Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 231647

Area Forecast Discussion
National Weather Service Charleston SC
1247 PM EDT Sun Oct 23 2016

High pressure will prevail through tonight. A dry cold front will
move through late Monday and be followed by strong high pressure
into the middle of next week.


No significant changes to the previous forecast. High pressure
will remain across the region today with a very dry atmosphere in
place as indicated by the 0.3 inch PWAT as per the 12Z CHS
sounding. Temperatures will recover this afternoon after a chilly
start this morning, likely peaking in the lower to mid 70s inland,
warmest over southeast GA. High temps at the beaches will remain
closer to 70 as winds will become onshore there.


Tonight: The boundary layer will decouple away from the coast this
evening as surface high pressure retrogrades to the southwest. The
pressure gradient looks to tighten a bit over northern areas after
midnight in response to the development of a weak lee-side trough
over the North Carolina Foothills and South Carolina Upstate, but
point soundings in this area suggest the nocturnal inversion will
likely be too strong by this time to induced any significant
recoupling of the boundary layer. Generally favored the cooler
side of the various 23/00z guidance packages, especially across
interior Southeast Georgia. Lows will range from the lower 40s
inland to the mid-upper 50s at the beaches.

Monday: The large scale features an amplified pattern, with the mean
trough position over the eastern states and ridging from the Gulf of
Mexico through the Plains and Rockies. Surface high pressure
retrogrades west along the Gulf coast, eventually becoming absorbed
by a much stronger and larger high building S-SE from southern
Canada and the Midwest. As this transpires it allows an east-west
cold front void of any moisture to approach from the north by late
in the day. The front gets a push from the building high from the
western Great Lakes, Midwest and Ohio Valley, reaching near the
Savannah River after midnight, clearing south of the Altamaha by
daybreak Tuesday. Warm advection in advance of the front and plenty
of sunshine, aside from few-scattered cirrus clouds, will support
high temperatures in in the lower 80s for most areas away from the
immediate coast. Cold advection lags a little behind the cold front
Monday Night, but still allows for lows to be a few degrees below
normal for this time of year.

Tuesday: A modified Canadian air mass encompasses the region, with
high pressure covering much of the area from the Plains east into
the Atlantic. Clockwise flow around the high from off the ocean and
the associated low level convergence as the synoptic flow veers to E-
NE will allow for a little marine-induced stratocumulus over coastal
Georgia, otherwise mostly sunny/sunny skies elsewhere. Highs should
reach the normal values for this time of year. A pinching of the
gradient along the coastal corridor will allow for breezy
conditions. Low temperatures should be near normal.

Wednesday: Broad high pressure will encompass the mid-Atlantic
states down to the deep south. Low pressure moving over or near Iowa
will be enhanced by an upper level disturbance moving over the
northern half of the Mississippi Valley. The high will gradually
weaken and move northward during the day as the aforementioned low
moves eastward. But dry conditions should prevail. Though, a few
light showers are possible along the Gulf Stream. Despite increasing
high clouds and northeast surface winds, thickness values should
support near normal high temperatures.


Models have come into better agreement in the long term. Broad high
pressure will shift offshore Thursday while an upper level trough
and associated surface low pressure move across the Great Lakes. A
cold front should stretch down into TN Valley, but it`s expected to
weaken as it approaches our area. The front is forecasted to pass
just north of our area Thursday night into Friday. There is not much
moisture with this front and the model trend has been drier. We
lowered POPs across the area, limiting the slight chance POPs to the
Charleston Tri-County area. But this may still be overdone given the
trend. High pressure builds across the Southeast on Saturday.
Temperatures may be a few degrees above normal in the long term.



Extended Aviation Outlook: VFR.


Today: North to northeast winds this morning will become more
onshore this afternoon as the center of surface high pressure
moves offshore. Speeds will diminish through the day with seas
slowly subsiding from 1-3 ft in the near shore waters and 3-4 ft
beyond 20 nm.

Tonight: Weak offshore winds will prevail tonight as the surface
high retrogrades to the southwest and a weak lee side trough
develops over the North Carolina Foothills. The highest winds, up
to 10 kt, will occur over the the Charleston County waters where
the pressure gradient will be the tightest. Overall, mariners can
expect any major impacts overnight with seas averaging 1-2 ft
across the entire marine area.

Monday through Thursday: High pressure will move west on Monday as a
dry cold front approaches from the north. A weak gradient will
equate to light winds and low seas. The dry cold front will move
southward and through the waters Monday night. The combination of
rising pressure and cold air advection behind the front along with a
pinching of the gradient along the nearshore waters will allow for
increasing winds and seas. Small Craft Advisories may be needed
Monday night into Tuesday morning. The gradient will weaken Tuesday
afternoon and remain weak into Wednesday as broad high pressure
encompasses the mid-Atlantic states down to the deep south. Winds
may increase on Thursday as a front approaches from the north.




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