Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 080257

Area Forecast Discussion
National Weather Service Charleston SC
957 PM EST Wed Dec 7 2016

High pressure will build over the region through mid week. A strong
cold front will sweep through the area Thursday night, followed
by cooler high pressure filling in over the weekend. Another
cold front should push through the area early next week before
high pressure returns mid week.


Late evening 11-3.9 micrometer satellite imagery and surface
observations show stratus continues to expand and thicken. A few
sites are already reporting some locally dense fog, which is
what RAP soundings have been suggesting for the past several
hours. While conditions do not look ideal for widespread stratus
build-down yielding significant dense fog overnight, areas of
fog, locally dense, look plausible north of the I-16 corridor.
Farther south, winds look to increase a bit too much after 2-3
am with the approaching a cold front from the west to support
much more than patchy fog.

Temperatures have fallen a bit quicker than expected east of a
Moncks Corner-James Island line due to nearly ideal radiational
conditions. Expect temperatures to level off and then rise
shortly as stratus intrudes so have made adjustments in the
hourly dewpoint and temperature forecasts to reflect this. Opted
to nudge overnight lows up by several degrees for most
locations given thermal declines will diminish as stratus
thickens. However, still kept temperatures a degree or so cooler
than some warmer guidance members would suggest. Further
adjustments will be made as the night progresses.

Expect dry conditions overnight despite some models generating
some very light QPF across the CWFA. Suspect these models are
having a hard time resolving the low-stratus and may be
erroneously generating light shower activity.


Thursday: A broad mid and upper trough will push east through
northeast quad of the nation, sending a an arctic cold front toward
the area Thursday, before pushing into the Atlantic early Thursday
night. Downslope flow and compressional heating in advance of the
front will warm the region into the lower and middle 60s, with the
warmest temps near and east of I-95. The cold front will come
through dry, with generally no more than scattered-broken layered
clouds. A huge 1045 mb arctic high will build from the Great Plains
behind the cold front, resulting in modest cold advection into our
CWFA Thursday night. The 850 mb 0C isotherm penetrates down into
our northern and western counties, enough to cause low temps by
Friday morning to fall to the mid and upper 30s most communities
aside from the immediate coast.

Friday through Saturday: The coldest air mass of the season will
encompass southern South Carolina and southeast Georgia as the core
of the arctic high shifts east toward the Mississippi Valley Friday
and toward the coast of the Carolinas on Saturday. We anticipate
full sunshine Friday, with no more than some late day stratocumulus
from off the Atlantic in coastal Georgia late Saturday. With this in
mind our forecast has leaned heavily in regards to the low level
thickness prognosis, which supports max temps both days about 10-12F
below normal. Friday night into Saturday morning is still supporting
a solid freeze all the way to near the intra-coastal, with a
potential for a hard freeze in parts of the northwest tier from
inland Berkeley County to Allendale, Screven and Jenkins counties.
Often times the models are a little too cold in these types of
scenarios, and depending upon how much decoupling transpires and how
quickly the synoptic flow veers to northeast will be strong factors
in actually how low are temps are by dawn on Saturday.


Cold high pressure will persist into Saturday night with temps
dipping into the low/mid 30s away from the coast. By Sunday, high
pressure will begin to shift off the Mid-Atlantic Coast while a
coastal trough develops along the Southeast Coast. A few showers
could drift onshore across Southeast South Carolina and Southeast
Georgia, but confidence is too low to mention in the forecast at
this time. A low pressure system is then expected to track over the
Great Lakes region and Northeast with a southward extending cold
front that reaches the area Monday into early Tuesday. Deep moisture
should provide chances of showers as fropa occurs across the region.
Dry high pressure will then build across the region behind the
departing cold front with temps near normal through midweek.


Stratus will fill back in at the terminals this evening. Expect
IFR cigs to return to KCHS by 08z, but current indications are
that cigs will remain MVFR at KSAV. Limited cigs to high end IFR
and above alternate minimums at KCHS, but there are signals in
the latest RAP soundings that a few hours of cigs below
alternate minimums may occur. Will need to watch trends before
the inclusion of such conditions. VFR will return to both
terminals 11-15z, earliest at KSAV, as winds begin to increase
ahead of cold front. Winds will shift northwest and become gusty
by early-mid afternoon as the cold front shifts offshore.

Extended Aviation Outlook: VFR.


Tonight: A weak gradient across the waters will limit winds to
5-10 kt maximum. Seas will range from 1-2 ft.

Thursday: A gentle to moderate offshore flow will prevail in advance
of the arctic cold front which arrives from the northwest during
Thursday night, followed by strong high pressure building from the
Great Plains. Modest cold advection and steady isallobaric pressure
rises in wake of the cold front will lead to Small Craft Advisory
conditions across most of the marine area, but given that it`s still
the third period of the forecast we will refrain from hoisting any
advisories at this stage.

Friday through Saturday: Continental high pressure will cover the
local waters late this week, as the center of the high heads east
and reaches the coast of North and South Carolina by 00Z Sunday.
Winds and seas will remain elevated given favorable fetch and some
pinching to the gradient, although the most favorable time for any
advisory conditions would be Friday morning during the best cold
advection pattern.

Sunday through Monday: High pressure slides into the Atlantic Sunday
as a coastal trough develops over or near the area, before
dissipating by Monday, and gives way to another cold front that
approaches late Monday. Winds and seas are forecast to remain below
any advisory thresholds early next week.




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