Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 210856

Area Forecast Discussion
National Weather Service Charleston SC
456 AM EDT Fri Oct 21 2016


A cold front will move offshore this afternoon and be followed by
another dry cold front Monday night. Otherwise, high pressure
will persist into next week.


Through Sunrise: Area webcams and surface reports indicate the fog
has become widespread and dense enough to justify a Dense Fog
Advisory for much of Southeast South Carolina. The advisory will
run through 9 am. Dense fog may ooze into the Savannah Metro area
and areas along the Savannah River prior to sunrise, so trends
are being monitored for a possible expansion west and south. Also
watching portions of Long, Liberty and McIntosh Counties where fog
is also becoming a bit more widespread and dense.

Today: A strong cold front will sweep across the area mid-late
afternoon as the associated upper trough digs across the eastern
United States. Moisture advection along/ahead of the front is
extremely limited with veered low-level trajectories keeping both
the Gulf and Atlantic closed as significant moisture sources.
Despite deep DPVA ahead of the upper trough, the lack of
significant moisture will keep the front mostly dry as it passes
offshore. There are signals in the various CAM members that a
broken line of low-topped showers could develop across North
Carolina into central/northern South Carolina late this afternoon
as the upper trough becomes slightly negatively tilted, but
current thinking is any such activity should pass to the north,
possibly brushing far northern Berkeley County. Will show some
low-end of 5-10% for parts of the Charleston Tri-county area,
highest over northern Berkeley County, to account for the
proximity of these showers, but 0% pops look reasonable for the
remainder of Southeast South Carolina and Southeast Georgia where
model point soundings show a persistent 700 hPA capping inversion
holding until FROPA.

Highs this afternoon will be a function of frontal timing and
pre-frontal cloud cover. Current data suggest much of the frontal
cloud band will dissipate as it moves east and encounters slight
downslope trajectories induced by low pressure moving offshore of
the Southeast U.S. coast. This trend is captured well in the
various model simulated cloud cover progs and RH time-sections.
Will lean closer to the warmer side of the various temperature
guidance showing highs from the upper 70s across far interior
Southeast Georgia with lower 80s elsewhere. Could see a few mid
80s just inland from the lower South Carolina coast where the
front will be the last to clear prior to the onset of cold air
advection. Breezy conditions will develop after FROPA with gusts
nearing 25-30 mph at times.


Tonight: Breezy conditions will linger this evening within a belt
of strong post-frontal cold air advection. Winds will diminish
after midnight as the intensity of the cold air advection wanes,
but full boundary layer decoupling is unlikely. Lows will range
from the upper 40s well inland to the mid 50s at the beaches.

Saturday through Monday: A trough over the east coast Saturday
morning will lift northward by Sunday, followed by general west
northwest flow aloft into Monday. At the surface, high pressure
centered over the lower Mississippi Valley Saturday morning is
forecasted to build eastward, allowing cold air advection to
gradually lessen into the afternoon. Despite sunny skies, low
thicknesses across the region will lead to cooler than normal
daytime temperatures. Many SC locations may not reach the 70
degree mark while GA locations will struggle to make it to the
lower 70s. Saturday night will be the coldest of the short term
due to clear skies allowing excellent radiational cooling. Lows
will fall close to the 40 degree mark well inland while the
immediate coast can expect the upper 40s. Sunday the surface high
will build across the deep south allowing our local air mass to
modify in abundant sunshine. Temperatures should reach the lower
to mid 70s, warmest in GA. Likewise, Sunday night lows will range
anywhere from 5-10 degrees warmer. By Monday a dry cold front will
approach from the north. Warmer temperatures ahead of the front
should top out in the upper 70s in SC, and the lower 80s in GA.


A dry cold front will cross through the region by Tuesday, leading
to cooler temperatures behind the front due to a northerly component
in the winds. High pressure is forecasted to build over the area
from the north Wednesday, allowing dry conditions and seasonal
temperatures. As the high shifts offshore Thursday temperatures will
start to moderate across the Southeast, rising to near normal for
this time of year. Additionally, a few showers are possible
offshore, near the Gulf Stream.


Shallow ground fog will make for a tricky vsby forecast through
sunrise for both terminals. The nature of the fog will be such
that large swings in vsbys are possible. Best fog parameters are
at KCHS, but KSAV not too far behind. Went ahead and introduced
IFR vsbys at KCHS 09-12z to account for the possible development
of a shallow, dense fog layer over the terminal. VFR will quickly
return after sunrise. Cold front will pass the terminals 19-20z
with winds ramping up quickly behind it. Gusts to 25 kt possible
at both terminals from mid-afternoon through the evening hours
within a strong post-frontal cold air advection regime.

Extended Aviation Outlook: There are no concerns.


Watching visibility trends carefully in Charleston Harbor.

Today: Light winds this morning will becoming west to southwest
between weak low pressure well north of the Bahamas and an
approaching cold front. Speeds will increase by mid-morning as the
pressure gradient begins to tighten. The cold front will push
offshore mid-late afternoon with winds increasing quickly closer
to sunset with the onset of cold air advection. Seas will average
2-4 ft nearshore waters and 4-5 ft offshore waters.

Tonight: Conditions will rapidly deteriorate this evening as
strong cold air advection spreads over the western Atlantic in the
wake of a cold front. Model soundings show deep mixing profiles
will develop as cold air flows over the relatively warm late
October waters within 60 nm. Winds will increase to 20-25 kt with
gusts to 30 kt nearshore with 25-30 kt with frequent gusts to 35
kt gales over the Georgia offshore waters. A Small Craft Advisory
remains in effect for all nearshore waters and will be expanded to
include the Charleston Harbor where similar conditions will occur.
Per coordination with the Ocean Prediction Center (OPC), A Gale
Watch will be posted for the Georgia offshore leg with the
anticipation that it will be upgraded to a Gale Warning later
today. There are signals gusts to gales could occur over the
eastern portion of the Charleston County waters, but the impacted
area looks to remain rather small so will hold off on a Gale
Watch there. Seas will build to 4-6 ft nearshore, a bit lower near
the immediate coast due to strong offshore influences, with 6-8
ft offshore waters. Its interesting to see the nested NWPS output
for Charleston Harbor depict a rather large area of waves building
1-2 ft where the low-level flow will be maximized across the
entire length of the harbor.

Saturday through Tuesday: Winds and seas will quickly trend downward
on Saturday as high pressure starts building into the region,
causing a weakening of the cold air advection and the pressure
gradient. The gradient will remain weak into Sunday night as the
high builds across the deep south. Winds should increase a bit on
Monday, as a dry cold front approaches from the north. The front
will cross by Tuesday and high pressure is forecasted to build
behind it, strengthening the pressure gradient and causing an
increase in winds and seas.


SC...Dense Fog Advisory until 9 AM EDT this morning for SCZ042>045-
     Lake Wind Advisory from 5 PM this afternoon to 8 AM EDT
     Saturday for SCZ045.
MARINE...Small Craft Advisory from 5 PM this afternoon to noon EDT
     Saturday for AMZ350-352-354.
     Gale Watch from this afternoon through Saturday morning for
     Small Craft Advisory from 5 PM this afternoon to 8 AM EDT
     Saturday for AMZ330.



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