Area Forecast Discussion
Issued by NWS Charleston, SC
FXUS62 KCHS 102348
Area Forecast Discussion
National Weather Service Charleston SC
648 PM EST Sat Dec 10 2016
High pressure will prevail until the passage of a coastal
trough into Sunday night. A weak cold front will then shift
into the area Tuesday before lingering in the vicinity through
mid week. The front will push offshore Wednesday night and be
followed by cooler temperatures for late week.
.NEAR TERM /THROUGH SUNDAY/...
Evening update: No changes to the going forecast. Previous
discussion follows below.
Cool, dry high pressure will hold firm across the region
tonight as the flow becomes increasingly more zonal aloft.
Models show the development of a weak coastal trough off the
coast of Georgia and Northeast Florida after midnight. Models
look too aggressive in the development of nocturnal shower in
the vicinity of this feature owing to difficulties in resolving
marine-based closed cell stratocumulus. A rain-free forecast
will be maintained.
Expect solid radiational conditions to prevail inland where the
combination of low dewpoints, clear skies and light/calm winds
will allow for excellent radiational cooling. Radiational
processes will be tempered somewhat at the coast and across the
far southern portions of the forecast area where a tighter
pressure gradient between high pressure to the north and the
developing coastal trough should keep the boundary layer a bit
more mixed. Lows will range from the upper 20s well inland to
the mid 40s along the Georgia coast with temperatures over some
of the exposed communities such as Tybee Island, Hunting Island
and Folly Beach holding in the upper 40s through the night.
Expect some increase in stratocumulus along the Georgia coast
and areas south of the I-16 corridor after 3-4 am, which will
also help keep temperatures up a bit in those areas.
.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
Sunday and Sunday night: A coastal surface trough will tighten up a
bit Sunday into Sunday night before weakening late and lifting
north. Moisture will be on the increase in the lower levels of the
atmosphere and isentropic lift will be on the increase as well. Have
fairly good confidence that clouds will be on the increase Sunday
and that by Sunday night it will be quite cloudy. Have low
confidence on the threat for rain during this period. NAM is quite
aggressive with the MET guidance ramping rain chances to above 70
percent for all areas by Sunday night. ECMWF 12Z guidance is on the
other extreme indicating basically no precipitation. Canadian seems
to be more in line with the NAM and the 12Z GFS is on the lower end
of the extremes. Ensemble guidance is not much help either, ranging
from basically no threat to likely or categorical. One of the
limiting factors to rain is that the moisture is fairly shallow, so
as a result of this generally went with the lower end of the
precipitation range for most areas. It does appear that the best
chances for rain will be along and near the coast in the Charleston
tri-county area. It does appear that the setup for an advection fog
event Sunday night is quite high given the high dewpoint air that is
expected to be trying to move in. As a result, raised the fog
coverage to areas for much of the later part of the night. Feel that
widespread fog is a definite possibility as well as areas of dense
fog. Would not be surprised if a dense fog advisory is needed for
some areas as the event gets closer/unfolds. More on that in
subsequent forecasts. High temperatures on Sunday should range from
the upper 50s northwest to the mid 60s south. A non-diurnal
temperature trend is expected Sunday night with lows from the mid to
upper 40s northwest to the mid and upper 50s south likely before
midnight, followed by steady or slowly rising temperatures.
Monday through Tuesday: Any morning fog should dissipate by mid to
late Monday morning. Moisture will continue to be on the increase
during this period. Despite rather warm and humid conditions for
this time of the year, the lack of any well defined surface features
and only subtle disturbances in the fast zonal westerly upper level
flow, maintained rain chances mainly between 20 and 40 percent. High
temperatures on Monday will mainly be in the lower to mid 70s, with
temperatures a couple degrees cooler on Tuesday, but still well
.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
Low pressure will likely move along the stalled front across the
area keeping it unsettled through Wednesday. Drier conditions are
then expected Wednesday night as high pressure builds in from the
northwest. High pressure should then keep things cool/dry into
Friday before another coastal trough potentially develops and brings
more clouds/showers back into the area. Temperatures should mainly
be near or above normal through Thursday night before possibly
falling below normal Friday, then back near or above normal Saturday
assuming the coastal trough pushes northward as a warm front.
.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
VFR conditions are expected to prevail through 00z Monday. A
coastal trough will begin to sharpen overnight and low level
moisture will begin to push onshore. This will primarily turn
out to be increasing stratocumulus through the day, increasing
from south to north. Cloud heights will be in the 2500-3500 ft
range and there is some potential for periods of MVFR ceilings,
mainly at KSAV. Also can`t rule out a shower approaching the
terminals, but likelihood is too low at this point to include in
the forecast. Northeast winds tonight will veer to more easterly
or even southeasterly in the afternoon.
Extended Aviation Outlook: MVFR cigs/vsbys are very likely after
midnight Sunday night into Monday morning with IFR or lower
restrictions quite possible due to low clouds/fog. Some restrictions
likely into Wed due to low clouds/showers.
Tonight: Elevated winds will remain in place tonight as the
pressure gradient tightens between high pressure centered well
to the north and a development of a weak coastal trough off the
Georgia and Northeast Florida coast. Northeast winds of 15-20
kt can be expected for all legs except the Charleston Harbor.
Seas will average 2-3 ft nearshore waters and 3-4 ft offshore
waters. Could see a few gusts near 25 kt from Edisto Beach south
where the pressure gradient is forecast to be the tightest
overnight. However, gust durations do not look long enough to
justify a Small Craft Advisory at this time.
Sunday through Thursday: A coastal trough will shift north through
the area through Sunday night while weakening. A weak cold front
will then move into the area Tuesday before stalling out over or
near the area through mid week. Prior to a significant cold front
moving through the waters late Wednesday or Wednesday night,
conditions will be characterized by winds at or below 15 knots and
seas generally from 2 to 4 feet. In the wake of the cold front
conditions could possibly near Advisory levels for at least a
portion of the local waters, especially the Charleston County and
offshore GA waters. Sea fog will be possible over the chilly near
shore waters, mainly Monday and Monday night.
The upcoming lunar perigee and full moon along with periods of
onshore winds could push tides to Coastal Flood Advisory
levels near times of the early morning high tides Sunday into
the middle of next week, especially along the South Carolina