Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
743 am EST sun Dec 8 2013
..dense fog advisory for the Charleston Harbor until 11 am EST...
a cool wedge of high pressure will linger over the area through
tonight. A warm front will lift north through the area Monday...
then a cold front will move into the region from the northwest
Tuesday. High pressure will slowly build from the west during the
middle to late week period. More unsettle weather is possible by next
Near term /until 6 PM this evening/...
classic cold air damming is rapidly maturing this morning...
characterized by low clouds...stiff northeast winds...areas of fog
and periods of drizzle. Models show The Wedge will change little
in orientation today as ongoing isentropic assent atop the shallow
cold air will produce patchy areas of light rain and periods of
drizzle. The resulting diabatic cooling will yield little change
in temperatures with the local forecast area likely seeing as
20-25 degree thermal gradient with afternoon highs. Statistical
guidance is likely underestimating the amount of diabatic cooling
that will take place so we continue to undercut the various
guidance packages by as much as a category. Expect highs to range
from the upper 40s across interior southeast South Carolina to the
around 70 across inland McIntosh County. There is obviously a big
bust potential here as heavier amounts of drizzle and/or more
scattered/widespread light rains will yield cooler temperatures
while the lack of significant precipitation/drizzle will allow
temperatures to recover somewhat...especially across the southern
portions of the forecast area. The fog and drizzle with a few
showers mixed in will linger well into the afternoon...although the
better chances for measurable rainfall will gradually shift north
Short term /6 PM this evening through Wednesday/...
tonight...solid wedge conditions will remain in place
overnight...although models show the parent high remaining
transient with the lack of a confluent flow aloft over the Ohio
Valley and middle-Atlantic regions. Temperatures will move little
overnight with lows ranging from the middle 40s well inland to the
Lower-Middle 60s across the far south. There will be a continued risk
for fog...isolated shower activity and possibly some drizzle. The
fog could become locally dense with lighter winds and a better
potential for stratus build down.
Monday...the warm front will be north of the forecast area by the
morning hours...with solid southerly flow prevailing across the
southeast states in its wake. A cold front will be taking shape
across the far western Carolinas and northern Georgia...yet make
only slow progress eastward as the day progresses due to the
blocking effects of persistent Atlantic ridging. Temperatures will
rebound significantly from the previous day and peak in the middle to
upper 70s...greater than 10 degrees above seasonal normals. Have
remained conservative with rain potential during the daytime
hours...since the best forcing will remain just upstream near the
Monday night....have indicated isolated shower coverage increasing
across northern and western zones from the evening hours into the
overnight period...as the front gradually slips closer to the
forecast area. Fog development is possible overnight with
dewpoints in the low 60s...however stronger wind profiles could
limit coverage compared to previous nights. Will still need to
monitor whether any sea fog just off the coast can spread inland.
Low temperatures could be almost 20 degrees above seasonal normals
within southerly flow and decent low level moisture. Expect
temperatures to only fall into the low 60s.
Tuesday...elongated shortwave energy shifting east will
eventually be able to dampen Atlantic ridging enough to allow the
cold front to cross offshore by Tuesday evening. High temperatures
will still rise above seasonal normals due to the warming effects
of pre-frontal compression early in the day...while cold advection
is delayed late in the day with high pressure remaining well to
the west. Expect temperatures to peak in the low to middle 70s
despite thick cloud cover and scattered shower coverage by the
afternoon hours. Rain potential will be highest during the
afternoon/early evening time frame when the frontal zone is
directly over the area...then decrease from west to east as the
front pushes offshore. Prefer not to mention thunder in the
forecast at this time...but subsequent model runs will be
monitored for slower frontal progression and the potential for
some slight destabilization. Low temperatures behind the front
will range from the middle 40s northwest to the low 50s southeast.
Wednesday...nearly zonal flow will prevail aloft...and lacking
stronger shortwave energy or a more amplified upper trough
following frontal passage...high pressure will struggle to solidly
build over the area. The front will thus settle just to the south
and east over the waters...while a wave of low pressure tracks
northeast along the feature. Have had to maintain a slight chance
of rain for portions of the forecast area to account for lingering
moisture convergence and isentropic ascent along and behind the
offshore front. Temperatures will be notably cooler as northeast
flow strengthens across the area...while cold advection
eventually initiates late in the day once the coastal wave shifts
away and allows high pressure to press toward the region. High
temperatures will be suppressed in the low/middle 50s north to the
low 60s farther south closer to the meandering boundary.
Long term /Wednesday night through Saturday/...
a more well-defined shortwave trough sweeps across the East Coast
by Thursday and allows drier high pressure to press into the middle
Atlantic and southeast states through Friday. Temperatures will
still be below normal for Thursday...then moderate into the upper
50s to lower 60s Friday as the surface high tracks toward the
coastline. Have confined rain chances to the coastal waters
through Friday afternoon. Shortwave energy will then track
northward from the Gulf Coast states late Friday...with a wave of
low pressure or coastal trough developing on the southern
periphery of the retreating high pressure over the weekend. There
is considerable discrepancy among model solutions regarding
details next weekend...however the scenario looks generally
unsettled. Will keep high temperatures within a few degrees of
seasonal normals...while indicating solid rain chances from Friday
night through Sunday.
Aviation /13z Sunday through Thursday/...
classic cold air damming with associated overrunning will keep
widespread IFR ceilings in place for the next 24 hours. Ceilings may
occasionally drop to airfield minimums...mainly at kchs...but
durations will be fairly short. Probabilities for LIFR will
increase by late evening as the boundary layer saturates and
stratus build down will eventually lead to areas of fog...
especially at kchs. Will hold visibilities at MVFR thresholds for now
given some model discrepancy this far out...but dense fog is
certainly a possibility.
Extended aviation outlook...MVFR/IFR or even lower conditions will
occur Sunday into early Monday...then again Monday night into
early Tuesday. Low-moderate risk of occasional flight restrictions
with a cold front and associated rain showers Tuesday afternoon/evening.
reports from the Charleston Harbor pilot office indicate visibilities
have dropped to 1/2 nm in the Charleston Harbor. Have therefore
issued a dense fog advisory through 11 am.
Today...seas at 41004 are up to 8 feet and up to 5 feet at 41008.
Have expanded the Small Craft Advisory to include the Georgia
offshore waters and it could be marginal for the Georgia nearshore
waters. Will hold off on expanding for the Georgia nearshore leg
for now and cap winds at 20 knots with seas 3-5 feet.
Tonight...considerably lighter winds will occur as The Wedge front
tries to move north as a weak warm front. It does appear northeast
winds will hold for much of the night and the models are likely
too aggressive in the erosion of the inland wedge. The only
exception will be over the Georgia offshore waters where northeast
winds will turn more east/southeast with the passage of the
Monday through Thursday....the main marine concern for Monday will
be impacts of sea fog forming over the cooler shelf waters...as
southerly flow enhances low level moisture across the region.
Winds will then veer to the north/northeast behind a cold front
crossing the marine zones late Tuesday. Strongest cold advection
and the tightening pressure gradient will be delayed until late
Wednesday and Thursday...when Small Craft Advisory conditions
could develop across portions of the marine zones.
Marine...dense fog advisory until 11 am EST this morning for amz330.
Small Craft Advisory until 10 am EST this morning for amz352-
Small Craft Advisory until 1 PM EST this afternoon for amz350.