Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 270133

Area Forecast Discussion
National Weather Service Charleston SC
933 PM EDT Mon Sep 26 2016

A low pressure will move up the southeast coast overnight. A cold
front will slowly approach the region Tuesday, then stall over the
area Wednesday. Low pressure will form over the Mid- Atlantic
Wednesday night, ushering the front off the coast Thursday
morning. High pressure will build into the region by the end of
the week.


Rain is beginning to expand over the coastal waters as expected
with a band of mainly light rain moving across parts of Charleston
and Berkeley Counties. The overall thinking has not changed too
much, but did nudge pops up across the coastal counties,
especially from Saint Catherine`s Sound northward. The rest of the
forecast is on track.

Expect shower and some tstm activity to redevelop over the coastal
waters later tonight as another lope of vorticity embedded in a
moisture channel aloft approaches from the southeast and weak
surface low pressure begins to pinch off along a well defined
coastal trough that is in place off the Northeast Florida and
Georgia coasts. Most of the high resolution models keep the
strongest moisture convergence pinned along the lower South
Carolina coast from late evening through early Tuesday morning,
which seems reasonable given the expected evolution of weak
cyclogenesis. The highest pops of 50-60% will be shown in these
areas with rain chances diminishing quickly inland away from the

The greater potential for fog and stratus development looks to be
across interior Southeast Georgia where a decoupled boundary layer
will be juxtaposed with some clearing of the thicker cloud canopy
aloft. Would not be surprised to see some locally dense fog
develop in this area if enough clearing can occur. Lows from the
upper 60s across interior Southeast Georgia to the mid 70s at the
beaches look reasonable.


Tuesday: A stacked/cutoff low will be moving over the Great Lakes
region with a cold front stretching along most of the east coast.
We will be in the prefrontal regime with warm air advection and
higher PWATs (generally no higher than 1.75"). The result is a
chance of showers and thunderstorms across our area. Lift is not
overly impressive, the best being with the front further to our
west. Instability is minimal, which would limit the thunderstorm
potential. As a result, we`re going with a slight chance.
Precipitation could be initiated from vort pulses moving around the
southern periphery of the upper level low, subtle waves in the
inverted trough along the coast, or the remnants of other convective
complexes that will be moving across the region ahead of the front.
It`s too difficult to time these features at this time, so chance
POPs will cover the forecast and refinements will be made as we get

Wednesday the stacked/cutoff low should still be over the Great
Lakes/Midwest region. The cold front will have stalled just to our
west, keeping our area in the warm sector. Models indicate the axis
of highest moisture should be offshore at this time. But with the
increased proximity of the front and slightly better instability,
chance thunderstorms are in the forecast to accompany the chance
showers. Weak low pressure will develop along the front Wednesday
night and then move offshore of the Carolinas by Thursday. There
remain differences between the models over the location of the low
and how fast it moves offshore. The general consensus is it`s
movement towards the coast will help pull the front through our area
by Thursday morning. Some remnant showers are possible through the
afternoon, mainly along the coast. High pressure building in through
the day Thursday will bring drier conditions and fall-like
temperatures to the area.


High uncertainty continues into the long term. Cut-off upper low
will continue to reside over the eastern U.S., though high pressure
trying to build in at the surface should keep conditions dry
locally, at least over land, Friday into the weekend. However, given
the close proximity of the front which will remain stalled over or
near the coastal waters and the area of destabilization under the
core of the upper-level low, there is potential for at least
nuisance showers mainly in the afternoon through the long term. Have
opted for a dry forecast at this time given the high uncertainty,
with temps near to a couple degrees below normal through the period.


Generally VFR. Most of the shower/tstm activity is expected to
remain offshore tonight, but could get close to KCHS at times,
mainly after 09z. Better fog/low-stratus potential looks to occur
across interior Southeast Georgia some of which could ooze into
the KSAV terminal. Will carry a tempo group 10-13z for low-end
MVFR cigs to cover for now since the bulk of this is expected to
remain west of that terminal. The introduction of IFR or lower
cigs may eventually be needed for the KSAV terminal and will be
reevaluated for the 06z TAF cycle.

Extended Aviation Outlook: Showers and thunderstorms associated with
a cold front may bring brief flight restrictions Tuesday afternoon
through Wednesday evening.


Tonight: East to northeast winds will persist tonight as weak low
pressure begins to develop off the Georgia and Northeast Florida
coasts. Winds look to stay about 10-15 kt, but will need to watch
for higher winds from roughly the Savannah River north to see if
the pressure gradient tightens more than expected. Seas will
average 2-4 ft.

Tuesday through Friday: A coastal trough/weak coastal low will
persist over the waters Tuesday as a cold front approaches from the
west. This front will stall just west of the area while the coastal
trough weakens. Low pressure will form over the Mid-Atlantic
Wednesday night, helping assist the front off the coast by Thursday.
High pressure will build into the region by the end of the week.
Winds will generally be less than 15 kt, with 2-4 ft seas. Locally
higher winds and seas are possible in showers and thunderstorms.





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