Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 181141

Area Forecast Discussion
National Weather Service Charleston SC
641 AM EST Sat Nov 18 2017

High pressure will prevail today, then a cold front will pass
through late tonight into Sunday morning. High pressure will
then return through much of next week, though a Gulf low
pressure system could impact the area mid to late week.


Today: Aloft, the forecast features westerly flow ahead of a
sharpening trough within a progressive overall pattern in the
mid and upper levels. At the surface, we will start out in a
weak pressure pattern around high pressure off the North
Carolina Outer Banks. By the afternoon, the high is progged to
push offshore and its influence will wane as a strong cold front
pushes into the Tennessee Valley. Southerly flow will steadily
strengthen through the day, resulting in a warm and breezy
conditions by the mid to late afternoon hours. Scattered
stratocumulus clouds in the morning will decrease in coverage by
the afternoon and the forecast is dry as the area of
convergence along the approaching front will remain well to the
west. Temperatures will be notably warmer than previous days,
with forecast highs in the low to mid 70s across most of the
area. There could even be some upper 70s in portions of
southeast Georgia.

Tonight: The cold front will move closer in the evening and
begin to push into the far inland zones in during the early
morning hours. The front continues to look relatively moisture
starved as it approaches the forecast area with a weakening band
of showers expected. In fact, it`s hard to justify more than
mid range chance PoP`s with this boundary, and it is likely that
some areas do not receive any measurable rainfall. The front
should be positioned just offshore by sunrise Sunday which means
the bulk of the cold advection will be delayed beyond the
tonight period. This will result in low temperatures only
falling into the mid/upper 50s for most areas. Though there
could be some low 50s well inland right at sunrise. Winds will
remain elevated through the night and will actually strengthen
immediately ahead of the front with the tightest pressure


The pronounced upper shortwave will push the cold front quickly
offshore Sunday morning. Residual showers and cloud cover
expected to be out of the area by noon Sunday. Weak downslope
flow combined with ample sunshine will help to offset the cold
air advection, pushing high temps into the mid 60s.

Expansive dry high pressure will expand over the area Sunday
night, yielding clear skies and light winds. Strong radiational
cooling will result, allowing temps to drop into the mid 30s
inland and upper 30s to lower 40s closer to the coast. We may
need to consider a Frost Advisory for Sunday night for a few far
inland zones where the winds would be more likely to go calm.

Cool high pressure will linger on Monday, keeping highs in the
upper 50s to mid 60s. A developing longwave trough over the
eastern United States Monday night into Tuesday will yield warm
air advection and increasing moisture across the area. A weak
coastal trough may develop, supporting isolated showers along
with increasing low clouds.


The period looks potentially unsettled though there are
substantial differences in the global models. A longwave trough
will anchor over the eastern half of the country with a series
of surface lows possibly shifting up the coast from the Gulf.
Given the large model spread, we started by bumping PoPs into
the 20-30% range late week.


VFR conditions are expected to prevail through 06z Sunday. Late
tonight a cold front will approach from the west and bring with
it a weakening band of showers as well as some potential for
temporary MVFR ceilings. No visibility reductions are expected
with the showers and confidence in ceilings is too low at this
time to include in the forecast.

Extended Aviation Outlook: VFR.


Today: The day will begin with high pressure off the North
Carolina Outer Banks and weak 5-10 knot onshore flow this
morning. As we move into the afternoon, the high will move
further offshore and the pressure gradient will tighten as a
strong cold front begins to approach from the west. The
increased gradient will result in strengthening southerly flow,
topping out in the 15-20 knot range by the late afternoon hours
across the local waters. Seas are expected to be in the 2-4 ft
range through the day.

Tonight: Winds will continue to increase and turn a bit more
out of the southwest, such that Small Craft Advisories are going
to be needed for all waters including Charleston Harbor. The
strongest winds will first impact the Charleston County waters
in the evening, but will steadily increase across all zones
through the early morning hours. Gusts to around 30 knots are
expected in the Charleston County waters, with a more solid
20-25 knots of flow elsewhere. Seas will increase in response,
becoming 4-6 feet beyond about 10 nm offshore late tonight.

Low-level winds will quickly diminish on Sunday behind the cold
front. We expect the Small Craft Advisory winds to subside by
late Sunday morning. High pressure building from the northwest
will produce an extended period of northeast winds next week.
The gradient is expected to increase mid to late week, with
marginal Small Craft Advisory winds and/or seas possible over
portions of the waters Wednesday night through Friday.


MARINE...Small Craft Advisory from 2 AM to 7 AM EST Sunday for AMZ330.
     Small Craft Advisory from 11 PM this evening to noon EST
     Sunday for AMZ352-354-374.
     Small Craft Advisory from 6 PM this evening to noon EST Sunday
     for AMZ350.



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