Gale Watch 1, NWS Charleston, SC

Area Forecast Discussion
National Weather Service Charleston SC
101 PM EST Tue Jan 2 2018

.SYNOPSIS...
Arctic high pressure will continue through the rest of the
week. A storm system passing offshore will likely bring some
wintry weather to the area Wednesday. A cold front could affect
the area early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Cold, dry high pressure will continue to be the main weather
feature today. After a chilly start, temperatures have only
slowly been rising despite full sunshine. Based on trends and
the latest high-resolution temperature data, we bumped high
temps downward, with mid 30s north to upper 30s south.

Tonight: Arctic high pressure will remain centered north of the
forecast area. Scattered clouds should remain through this
evening, then filling in to BKN to OVC after midnight.
Temperatures are forecast to steadily cool from sunset through
midnight, ranging in the mid 20s. Near term guidance have
trended a little slower with the arrival of the deeper moisture
and measurable precipitation tonight. Using a blend of
operational and ensemble guidance, I will keep mentionable PoPs
limited to the pre-dawn hours Wednesday. Forecast soundings
indicate that the initial hours of precipitation, hydrometeors
will encounters a pronounced warm nose between 2-4 kft.
Following the top down method and partial thicknesses,
precipitation should favor FZRA across the coastal region and
SN/IP inland. It is possible that portions of the Coastal Empire
of GA could see a light glaze ice around dawn Wednesday.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
Confidence increasing in wintry weather across most of the forecast
area Wednesday. Arctic high pressure will persist while a strong
upper disturbance shifts through the region helping a surface low
strengthen and move north off the Southeast U.S. coast. Models are
in pretty good agreement indicating the low staying a good distance
offshore with the NAM remaining the strongest and most westward with
the system. Thus, it has more moisture farther inland but also
warmer low-level air closer to the coast. This would mean more
snow/sleet inland and more freezing rain near the coast. We
preferred more of a GFS/ECMWF solution which doesn't have much
precipitation inland, especially toward the CSRA, but just enough
moisture and cold air near the coast to get a snow/sleet/freezing
rain mix. Our latest snow/sleet accumulation forecast is for around
1-3 inches, highest near the SC coast. However, given the
uncertainty, a Winter Storm Watch will be posted for most of the
area for the potential for at least 2 inches of snow and/or 0.25
inches of ice. The precipitation will end from southwest to
northeast during the afternoon and should be completely out of
northern portions of the Lowcountry by around midnight.

Temperatures will be much below normal through the period with wind
chills likely near Advisory levels near the coast (15 degrees) each
morning.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Upper troughing will dominate the region into Saturday with cold and
dry surface high pressure prevailing. The high will shift offshore
early next week bringing some milder air into the region and another
chance of some rain by the end of the period as a cold front
approaches, possibly moving through the area as early as Monday
night.

Temperatures will remain below normal through Sunday before getting
back closer to even above normal Monday, then possibly falling back
below normal if the cold front has already pushed through.  Wind
chills could be near 15 degrees for most of the area through
Saturday morning which could necessitate another Wind Chill Advisory
for coastal areas.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
VFR conditions will prevail through late tonight. A large and
powerful low pressure system will push deep moisture onshore
during the pre-dawn hours Wednesday morning while cold Canadian
air remains in place. Increasing confidence that freezing rain,
mixed with snow and or sleet will overspread the TAF sites mid
to late Wed morning. Best indications are that the precipitation
will transition to predominantly snow and sleet during Wed
afternoon, continuing into the evening before tapering off,
first at KSAV, then KCHS. The time of precipitation-type
transition is key for pinning down accumulations of each.

Latest forecast is showing around a tenth of an inch of ice
accretion from freezing rain, followed by 1-3" of snow/sleet.

Extended Aviation Outlook: Restrictions likely Wednesday
afternoon/evening with wintry precipitation ongoing.

&&

.MARINE...
The gradient will relax somewhat today, though remain tight
enough to support Small Craft Advisories in all waters outside
Charleston Harbor. Tonight, as low pressure develops well
offshore and strong high pressure remains to the north, the
gradient will again tighten up, with Small Craft conditions
persisting through at least Wednesday evening (into Thu night
for the offshore GA waters). Cannot completely rule out some
Gale force gusts over the offshore GA waters closer to the Gulf
Stream Wed into Wed night.

&&

.TIDES/COASTAL FLOODING...
Coastal flooding is possible around the time of the morning
high tide Wednesday.

&&

.CLIMATE...
Record lows for 3 January:
KCHS: 19/1957
KCXM: 23/1918
KSAV: 18/2001

Record snowfall for 3 January:
KCHS: Trace/2002
KCXM: no record available
KSAV: no snow ever recorded on this date

&&

.EQUIPMENT...
The temperature/dewpoint sensor on the KCHS ASOS is
malfunctioning once again. Use data with caution. The observer
is manually augmenting the observation until the problem can be
resolved. Parts are on order.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Winter Storm Watch from Wednesday morning through Wednesday
     evening for GAZ101-116>119-137>141.
SC...Winter Storm Watch from Wednesday morning through Wednesday
     evening for SCZ043>045-047>052.
MARINE...Small Craft Advisory until 6 AM EST Thursday for AMZ350-352-
     354.
     Small Craft Advisory until midnight EST Thursday night for
     AMZ374.

&&

NEAR TERM...JRL/NED
SHORT TERM...RJB
LONG TERM...RJB
AVIATION...JRL/RJB
MARINE...JRL
TIDES/COASTAL FLOODING...
CLIMATE...
EQUIPMENT...

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