Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 180920

Area Forecast Discussion
National Weather Service Charleston SC
420 AM EST Mon Dec 18 2017


A stationary front will linger over the area through Tuesday.
Low pressure will pass over or north of the region late
Wednesday, followed by inland high pressure Thursday into
Friday. The high will shift offshore next weekend and then a
cold front will slowly approach from the west.


A weak, quasi-stationary front will bisect the forecast area
today as moisture surges north out of the Gulf of Mexico. The
strongest isentropic assent and H9-H8 moisture transport will
occur in the vicinity of the front from late morning into the
mid-afternoon hours and this will be where the greater chances
for measurable rains will be found as isentropic lift is
augmented by weak DPVA induced by a passing H5 shortwave. Short
term guidance is still showing some discrepancies on where
exactly the front will wavier through the day, but there is
enough agreement among the various data sets to support
greater rain chances between roughly the I-16 and I-26
corridors, including the Beaufort, Savannah Metro Area and
possibly the Charleston Metro Area. Pops up to 40-50% will be
highlighted for this area with 20-30% pops elsewhere. Higher
pops may eventually be needed if guidance comes into slightly
better agreement later in the day. Either way, QPF will be light
with maximum amounts only averaging 0.1 inch or less.

The high temperature forecast will be a tricky one as much will
depend on both frontal and precipitation placement. Generally
favored a blend of the higher resolution NAM12, RAP and H3R to
construct afternoon highs with some influence from both the
18/00z GFS and ECMWF MOS. This yields highs from the mid 60s for
areas bordering the CSRA, Midlands and Pee Dee to near 70/lower
70s along/south of the I-16 corridor. There are some signals
that temperatures could end up slightly cooler across the far
interior and warmer across the far south, but again, this will
be highly depend on both frontal and rain placement, which are
both still a bit uncertain. Adjustments are likely for the next
few enhanced short term forecast cycles.


Tonight: Lingering coastal showers will end during the early
evening hours with mainly dry conditions prevailing through the
night time period. The very soupy, moisture laden airmass
currently producing widespread dense fog from Texas east into
portions of the Deep South this morning will be positioned
across the Southeast U.S. tonight. Widespread stratus formation
is likely once rain exits offshore with the stratus building
down and gradually transitioning to areas of fog through the
night. In addition, increasing dewpoints advecting over the cool
shelf waters could support the formation of sea fog across the
coastal waters, some of which could impact portions of the
coast before winds shift offshore late. All in all, conditions
look to favor fairly foggy conditions overnight with much of the
00z guidance supporting some degree of dense fog. Will include
"areas of fog" in the grids and text products, but the
introduction of "dense fog" and a widespread qualifier may
eventually be needed in later forecast cycles as fog trends
become clearer. Lows will range from the lower 50s well inland
to the lower 60s south of a Reidsville-Savannah line.

Tuesday: The mid/upper levels will consist of zonal flow over the
Southeast. At the surface, a weak stationary front is forecasted to
be dissipating to our south in the morning. At the same time two
weak areas of high pressure, one generally located over FL and
another in the lee of the Southern Appalachians will stretch into
our area, negating any lasting impacts from the aforementioned
front. The FL high will strengthen a bit into the afternoon while it
trends more into the Southeast. But by the evening the high will
retreat offshore, allowing low pressure developing somewhere along
the Lower MS Valley to advance towards the Southeast. Moisture will
be low during the day, then trend upward in the evening and
overnight, with PWATs >1.0". Models indicate most of our area
remaining dry overnight. The exception is our westernmost zones
possibly experiencing some light showers, especially towards
daybreak. Hence, the POPs reflect this minimal threat. Temperatures
within this warm air mass will be well above normal.

Wednesday: The mid/upper levels will consist of zonal flow over the
Southeast in the morning. Meanwhile, a disturbance over the
lower/mid MS Valley will move eastward, passing mainly to our north
in the evening, then rapidly moving offshore overnight. At the
surface, low pressure generally located over the TN Valley in the
morning will rapidly move eastward, passing just to our north in the
evening as it broadens out, and then zooming offshore overnight. The
disturbance and a jet streak passing overhead could help enhance the
lift. There is some instability in place ahead of the low, so a few
thunderstorms are not out of the question in the afternoon. Added
these to the forecast to account for this potential and to match up
with some of our neighboring offices. Decent moisture will be in
place ahead of the low with PWATs approaching ~1.5" (or 2 standard
deviations above normal per NAEFS). Expect showers to overspread the
area from west to east during the day, with the highest POPs far
inland and across the Charleston Tri-County area. Locally heavy
rainfall is not out of the question. The fast movement of the low
should allow showers to quickly dissipate from west to east
overnight. Though, some remnant showers may last along the immediate
coast until daybreak. Temperatures will be well above normal.

Thursday: The mid/upper levels will consist of zonal flow over the
Southeast. At the surface, a wedge of high pressure will build
inland during the day. Other than some remnant showers along the
coast early in the morning associated with the previous days
departing low, dry weather is expected to overtake the entire area.
Mostly cloudy skies and northeasterly flow should keep high
temperatures close to normal.


The inland wedge will prevail Thursday night into Friday, then move
offshore. Models then show a front slowly approaching from the west
this weekend. The problem is the location of the front is different
for each model, which gives our area vastly different weather. Given
this problem, we opted to go with low end POPs until the models
narrow down the location and timing of the front more. Simply put,
expect more changes to the long term forecast for the upcoming
holiday weekend.


Primary concerns:
* Cigs and late night fog
* Rain chances

Low confidence forecast for the next 24 hours as a warm, soupy
airmass will be in place. Guidance is all over the place with
respect to cigs/vsbys through daybreak and again overnight
leading to lower than normal confidence. Statistical guidance
shows LIFR cigs developing prior to daybreak, while SREF
probabilities show a less than 5% chance of occurrence. NARRE-
TL output favors the SREF so will not include prevailing IFR
cigs at this time for any of the next 24 hours, although
amendments will likely be needed as shorter term trends become
more apparent. Will limit cigs to MVFR for now, mainly from
sunrise on. A band of scattered showers will likely move across
the region later this morning with the potential for steadier
rains late this afternoon. Coverage/timing is a bit uncertain
with high resolution guidance showing slightly differing
positions on the locations of the primary rain axis. The risk
for late night fog will increase as dewpoints rise and scattered
rains end. Better fog chances will hold off until after 06z, so
no mention will be included at either KCHS or KSAV attm.

Extended Aviation Outlook: Flight restrictions are expected Tuesday
morning and again Tuesday night into Wednesday due to low pressure
moving over the region.


Today: Light winds this morning will become more uniformly
southwest through the day. Winds will average 10 kt nearshore
with 10-15 kt over the offshore waters by this afternoon with
seas 1-2 ft throughout. Increasing dewpoints over the cold shelf
waters could support some patchy sea fog over the nearshore
waters by late afternoon, however the better chances look to
hold off until after sunset.

Tonight: Southwest winds will gradually back west through the
night with speeds generally 10 kt or less. Seas will average
1-2 ft nearshore and 2-3 ft offshore. The biggest forecast
concern centers on the potential for sea fog. Increasing
dewpoints advecting over the chilly shelf waters will likely
support some degree of sea fog overnight, initially beginning
over the nearshore waters, then advecting into the Charleston
Harbor. The fog could get into the offshore waters closer to
sunrise as winds veer more westerly and even more fog from over
land moves into the nearshore waters becoming trapped beneath
the strong marine inversion. As is typical with marine fog
events, it is unclear exactly how widespread the fog will
become, but conditions look at least marginally favorable for
some degree of fog. Will highlight "patchy fog" this evening
with "areas of fog" overnight. A Marine Dense Fog Advisory may
very well be needed at some point tonight.

Tuesday and Tuesday night: Weak high pressure to our south will
prevail. Winds and seas will stay well below any advisory
conditions, but sea fog could again be a problem with the warm and
humid air atop the "cooler" shelf waters.

Wednesday through Friday: Low pressure will travel offshore late
Wednesday, probably over or just north of our northern waters,
followed by a cold front later at night. An inland high pressure
wedge then develops Thursday into Friday. Conditions could be close
to advisory thresholds for at least a part of this time. Sea fog is
still possible into Wednesday morning, followed by at least a low-
end potential for t-storms late Wednesday into early Wednesday




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