
Thursday morning. However, given the uncertainty in terms of the
weakening parent low near the Great Lakes, the front may drag its
feet getting through the area, moving over the waters and stalling
out well to our south and west by midday Thursday. Either way,
strong, dry continental high pressure will fill in behind the front
and quiet weather will ensure for the second half of the week. Temps
will be near to a few degrees below normal through the period.

&&

.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
Conditions should remain VFR until late night when the potential
for both low stratus and fog will increase toward daybreak,
especially around the KCHS terminal where models shows a bit drier
air atop the boundary layer. We prevailed MVFR conditions at KCHS
leading up to 12Z but cannot rule out patchy IFR conditions. KSAV
could also see low clouds and fog but confidence is low with a
potential for mid and high clouds acting as a limiter. Otherwise,
VFR conditions will prevail with rain chances mostly south of the
terminal.

Extended Aviation Outlook: Flight restrictions will be possible in
showers and thunderstorms Monday through Wednesday.

&&

.MARINE...
Today: Onshore flow will gradually increase as high pressure
builds north of the waters. Speeds should increase to near
15 kt later with seas 2 to 4 ft. A coastal trough is forecast
to develop tonight and this could result in a pinching surface
gradient over the waters by daybreak on monday. A bit too early
to tell if and where the gradient will tighten a bit, we
continued to indicate 15 kt and seas 3 to 4 ft for the most part.

Monday through Thursday: The gradient will weaken through the day
Monday as the high pressure wedge inland weakens. A coastal trough
over the waters will keep flow mainly 10 kt or less through midweek,
while keeping thunderstorm chances in the forecast, especially
during the early morning hours. A cold front will move over the
waters Thursday. Seas will generally range 2-4 feet through early in
the week thanks to a combination of some lingering swell from Karl
in the open Atlantic and local wind swell.

Rip Currents: Onshore flow in the 10 to 15 kt range and residual
swell from Karl will lead to a moderate risk of rip currents for all
beaches Today.

&&

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

&&

$$
SYNOPSIS...
NEAR TERM...
SHORT TERM...CEB
LONG TERM...CEB
AVIATION...CEB
MARINE...CEB



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