Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Charleston SC
740 am EDT Sat Apr 19 2014

Synopsis...
low pressure will shift off the southeast coast late today into
tonight. Then...the low will continue to gradually move away from
coast Sunday. High pressure will briefly build into the region
Monday. A weakening cold front will push through the area by
Wednesday... followed by high pressure and warmer temperatures
through late week.

&&

Near term /through tonight/...
widespread stratus clouds and occasionally drizzly conditions blanket
the region early this morning under a wedge of subsidence ahead of
the Deep Cut-off upper low in the Florida Panhandle. Temperatures are
mild...55 to 60 degrees with windy conditions lingering along
upper coastal South Carolina.

An interesting and complex day on tap for our region as the upper
cut-off low traverses northern Florida. Models indicate the middle
level cold pocket will move over the forecast area today. Very
unstable elevated instability was noted on the 00z model guidance
today...however the low pressure waves along The Wedge/coastal front
remain just off the coast. The main question for south Georgia
today will be if low clouds manage to break allowing surface-
based instability to develop. European model (ecmwf)/NAM agree that steep lapse
rates and mu convective available potential energy nearing 1000 j/kg will pose a risk for some
robust elevated convection across southeast Georgia but the severity of
the convection will hinge on whether we see any surface heating
and breaks in the low cloud fields during the morning/early
afternoon. If temperatures can break into the Lower/Middle 70s to the south
of I-16...we could see isolated strong/severe thunderstorms with a chance for
large hail given low web bulbs and forecast sounding profiles.
The chance for that occurring at this time appears low. Even
without a lot of heating...the synoptic setup still suggests a
risk for small hail and scattered downpours in showers and a few
thunderstorms by afternoon. Scattered to numerous convective rains in
Georgia will likely spread up the coast into southeast South Carolina by
late day and this evening but widespread heavy rainfall potential
is lower today as the conveyor belt of deepest moisture offshore.

We did not alter too much from our previous package given good
model agreement on the main features and precipitation timing. Unsettled
cloudy weather with decent rain chances on tap through tonight.

&&

Short term /Sunday through Tuesday/...
the cut-off upper low will roll only slowly east/northeast Sunday
through Monday...supporting only gradual improvement/lowering probability of precipitation.
Capped maximum probability of precipitation around 50 percent Sunday morning across northern
coastal counties/adjacent coastal waters...but higher probability of precipitation could be
required Sunday depending on the distribution of wrap-around
moisture. Also of note...a tight surface pressure gradient between
the offshore low pressure and high pressure building from the north
will support breezy/windy conditions and below-normal temperatures
mainly in the 60s Sunday...followed by temperatures in the 70s and
lighter winds Monday.

Tuesday...an upper trough and associated surface cold front will
approach the region from the west/northwest. Ahead of these
features...temperatures should warm into the lower 80s at many
locations. During the afternoon/evening...guidance continues to
depict a band of enhanced 850-500 mb moisture pushing through the
region. The 19/00 UTC GFS depiction of instability featuring sbcapes
as high as 900-1000 j/kg and climatology justify a slight chance of
thunderstorms during this period.

Lake winds...gusty winds will likely necessitate a lake Wind
Advisory Sunday into Sunday night.

&&

Long term /Tuesday night through Friday/...
the upper trough and associated surface cold front will push through
the region Tuesday night. Surface high pressure will then build over
the region from the north...supporting rain-free conditions and
temperatures ranging from the the middle 70s lower 80s Wednesday and
upper 70s to middle 80s Thursday. As the high shifts offshore
Friday...temperatures in the lower to middle 80s will be common away
from the immediate coast.

&&

Aviation /12z Saturday through Wednesday/...
IFR/LIFR ceilings across the coastal corridor at daybreak with the
dry slot aloft providing an impetus for low clouds to build
down early this morning. Where the clouds were below 500 feet...
light fog reported. Forecast soundings do not provide much
hope that ceilings lift quickly or scatter out today. There is
great doubt that ceilings break at kchs and we retained IFR through
the taf cycle but stronger low level winds by tonight could
allow ceilings to lift to low end MVFR. Scattered showers developing
across southeast Georgia should expand to southeast South Carolina by later
this afternoon and evening. There is a chance for thunderstorms and rain later
today but overall potential not there for taf inclusions.

Extended aviation outlook...flight restrictions into Sunday. VFR
thereafter. Gusty winds possible Sunday.

&&

Marine...
there has been very nasty/hazardous weather at times over across
the South Carolina waters overnight with winds gusting over 35 knots
at times along the immediate coast and heavy rains reducing visibilities
below 2 nm with stronger thunderstorms well offshore. The rains were
slowly diminishing this morning but a packed pressure gradient along
the coast from fort Pulaski north continues to produce gusts winds
to 30 knots at times. Small Craft Advisory conditions intact all
waters with seas 5-8 feet near shore and perhaps up to 10 feet well
off the Georgia coast seaward from the Savannah River entrance.
At daybreak...winds along the coast have diminished below 20 knots
and should remain so this morning.

Conditions not looking to improve too much today as low pressure moves
off the NE Florida coast late. The gradient will begin to pack
again tonight producing increasing north to NE flow and winds gusting
over 30 knots again...especially after midnight. Small craft advisories
will continue all waters including the Charleston Harbor through
tonight.

An impressive surface pressure gradient and 35-40 knots winds as low as
the 950-925 mb layer suggest the potential for gales outside
Charleston Harbor Sunday into Sunday night. Thus...raised gale
watches for all zones outside The Harbor beginning 12z Sunday and
continuing through Sunday evening across nearshore waters and
through Sunday night across amz374. Solid Small Craft Advisory
conditions will prevail on Charleston Harbor during this period.
Expect a transition to Small Craft Advisory conditions outside The
Harbor when gales end overnight Sunday night into Monday. Small
Craft Advisory seas could persist across amz374 through Monday
night/Tuesday morning.

Winds/seas are expected to remain Small Craft Advisory levels ahead
of an approaching cold front Tuesday. This front will advance
through the waters Tuesday night...and a period of Small Craft
Advisory winds is possible especially across amz374 through
Wednesday. Then...winds/seas should remain below Small Craft Advisory levels through
late week.

Rip currents...moderate to high risk today...highest along the
Georgia coast. An enhanced risk for rip currents will persist
through the weekend and perhaps into early next week.

&&

Tides/coastal flooding...
tides will continue to run high through the weekend and coastal
flood advisories could be needed during the nighttime high tide
cycles.

&&

Chs watches/warnings/advisories...
Georgia...none.
SC...none.
Marine...Small Craft Advisory until 6 am EDT Monday for amz330.
Gale watch from Sunday morning through Sunday evening for
amz350-352-354.
Small Craft Advisory until 8 am EDT Sunday for amz350-352-354-
374.
Gale watch from Sunday morning through late Sunday night for
amz374.

&&

$$

Synopsis...
near term...
short term...Spr
long term...Spr
aviation...
marine...Spr
hydrology...
tides/coastal flooding...