Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
745 am EDT Wednesday Sep 17 2014
a cold front will move southward across the region through tonight.
An inland wedge of high pressure will then prevail in the wake of
the front into the weekend. Another cold front will approach from
the west early next week.
Near term /through tonight/...
weak cyclogenesis will occur off the Georgia and South Carolina
coasts this morning as increasing cyclonic flow aloft spreads over
the southeast states today. Deeper moisture and enhanced upper
forcing will also be associated with the exiting low pressure off the
coast. In the low levels a backdoor surface cold front is forecast
to slide through the forecast area this morning in the wake of the
Atlantic low pressure moving out.
It looks like mainly isolated convection at best for the morning
hours with a round of scattered showers and storms for this
afternoon and evening as a series of middle level short wave troughs
ripple east and southeast through Georgia and South Carolina. Coverage/duration
should be less over South Carolina today but we will have to watch
for a region of upper difluence prognosticated by just about all the models
late day. Upper forcing could enhance the convective potential during
the late afternoon hours across parts of east central or NE South
Carolina. We raised probability of precipitation into the 40 percent range NE of Walterboro
to account for this potential. Otherwise... typical late Summer
thermodynamics/instability anticipated with just enough middle/upper
flow to suggest some multi-cell organization at times. We cannot
rule out isolated strong thunderstorms either but confidence with the
mesoscale is quite low with the weakly sloped frontal zone in the
area. Some of the thunderstorms will also produce downpours which will
will be slow to run off around high tide if storms crop up along
the coast toward middle afternoon. Deep layered steering is also
generally toward the coast as well.
Tonight...later shifts can take a harder look at late night fog
potential but chances seem to be there nearing dawn on Thursday.
Probability of precipitation shift offshore later in the evening with partly cloudy
skies most area.
Short term /Thursday through Saturday/...
Thursday...the cold front will steadily progress offshore and
eventually pull north/northeast...as a wave of low pressure
develops along the feature and lifts ahead of shortwave energy in
the base of the upper trough. High pressure over the Great Lakes
region will slowly build toward the Middle Atlantic States and
Carolinas behind the departing front...introducing a notably drier
air mass into the forecast area. Any isolated shower activity on
Thursday will likely be confined to the Charleston tri County
region and SC/Georgia coastal counties...where seabreeze and coastal
convergence could provide some forcing for weak convection. High
temperatures will reflect a cooling trend...peaking in the low to
middle 80s under partly cloudy skies. Low temperatures will also be
notably cooler...falling into the middle to upper 60s overnight. Best
rain chances during the nighttime period will be found over the
marine zones...however an isolated shower could move into the
coastline within onshore flow.
Friday and Saturday...as the center of high pressure to the north
moves over the New England states Friday and shifts off the
northern coastline Saturday...an inland wedge will amplify down
the middle Atlantic and southeast states. The upper trough will
gradually diminish late week into the weekend...while a weak
upper low takes shape just off the southeast coast. Expect the
energy within this feature to support the development of a well-
defined coastal trough at the surface. Best rain potential will
mainly be found along the coast and over the marine
zones...closest to moisture and convergence within the coastal
trough. Have thus indicated a distinct west-east gradient in rain
chances Friday and Saturday...with slight chances less than 25
percent across inland zones that increase to solid chances in the
30 to 40 percent range for coastal counties...highest during the
afternoon periods. Cool northeast flow within the inland wedge of
high pressure will suppress temperatures below seasonal
normals...rising only into the lower 80s each day. Low
temperatures Friday night will trend even cooler than the previous
day...ranging from the low to middle 60s away from the locally warmer
Long term /Saturday night through Tuesday/...
a coastal trough on the periphery of inland high pressure will
lift north/northeast across the Atlantic waters Sunday...while
the inland wedge eventually erodes ahead of a strong cold front
upstream. Precipitation potential on Sunday will be similar to
previous days...mainly focused across the coastal counties with
rain-free conditions farther inland. The cold front will continue
to approach from the northwest on Monday...as a pre-frontal surface
trough shifts over the East Coast states. The cold front then
steadily moves through the forecast area early Tuesday...followed
by expansive high pressure with origins from the northern plains.
Medium range model solutions are not indicating significant
precipitation associated with the frontal passage...and have thus
capped rain chances generally below 30 percent for the early week
period. Temperatures could rebound ahead of the front on Monday and
peak in the middle to upper 80s...before a notable cooling trend
occurs in the wake of the front Tuesday and Wednesday.
Aviation /12z Wednesday through Sunday/...
the arrival of a backdoor cold front has disrupted stratus formation
and diminished the risk for IFR ceilings at the terminals. A band of
MVFR/IFR ceilings to the north of ksav associated with the low level front
could make for low ceilings there for a couple hours this morning. At
kchs...stratus is generally scattered and there could be patches of
broken IFR ceilings until middle morning but looking unlikely.
VFR this afternoon and evening outside of any convective rains. Thunderstorms
are possible...especially at kchs but timing/coverage windows are
most uncertain at this point.
The next item to watch out for is a return of low clouds and some
fog late tonight/early Thursday morning. No introductions just yet but
the risk for IFR or lower ceilings will be investigated on later cycles
Extended aviation outlook...outside of rain showers and/or thunderstorms and rain...flight
restrictions will be possible each morning with fog and/or low
stratus shifting onshore.
ground swell from distant major Hurricane Edouard trickling into our
coastal waters early this morning...and additional swell energy is
to arrive as time progresses. Seas 2 feet near shore will build to
3 to 4 feet well offshore today into tonight. As a cold front moves
through this morning light and variable winds should become more
onshore but speeds mainly 10 knots or less through tonight. The
subtle long period swell could make for locally rough conditions
around the jetties...especially around times of low tide.
Thursday through Sunday...light northeast winds behind the cold
front on Thursday will begin to surge by Friday...as a wedge of
high pressure strengthens inland and a coastal trough amplifies
to the southeast. Winds will increase to 15 to 20 knots across
portions of the marine zones Friday and Saturday...while seas
build to 3 to 5 feet. Latest numerical model solutions suggest
conditions will remain just below Small Craft Advisory criteria
at this time. Conditions should moderate over the waters as the
inland wedge weakens by Sunday.
Rip currents...long period swell from Hurricane Edouard will occur
along the beaches today into Thursday. Despite only light
onshore winds...1-2 feet swells with periods near 14 seconds support
an enhanced period of rip currents along the South Carolina and
Georgia beaches. A moderate risk of rip currents today...and