Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 040250

Area Forecast Discussion
National Weather Service Charleston SC
950 PM EST Sat Dec 3 2016

High pressure will prevail with dry weather through tonight. A
storm system will affect the area Sunday through Tuesday. A
strong cold front will sweep through late next week with cold
high pressure building in by next weekend.


A closed mid level cyclone over the Gulf of California will
drop southeast along the coast of Mexico, inducing flat ridging
across the southeast states. Mid and upper level moisture will
feed into the area within the westerly flow aloft accompanied by
subtle mid level impulses, with water vapor imagery depicting
this quite well as the moisture feed taps into the Pacific. At
the same time, surface high pressure in West Virginia will
continue to ridge across the local area, maintaining a dry low
level air mass.

Isentropic ascent will continue to increase tonight, and since
the models oftentimes are too slow in the development of over-
running or isentropic generated rains, none of them are
depicting any of the rains already moving into our northwest
tier. These rains appear to be associated with a weak mid level
perturbation and based on the trajectory forecast they would
track due east through the night. But given the dry sub-cloud
layer below 7-10K ft we feel that some of those rains could
evaporate before making it to the coast of South Carolina. As a
result, we have adjusted the forecast to show up to 50% chance
of light rains from inland Berkeley to Allendale, with 20-30%
PoP from tidal Berkeley to Hampton, Screven and Jenkins
counties. Little to no chance of light rains elsewhere. Where it
does rain the QPF will be light; no more than a few hundredths
of an inch.

Mid and high level clouds will be the rule through the night,
as the atmosphere gradually moistens from the top on down.
Combined with some influx of low level moisture off the Atlantic
across extreme southeast Georgia, this will equate to overcast
skies. Although there is very weak cold advection from the
north, temps won`t drop too much between now and daybreak
Sunday, as we look for readings near or a tad higher than what
is typical for early December.


Sunday will be a transition day into a more active weather pattern.
Cool and dry high pressure to the north will extend a wedge of cool
high pressure down the Atlantic coast while an upper level digging
trough to our west and 850 mb high shifting off the Southeast coast
will allow for efficient moisture transport over our area throughout
the column above the boundary layer. Isentropic lift will prompt
light rainfall to begin inland and work toward the coast through the
late afternoon and evening, with the moistening of the dry surface
layer being the main inhibitor to precip coverage.

By Sunday night, abundant moisture advection will be occurring on
the back of a strong low level jet. The warm front along the edge of
the eroding wedge is expected to lift north Sunday night as the
initial surface high gets shunted to the east, allowing temps to
rise modestly through the night. Weak disturbances embedded in the
mid-level flow could lead to periods of enhanced rainfall Sunday
night and Monday morning, especially along the front.

The progression of the warm front will be short lived however, as
surface high pressure sliding from the Midwest to New England will
quickly reintiate the CAD regime. 850 mb flow will shift more
westerly Monday afternoon and evening, retarding the moisture tap
and bringing a temporary reduction in shower coverage and intensity.
However, given the backdoor cold front still residing somewhere near
or south of the Savannah river and the moist airmass already in
place, likely POPs have been kept in the forecast.

Tuesday is the most uncertain, and likely the most active day of the
period. Falling heights and divergence aloft will lead to
destabilization of the column as a shortwave passes near the area in
the afternoon. Phased low pressure at the surface will slide up
Tennessee Valley through the day and ample moisture advection will
ensue on the back of a nearly 40 kt LLJ. PW values Tuesday are 2 to
3 standard deviations above normal and moderate to heavy rainfall is
possible at times, prompting WPC to extend the slight chance of
excessive rainfall into our inland counties, with marginal
elsewhere. The key to how active the day will be rests on the
ability of the warm sector to expand across the CWFA from the south.
If the warm sector does encompass the area, ample low level shear
and CAPE values of 1000 J/kg or higher will make thunderstorms
producing damaging wind gusts, and isolated tornados a possibility.
High uncertainty remains in the erosion of the CAD, a feature which
is notoriously poorly handled by the global models.


Mainly dry and mild Tuesday night through Wednesday night before
another cold front sweeps through Thursday or Thursday night.
There are still some model differences regarding timing and
precipitation associated with this front though the trend is
drier. All indications are that a polar airmass will reach the
area Friday into Saturday with a marked cooldown.


VFR ceilings will prevail at KCHS and KSAV through most if not
all of the valid 00Z TAF cycle as surface high pressure remains
atop the area from the north. However, moisture and lift will
increase in advance of a warm front that heads east-northeast
along the northern coast of the Gulf of Mexico. This will allow
for at least some potential of MVFR ceilings along with light
rain for late Sunday, especially at KCHS. Sporadic rains could
develop prior to then, but no sub-VFR conditions would occur.

Extended Aviation Outlook: A storm system and associated
precipitation will likely produce periods of flight restrictions
through Tuesday. VFR weather returns for Wednesday and


Tonight: A modest NE-E gradient will prevail around the
circulation of 1027-1028 mb high centered in West Virginia.
Winds will be as high as 8-12 kt in Charleston Harbor, up to
15-20 kt over the open ocean. The favorable fetch allows for
seas to reach as high as 3 or 4 ft, greatest on the Georgia

Sunday through Thursday: Marine winds will oscillate between
southwest and northeast Sunday through Monday night as a wedge
of high pressure to the north gets replaced by a secondary
wedge. Mainly 2-4 ft seas in southerly windswell during this
time. Tuesday, a strong low level jet will fill in over the
waters, bringing SCA strength winds and seas near 6 feet,
especially for the offshore Georgia waters. Fairly weak winds
expected Wednesday through Thursday until strong cold advection
behind a late week front likely results in advisory conditions
for some or all of the waters.

Sea fog is possible Monday night and Tuesday morning mainly across
nearshore waters south of the Edisto River as very moist air moves
over the relatively cooler shelf waters.




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