Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 170844
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
344 AM EST Wed Jan 17 2018

.SYNOPSIS...
An arctic cold front will push offshore today. Cold, dry high
pressure will prevail across the region Thursday before
moderating temperatures occur this weekend into early next week.
A cold front will push through the area Monday, followed by dry
high pressure into the middle of next week.

&&

.NEAR TERM /THROUGH TODAY/...
A cold front will move rapidly offshore this morning as a
powerful upper trough digs across the eastern United States.
Strong quasi-geostrophic forcing ahead of the trough coupled
with a modest moisture feed from off the Gulf of Mexico will
support a band of light to moderate precipitation moving from
west-east across the Deep South and Southeast. The geometry of
the front is such that most of the frontal upglide will occur
behind the leading wind shift yielding more an anafrontal
structure. This will keep all of the precipitation post-frontal
and falling into the advancing arctic airmass. The greater
precip chances will be confined to areas north of I-16 and
closest to the CSRA and southern South Carolina Midlands. Pops
will be increased to 60-70% in the Metter-Walterboro-Moncks
Corner corridor and tapering to less than 20% roughly south of a
Ludowici-Richmond Hill line.

RUC and H3R soundings show the freezing level dropping to around
1000 ft across far interior Southeast Georgia by mid-late
morning as the bulk of the heaviest precipitation pushes
through. It appears the top of the precipitating cloud will be
cold enough to support ice nucleation for several hours before
drier air filters in from the west and precipitation quickly
ends. Surface temperature should remain above freezing through
the day, but a lowering freezing level and an increased
precipitation rate should support enough diabatic cooling to
allow for a rain/snow mix or even a transition to all snow for
areas roughly along/north of a Metter-Hampton line later this
morning. Ground temperatures are warm so expect most of any snow
that falls to melt, but there are signals in the various high
resolution guidance that there could be about a 1-3 hour period
of moderate snow across portions of Jenkins, Screven and
Allendale Counties which could briefly accumulate to a dusting
to as much as 0.5 inch on grassy areas. Will hold off on the
issuance of a Winter Weather Advisory per coordination with WFOs
Atlanta and Columbia since snow totals should remain less than
1 inch; however, trends will be carefully monitored in case a
short-term advisory is needed. A Special Weather Statement will
be issued to highlight possible accumulations on grassy
surfaces.

High temperatures will be tricky today as inland areas will be
influenced by diabatic cooling and coastal areas, especially in
the Beaufort-Charleston corridor, will be see a few hours of
warming prior to the onset of strong post-frontal cold air
advection. The high temperature forecast will show highs from
the upper 30s across far interior Southeast Georgia to the
lower-mid 50s over the upper portions of Charleston County.
Coastal temperatures will fall through the day with all areas
settling into the upper 30s inland to lower 40s coast by mid-
afternoon.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...
Tonight: Light precipitation looks to linger across the
Charleston Tri-County area into Colleton County this evening as
the base of the large, upper trough swings through. Freezing
levels will drop enough during this time to support a few light
snow showers or flurries, but no accumulation is expected as
moisture will be quickly thinning with time. Mostly clear skies
will prevail overnight the last of the clearing pushes off to
the northeast with the upper trough. The various statistical
guidance members look to be running a degree or two too warm
given gusty winds will keep the boundary layer well mixed
through the night. Expect lows to bottom out in the lower 20s
inland with mid-upper 20s at the coast. Wind chills will lower
to around 15 degrees across the coastal counties by daybreak
Thursday. The situation looks a bit too marginal for a second
period Wind Chill Advisory, so will defer to the day shift to
determine whether one will be needed. Inland wind chills look to
remain above advisory criteria.

Thursday through Saturday: Dry sfc high pressure will build
over the Southeast on Thursday behind a mid/upper lvl trough
departing off the eastern seaboard. The setup will support temps
well below normal Thursday afternoon with highs peaking only in
the mid 40s. Thursday night lows will be chilly under clear
skies and a light westerly wind, dipping into the mid 20s away
from the coast. On Friday, noticeably warmer conditions are
expected as dry high pressure becomes more centered near the
area Friday afternoon. In general, temps should peak in the mid
50s under a full sun and ridging aloft. Friday night lows should
dip into the lower 30s away from the coast. A mid/upper lvl low
is then expected to traverse the Southeast on Saturday, but
moisture will be very limited, especially in the lower levels
where dry high pressure remains. Given the setup, some mid/upper
lvl clouds are possible as the disturbance passes aloft, but
precip is not expected over the area. Temps will continue to
modify during the weekend. In general, highs should peak in the
lower 60s Saturday afternoon.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
High pressure will prevail over the Southeast United States late
weekend into early next week before a cold front arrives Monday. The
pattern will favor warming conditions under ridging aloft. In
general, afternoon highs will peak in the mid/upper 60s on Sunday.
By Monday, southerly winds will help advect deep moisture over the
Southeast and produce some cloud cover ahead of the approaching
front. This could limit overall heating potential, but a prevailing
southerly flow should still support temps in the upper 60s for most
locations in Southeast South Carolina to around 70 degrees in
Southeast Georgia ahead of cold front. Chances of showers should
arrive to most areas Monday afternoon as the front begins to shift
over the region. Dry high pressure will then return on Tuesday and
persist into the middle of next week. Temps will be slightly cooler
behind the front, but should remain a few degrees above normal. In
general, Tuesday and Wednesday afternoon temps should peak in the
lower 60s. Overnight lows should range in the low/mid 40s.

&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
Primary concern:
* low cigs, mainly at KCHS

VFR this morning with cigs lowering through the day as a cold
front pushes offshore. Best rain chances will remain west of the
terminals, but the tail end of an area of enhanced showers could
brush KCHS later this afternoon, roughly 20-23z. Expect MVFR
cigs to prevail at KCHS by early afternoon, but remain just
above MVFR thresholds at KSAV. It is still possible a period of
MVFR cigs could reach as far south at KSAV later this afternoon,
which will be monitored.

Extended Aviation Outlook: VFR conditions are expected at both KCHS
and KSAV terminals Thursday afternoon through Sunday.

&&

.MARINE...
Today: Conditions will deteriorate across the waters today
behind a cold front. Light winds this morning will give way to
northwest winds 15-20 kt with seas building 2-4 ft nearshore and
as high as 4-6 ft over the Georgia offshore waters. A Small
Craft Advisory will go into force by early afternoon for the
Georgia offshore leg.

Tonight: Winds will peak 20-25 kt with gusts to 30 kt across all
waters as an arctic airmass settles in across the region. A
Small Craft Advisory will be issued for all areas outside of the
Charleston Harbor. Conditions in the harbor look to peak just
shy of advisory thresholds. Seas will peak 3-5 ft nearshore
waters and 5-7 ft offshore waters.

Thursday through Monday: Winds/seas will decrease/subside Thursday
as cold air advection begins to wane over coastal waters. However, a
Small Craft Advisory will likely be ongoing to around noon while 6
ft seas linger in offshore Georgia waters. Dry high pressure will
then dominate/prevail over the coastal waters this weekend into
early next week with winds/seas remaining well below Small Craft
Advisory levels. In general west/northwest winds will remain at or
below 15 kt Friday through Sunday while seas are no higher than 2-3
ft. A southerly wind will then develop on Monday as a cold front
approaches from the west late in the day. Expect winds and seas to
increase/build, but remain below Small Craft Advisory levels during
fropa.

&&

.EQUIPMENT...
The KCLX radar is 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...Small Craft Advisory from 5 PM this afternoon to 5 AM EST
     Thursday for AMZ350-352-354.
     Small Craft Advisory from 1 PM this afternoon to 1 PM EST
     Thursday for AMZ374.

&&

$$



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