Area Forecast Discussion
Issued by NWS Charleston, SC

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842
FXUS62 KCHS 270524
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
124 AM EDT Tue Sep 27 2016

.SYNOPSIS...
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.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Scattered rains have been expanding to the east of I-95 early
this morning. Current trends on pops look decent per latest
radar trends but the most significant downpours will likely
be confined closer to the coast from Savannah north.

Shower and some tstm activity expanding over the coastal waters
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 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 coast.

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.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH THURSDAY/...
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
closer.

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.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
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.

&&

.AVIATION /05Z TUESDAY THROUGH SATURDAY/...
Overnight, isolated to scattered showers will pass through the
coastal corridor with periods of brief MVFR/IFR cigs and vsbys.
Late tonight, the better chance of showers will be near KCHS while
IFR or lower cigs are possible at KSAV as light north flow ensues.
After mid morning VFR conditions are expected at both terminals
with most of the rain over by that time.

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

&&

.MARINE...
Overnight: East to northeast winds will persist 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.

&&

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

&&

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



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