Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 220007
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
707 PM EST Sun Jan 21 2018

.SYNOPSIS...
High pressure will prevail into Monday, as a weak coastal
trough lingers offshore. A cold front will move through the
region Tuesday morning, followed by high pressure through
Saturday. Another cold front will likely impact the area next
Sunday.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Clear skies and light winds will prevail until later tonight.
We made some minor tweaks to temps and dew points on the early
evening update.

Overnight, as low level winds turn toward the south, increasing
moisture will translate to stratocumulus which will develop
over southern coastal waters and will spread into all but far
inland/northern sections of the forecast area by daybreak.
Various guidance even suggests that a few showers could develop
in the vicinity of a weak surface trough over southern waters
and approach the coast late tonight. Also, the subtropical jet
will push increasing high clouds into the region overnight.
Otherwise, low temperatures will range from the lower/mid 40s
inland to around 50F on the beaches. Temperatures could even
rise a few degrees where stratocumulus develops late.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
Monday and Monday night: A dynamic and vertically stacked low in the
central Plains at the start of the period will head through the
upper Midwest and into the western Great Lakes region by the end of
the period. Our region lies underneath mid level ridging initially
that slides offshore and gives way to a deep and moist SW flow in
advance of a cold front that heads rapidly east and reaches into our
forecast district by 12Z Tuesday. The cold front pushes high pressure
across the Carolina`s on Monday into the ocean at night, while a
very subtle inverted trough not far offshore gradually dissipates.
Increasing isentropic upglide and the nearby trough should be enough
to produce isolated to scattered showers, mainly to the east of I-95
in SC. But the lack of forcing elsewhere will keep other locations
to the west rainfree. Rain chances will ramp up during Monday night
in advance of the approaching cold front, aided by falling heights
aloft and upper difluence from a 130-140 kt upper jet not far to the
NW. It looks to be a high-Pop but low-QPF event due to the
progressive nature of the showers, with our latest forecast showing
60-70% chances but only about 1/10 inch of rainfall. Poor low level
lapse rates, quite a bit of CINH and the nocturnal timing supports
very little chance of thunder. But due to some elevated instability
and decent bulk shear, a little thunder is certainly possible in a
few locations. Despite considerable cloud cover Monday, warm
advection will boost temps into the upper 60s and lower 70s,
although the shoreline area will be considerably cooler with an
onshore flow. Monday night will be abnormally warm, only falling to
the mid and upper 50s. We have included patchy fog along the coastal
corridor due to some marine fog and the build-down of stratus, plus
much of the statistical guidance showing category 1 or 2 ceilings
and visibilities. But confidence on this is low given too much
mixing and only a limited time where the fetch is ideal for any sea
fog.

Tuesday and Tuesday night: The cold front pushes offshore by 15-18Z,
with rapid drying in its wake as a deep westerly flow develops
throughout the vertical. Increasing insolation plus some downslope
off the mountains to negate weak cool advection, temps should again
peak well above normal and pushing near 70F in many locations. As
mixing heights climb throughout the day, plenty of the 25-35 kt low
level winds will mix down, creating breezy conditions and probably
even the necessity for a Lake Wind Advisory, for wind gusts near 25
kt along the shores of Lake Moultrie. We will add mention to the
Hazardous Weather Outlook (HWO). Cold advection strengthens after
dark, and temps will be as much as 15-20F colder than the
previous night, but holding above freezing.

Wednesday: Cool high pressure will build from the west at the
surface, while zonal flow dominates aloft. A weak short wave will
bring some cirrus to the area as it moves through in the afternoon,
but nothing more with PWat down near 1/4 inch. We stayed close to
the low level thickness forecasting scheme regarding max temps, as
they reach the upper 50s to around 60F.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
High pressure will prevail into Friday, bringing dry conditions and
seasonal temperatures. The threat of showers returns with the
approach of the next cold front later Saturday into next Sunday.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
VFR through until late tonight/early Monday. Then, there is an
increasing potential for developing stratocumulus and MVFR cigs
are possible at both terminals on Monday. Confidence is lower
than usual as forecast soundings from the 18Z appear too robust
in low level moisture but the trends from the earlier TAF cycle
have been retained.

Extended Aviation Outlook: Flight restrictions are likely in
association with a cold front Monday night into Tuesday morning.
Gusty winds will develop behind the front for the balance of
Tuesday.

&&

.MARINE...
Tonight: Variable to onshore flow mainly less than 10 kt will
persist as a weak trough develops offshore. Seas of 1-2 ft,
dominated by a 9-10 second period easterly swell and highest
beyond 20 nm, will prevail.

Monday and Monday night: The western extension of broad Atlantic
high pressure will encompass the waters Monday, then pulls further
offshore as a cold front approaches from the east at night. A weak
coastal trough between the Gulf Stream and shelf waters will
dissipate, but could be enough to produce isolated to scattered
showers during the daylight hours. A much greater chance of showers
and maybe a few t-storms will arrive with the approaching cold front
after midnight Monday night. Conditions do not appear favorable for
widespread sea fog, but given the warm and humid air mass atop the
significantly cooler waters, there is at least some risk for patchy
fog and low stratus before winds become too strong Monday night. For
now it looks like winds and seas could come close to the requirement
of a Small Craft Advisory, especially over the AMZ350 and AMZ374
waters during the late evening and post-midnight hours.

Tuesday and Tuesday night: The cold front will progress through the
waters the first half of the day, followed by continental high
pressure building from the west and increasing cold advection,
especially at night. We;ll be close to advisory conditions on the
Charleston County and outer GA waters in the morning, and in
Charleston Harbor during the late morning and afternoon as gusty
westerly winds develop with increasing mixing.

Wednesday through Friday: High pressure will dominate, and at some
point there could be enough pinching where SCA`s could again be
required. But much of the extended period currently looks to stay
with winds and seas at or below 20 kt and 5 ft, respectively.

&&

.EQUIPMENT...
The KCLX radar remains out of service until further notice.
Repairs are ongoing. Adjacent radars include: KLTX, KCAE, KJGX,
KVAX and KJAX.

The Downtown Charleston observation site (CHLS1/KCXM) remains
out of service until further notice.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$
NEAR TERM...
SHORT TERM...33
LONG TERM...
AVIATION...
MARINE...
EQUIPMENT...



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