Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 041141

Area Forecast Discussion
National Weather Service Charleston SC
641 AM EST Sun Dec 4 2016

A storm system will affect the area through Tuesday. A strong
cold front will sweep through on Thursday followed by cold high
pressure into Saturday. Another cold front may affect the area
late next weekend.


Today: The pattern aloft will feature broadly anticyclonic flow
with high pressure at the surface centered near the Mid Atlantic
region. Plentiful mid/upper level moisture will persist, leading
to mostly cloudy skies through the day. Lower levels of the
column will begin to moisten up this afternoon as the center of
the 850 mb high shifts offshore and promotes better low level
moisture transport. The setup will feature developing cold air
damming across Georgia and the Carolinas with a inverted surface
trough developing along the coast in the afternoon. Models are
in fair agreement that the bulk of the precipitation will stay
to the west where the better upper jet dynamics will coincide
with low level convergence and isentropic ascent. By the
mid/late afternoon, models generally agree that light rainfall
will begin to break out mainly across the South Carolina zones
as the aforementioned trough starts to sharpen. As a result,
PoP`s feature a tight north/south gradient. Likely PoP`s are in
place for the northern half of the forecast area, while the
southern half mainly features slight chance to chance PoP`s.
Temperatures will be quiet tricky and will also feature a
difficult to depict gradient. Some inland locations will see
very limited temperature rise through the afternoon and highs
will range in the mid 50s in areas that border the Midlands.
However, if you go south to around the Altamaha, highs are
forecast to reach into the low 70s.

Tonight: The forecast becomes increasingly complex as the models
feature a weak surface wave developing along the inverted trough
and tracking northeastward directly across the forecast area.
The exact track and placement of this feature will have
significant implications for where precipitation sets up and for
temperatures. Model consensus brings this feature just inland of
the coast and right through the forecast area, which seems
reasonable given that is where the best temperature gradient
will reside. Continued isentropic ascent and convergence around
the wave should first favor the northern tier closest to the
Midlands for the best rain chances, and then expansion further
so the south and southwest is anticipated as the low moves to
the northeast. Temperatures could actually rise in some areas
overnight as the flow veers from northeasterly to more
southerly. Overall confidence in the details of the forecast
during the overnight are quite low given the presented
uncertainties. Also, it is interesting to note that there is
some evidence of possible elevated instability as showalter
values drop to around or just below 0 late. No thunder is in the
forecast at this point.


Monday: High pressure will rebuild toward the Mid-Atlantic
region, reinforcing a cold air wedge across the Carolinas and
extending into southeast GA. Meanwhile, an upper trough pattern
to the west will maintain a steady feed of Gulf moisture across
the Southeast. There may be some breaks in the rain on Monday as
the best forcing shifts to the north during the morning. The
highest QPF is expected Monday night as a potent upper shortwave
drives a surface low into central TN. This will yield a surge
of nearly 2 inch Precipitable Water values into the forecast
area atop the in-situ wedge. The wedge front is expected to
eventually lift north as a warm front late Monday night.

Tuesday could potentially be interesting as the warm front lifts
north and a cold front moves through during the afternoon. Shear
parameters are quite high along and south of the warm front and
there is the potential for at least marginal instability to
develop. At this point the big question mark is the instability
since we will likely have mostly cloudy skies in the morning
which would inhibit substantial surface heating. The SPC Day 3
outlook includes the area in a Marginal Risk due to the
potential for a few damaging wind gusts and possibly an isolated
tornado. Since we are so uncertain about the presence of enough
surface-based instability to spur severe weather, we have not
included mention of this in the Hazardous Weather Outlook at
this point.

Drier and slightly cooler air ushers in Tuesday night and
Wednesday, though downslope flow will still allow for mid/upper
60s highs.


There is better model agreement that the cold front Thursday
night will be accompanied by only isolated rain showers, then a
much colder airmass ushers in Friday into Saturday. Highs may
struggle to climb out of the 40s both days and we anticipate a
hard freeze Friday night. The approach of another front may
bring some precip and warming temps by Sunday.


VFR conditions with ceilings between 5-10 kft will persist
through the remainder of the morning. Focus then turns to late this
afternoon and the overnight. Model consensus is good that MVFR
ceilings with increasing rainfall chances will spread in late
this afternoon, especially at KCHS. The tricky part is that the
models then show a surface wave developing and moving through
the forecast area tonight. Models show steadily falling ceilings
starting this evening and reaching their lowest point around
sunrise on Monday. MVFR has been timed into the terminals around
00z with near IFR conditions about 6 hours later.

Extended Aviation Outlook: Flight restrictions, mainly from low
ceilings, are expected through Tuesday night as a storm system
affects the area. VFR weather returns Wednesday into late week.


Today: High pressure positioned to the north near the North
Carolina/Virginia state line will shift eastward to the
tidewater region and drive a primarily northeast flow this
morning. Then in the afternoon, a surface trough will begin to
develop across the waters which will turn the prevailing flow
more out of the east and southeast. Winds will primarily be 15
kt, though some periods of 15-20 kt will be possible for the
Charleston County waters at times. Seas will be in the 2-3 foot
range, with 4 feet possible in the outer Georgia waters.

Tonight: A surface wave is expected to develop and track along
the surface trough. As it does, winds will turn more
southeasterly and then southerly as the feature progresses to
the northeast. Depending on where the wave tracks exactly, the
pressure gradient could tighten enough to where a short period
of Small Craft Advisory winds will be possible. Confidence is
too low to introduce an advisory at this point. Seas will
increase as a result, becoming 2-4 feet across most of the
waters, with 3-5 feet possible in the Charleston County waters.

The best chance for advisory conditions will be Tuesday and
Tuesday evening as southwest flow increases ahead of a strong
cold front. We anticipate advisories at least for the offshore
GA waters, potentially also including the Charleston nearshore

Another round of Small Craft Advisory conditions is possible
Thursday night and Friday as strong offshore winds develop
behind another cold front.

Monday night and Tuesday, warm and moist air overrunning the
cool shelf waters may produce light sea fog across nearshore





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