Area forecast discussion
National Weather Service Charleston SC
822 PM EDT Wednesday Jun 19 2013
Synopsis...
a cold front will slowly progress through the region tonight. The
front will then stall just south of the area Thursday and linger
through Saturday...as high pressure builds from the north. The
center of high pressure will slip into the Atlantic early next
week.
&&
Near term /until 6 am Thursday morning/...
water vapor imagery shows a significant shortwave aloft shifting
into the southern appalachian region by early evening...with
abundant deep moisture over the southeast states downstream of the
associated trough axis. At the surface...a cold front has slipped
into the forecast area...as seen in weak north/northeast winds
across South Carolina counties and variable to southerly flow
south of the Savannah River. Short range model solutions indicate
the front descending southward as the night progresses...before
likely stalling around the Altamaha river along the southern
border of the forecast area.
Strong thunderstorms producing extremely heavy rain have mainly
shifted toward the coastline and into the marine zones by 8 PM
EDT...remaining somewhat aligned with the location of the front
and associated convergence. Convection west of the area is either
on a weakening trend and/or is progressing more southerly along
mesoscale boundary interactions. Thus...latest forecast will lean
toward rain chances diminishing from west to east during the next
several hours.
Depending on rather variable wind fields...fog could become a
concern later tonight where heavy rainfall occurred. Hourly
temperatures have also been impacted by rain-cooled air...yet
will still advertise lows in the upper 60s inland and low 70s
along the coast.
&&
Short term /6 am Thursday morning through Saturday/...
Thursday...expect a decrease in shower and thunderstorm coverage
Thursday morning...as a stalled front remains just offshore and
south of the forecast area. Models are fairly consistent that a
weak low/coastal trough will develop along the stalled boundary
during the day...thus providing a forcing mechanism for additional
showers/thunderstorms Thursday afternoon for locations where
abundant low level moisture lingers...mainly along coastal South
Carolina and southeast Georgia counties. Have therefore maintained
30-40 probability of precipitation over these locations. Overall high temperatures should remain
a degree or two below normal with increasing cloud cover and
precipitation over the area. Expect temperatures to peak in the middle/upper 80s.
Showers and/or thunderstorms may linger into the overnight period
as the surface low/coastal trough remains positioned near the
southeast coast. For this reason...have maintained 20 probability of precipitation mainly
over all South Carolina and Georgia coastal counties. Overnight
lows will range in the upper 60s inland to low/middle 70s along the
coast.
Friday and Saturday...surface high pressure will eventually build
over the midlands from the north by Friday...with the pattern
transitioning into an inland wedge and coastal trough type scenario.
Models continue to suggest weak waves of energy developing along the
coastal trough which gradually slide the trough near the immediate
coast Friday afternoon...thus providing another day of shower and/or
thunderstorms...with the greatest coverage along coastal counties.
By Saturday...high pressure will gradually strengthen over the
western Atlantic and likely force the coastal trough over land by
the afternoon or evening. Have therefore indicated a 30 percent
chance of coastal showers and/or thunderstorms Saturday morning
gradually shifting inland with the trough Saturday afternoon.
Overall highs will continue to remain a few degrees below normal on
both days...with temperatures expected to reach the middle/upper 80s...highest
over inland Georgia. Overnight lows will range in the upper 60s to
lower 70s.
&&
Long term /Saturday night through Wednesday/...
the inland wedge/coastal trough pattern will transition on
Sunday...as the trough dampens and/or moves inland while the
center of high pressure settles over the Atlantic. The forecast
area will then remain along the western periphery of the Atlantic
high pressure into early next week...with a weak inland surface
trough developing toward the middle week period. Have indicated a
more seasonal convective scenario....with thunderstorms generally
developing along and inland of the seabreeze each
afternoon...diminishing in coverage and/or re-developing over the
waters each night. 500 mb heights peak early next week...evidence
that the initially weak upper level pattern will transition into
broad ridging aloft. As a result...expect a gradual warming trend
through the period. High temperatures around 90 degrees away from
the coastline Sunday will increase into the lower 90s Monday
through Wednesday.
&&
Aviation /00z Thursday through Monday/...
showers and thunderstorms will linger near ksav...resulting in
MVFR or lower conditions at times through about 01z. Current
indications are that restricted ceilings will develop along and
immediately behind a southward sinking front. The forecast
indicates prevailing MVFR ceilings in the 09-14z time frame...but
it is very possible that ceilings could be in the IFR category for
part of that time. Additional showers and thunderstorms are
expected Thursday afternoon and have included vicinity
thunderstorms in the forecast.
Extended aviation...MVFR or lower ceilings/visibilities possible through
Saturday with isolated to scattered showers and/or thunderstorms
developing near a coastal trough. VFR conditions may return by
Sunday.
&&
Marine...
tonight...a cold front to the north will continue sinking southward
and into the vicinity of the waters through tonight. Most of the
marine zones will maintain west/southwest flow tonight...except
for the Charleston County waters where wind directions have
already begun turning northerly behind the front. Overall
conditions will remain well below Small Craft Advisory levels with
winds less than 15 knots and seas 2-3 feet within 20 nm and 3-4 feet
beyond.
Thursday through Sunday...east/northeast winds and seas are
expected to remain well below Small Craft Advisory criteria
Thursday as a cold front stalls over or near the waters. A coastal
trough is then expected to develop along the stalled boundary by
Friday while high pressure builds from the north inland. This may
produce a modest pressure gradient over the waters with wind
speeds peaking between around 15 knots and waves building to 3-4 feet.
Additionally...showers and/or thunderstorms are possible through
Saturday over all waters until high pressure builds over the
western Atlantic and shifts the coastal trough onshore. Winds/seas
are then expected to decrease/subside into Sunday as high pressure
continues to build over the Atlantic.
&&
Chs watches/warnings/advisories...
Georgia...none.
SC...none.
Marine...none.
&&
$$
Near term...wms
short term...dpb
long term...wms
aviation...jaq
marine...bsh/dpb
