Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 261125

Area Forecast Discussion
National Weather Service Charleston SC
725 AM EDT Mon Sep 26 2016

High pressure north of the region will weaken today while a surface
trough develops just offshore. A cold front will slowly approach
through the first half of the week before stalling over the area
Wednesday. Low pressure will form over the Mid-Atlantic Wednesday
night, ushering the front off the coast Thursday morning. High
pressure will build into the region by the end of the week.


Today: The evolving synoptic pattern favors a turn to more
unsettled weather across the forecast area. High pressure both at
the surface and aloft will shift off the New England and mid Atlc
coasts and this will induce deep easterly flow and Atlc moisture
advection in our area. A weak mid level disturbance off the
Florida coast will lift north and elongate, while broad upper
difluence spreads inland up the Savannah River after mid morning.
These complex but subtle features make for timing rain chances and
coverage a challenge but the overall pattern signals a shot for
periodic/scattered convective rains especially eastern zones and
north of the Savannah River later today. To the west of I-95, the
shower chances appear to be a bit lower but we continue to
indicate chance pops for the most part. We introduced some likely
pops between Charleston and Savannah and these will definitely
need some massaging today. We also cannot rule out a few tstms
later today, but instability is generally unimpressive with poor
lapse rates, even though the RAP13 does indicate negative
showalter indices to go along with about 1500-2000 J/KG MU Capes
for a brief window. An increasing canopy of mid to high clouds and
the showers around should keep temps down a bit in some areas but
upper 80s still a good bet over inland Georgia Zones with mid 80s

Tonight: Rain chances should diminish across much of Southeast
Georgia with a continuation of mostly cloudy skies. The South
Carolina forecast is tricky with a wave of low pressure developing
and tracking up the surface trough just off the coast. Models keep
plenty of moisture and weak upper forcing forecast along the
South Carolina coast even late tonight. We maintained chance pops
in this region. It looks like too many clouds around for any
significant fog chances, although the area of interest would be
far inland Georgia where clouds could be thinner.


The general trend through the period should be improving conditions,
through there is still considerable model disagreement on some
significant features that will determine how well the forecast

Heights falls aloft thanks to a robust stacked/cutoff low moving
over the midwest and warm air advection at the surface in a
prefrontal regime locally will result in a chance of showers and
thunderstorms for the entire area Tuesday. Though the entire day is
not expected to be a washout due to underwhelming atmospheric
moisture content (PW less than 1 standard deviation above normal),
pinning down the timing of convection is a challenge as convective
initiation could come from vort pulses moving around the southern
periphery of the upper level low, subtle waves in the inverted
trough along the coast, or the remnants of other convective
complexes that will be moving across the region ahead of the front,
so chance POPs are in the forecast for most of the day. Severe
threat is low given modest instability.

Wednesday should be active as well as the area remains in the warm
sector of a large low-level cyclone over the Great Lakes/Midwest,
though the axis of highest pre-frontal moisture will likely have
shifted offshore by his time. This is where the certainty in the
forecast takes a nose dive. Surface low pressure over the Midwest
will stall and fill rapidly Wednesday as the cold front occludes and
the baroclinic structure of the system weakens. The general pattern
amongst global model ensemble members is that, given the troughing
remaining aloft, low pressure will develop along the front-which
will have essentially stalled over the area Wednesday evening-over
the Carolinas and lift north along the front through the day
Thursday. While significant disagreement exists amongst guidance in
the strength, location, and timing of this feature, there is general
agreement that the formation of this low will give the stalled front
enough of a kick Thursday morning to get through the area and allow
high pressure to fill in through the day Thursday. This general
trend is represented in the official forecast, with the caveat that
if the low is weaker or slower with developing over the Carolinas,
the front will be slow to move through the area and at least the
first part of the day Thursday could be more unsettled.


High uncertainty continues into the long term. Cut-off upper low
will continue to reside over the eastern U.S., though high pressure
trying to build in at the surface should keep conditions dry
locally, at least over land, Friday into the weekend. However, given
the close proximity of the front which will remain stalled over or
near the coastal waters and the area of destabilization under the
core of the upper-level low, there is potential for at least
nuisance showers mainly in the afternoon through the long term. Have
opted for a dry forecast at this time given the high uncertainty,
with temps near to a couple degrees below normal through the period.


Mostly VFR but we could still see a period of MVFR cigs/vsbys at
KSAV prior to 15Z per 06Z NAM soundings. Periodic showers are
possible along the South Carolina Coast today as moisture advects
onshore to the the north of a weak low pressure area trying to
pinch off along the Gulf Stream. VCSH at KSAV this morning and
afternoon with small chances for thunder. Late tonight, light
north flow signals that we will need to watch for some low clouds
or fog late. At this point, we think upper level cloudiness will
preclude widespread IFR potential but will continue to monitor.

Extended Aviation Outlook: Brief flight restrictions will be
possible in showers and thunderstorms Tuesday and Wednesday.


Today: High pressure to the north will gradual erode as it moves
offshore. A coastal trough will sharpen but overall the pressure
gradient and pinching gradients are forecast to weaken as the
high loses influence. We think the models may be a bit quick
which is a typical trait due to the localized moisture gradient
complexities. Onshore flow 15-20 kt from Fort Pulaski up through
Charleston County should decrease to around 15 kt later today
and this steady flow will support choppy seas 3 to 4 ft with
a few 5 ft seas possible well offshore.

Tonight: A weak wave of low pres should traverse the offshore
surface trough and turn the flow more north later tonight. Speeds
should be less as the gradient continues to weaken with time. Seas
2 to 4 ft, highest offshore.

Tuesday through Friday: A coastal trough/weak coastal low will
persist over the waters Tuesday before weakening Wednesday as a cold
front approaches from the west. This front will enter and stall
somewhere near the waters Thursday morning, keeping conditions
unsettled through most of the period. Winds will generally be less
than 15 kt, with 2-3 ft seas, and locally higher winds and seas
temporarily possible in showers and thunderstorms.




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