Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 280754
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
354 AM EDT Sun May 28 2017

.SYNOPSIS...
High pressure will linger over the region today. A weak cold
front will gradually sag south through the area Monday through
Wednesday, then lift back north late week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: The ongoing area of convection (MCS) upstream of the
forecast area continues to diminish in coverage and intensity as
IR satellite imagery reveals warming cloud tops. However, it
will be interesting to see how the residual outflow from this
convection evolves through the morning and into the afternoon as
it could eventually serve as a initiation mechanism later
today. The pattern aloft will feature nearly zonal flow as the
forecast area is situated between the ridge and anticyclone over
the Gulf of Mexico and a upper low over the Upper Midwest and
central Canada. However, this will not be benign zonal flow as
models depict a series of potent shortwaves that will be poised
to translate eastward through the day. The good news is that
forcing associated with these embedded shortwaves appears to be
solidly north of the forecast area. At the surface, high
pressure to the south will result in westerly low level flow
with a late day sea breeze that will bring more southwesterly
flow along the coast. Despite the fact that the large scale
forcing will pass by to the north, strong heating and the
potential presence of the residual outflow boundary could help
kick off isolated thunderstorms this afternoon. Model soundings
show that any thunderstorms would mainly impact the South
Carolina zones where a much weaker cap would be in place
compared to southeast Georgia.

If storms are able to initiate, the near storm environment is
potentially quite interesting. Various model solutions show a
well defined elevated mixed layer with steepening mid-level
lapse rates in the -7 to -8 C range. DCAPE values could be as
high as 1000-1200 J/kg, with hail CAPE approaching 1000 J/kg.
Furthermore, a 50+ kt 500 mb jet is progged to extend west/east
across the northern half of the forecast area, which would
increase 0-6 km shear into the 40 kt range. With all this in
place, the forecast area is in a marginal risk area in the SPC
Day 1 outlook which seems quite reasonable. There are
uncertainties for sure, including the impact of lingering debris
clouds on surface temperatures and resulting buoyancy. Also,
the fact that the main slot of forcing will be north of the area
should temper expectations. The expectation is the coverage
will be isolated at best, but the convective mode should favor
discrete cells if they are able to develop. Damaging wind gusts
and large hail would be the main impacts, but given the
potential shear some rotating storms would be possible. Another
source of uncertainty is the feeling that current model
solutions are struggling with the current convection and any
subsequent convection later in the day. So, the risk is not
particularly high or widespread, but certainly something to
watch.

Tonight: If any storms develop in the afternoon, they should
diminish quickly in the evening with the loss of heating.
Through the remainder of the overnight, dry and mild conditions
should prevail. Lows will likely not fall below 70 for most
areas.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
The upper ridge will gradually weaken and shift east Monday
into Tuesday, allowing more shortwave energy to push into the
area. A weak cold front will move into northern GA and far
western NC/SC Monday afternoon, then approach the local area
Tuesday afternoon.

Monday looks rather toasty as warm low-level thicknesses reside
over the area, ample sunshine occurs, and low-level flow is
westerly. Highs expected to be in the mid 90s away from the
coast. Model soundings show a decent mid-level cap until the
afternoon which will probably preclude convective development
until later in the day. There won`t be much forcing for
convection on Monday though it`s possible that some activity
will develop over the SC Midlands and push east-southeast late
in the afternoon. Convective parameters would be conducive for
some severe thunderstorms given extensive dry air off the
surface and CAPEs of 1500-2000 J/kg.

Tuesday is looking more active than Monday given the continued
breakdown of the upper ridge and shortwave energy making its way
into our inland zones during the day. The surface front pushing
in late in the day will also provide a trigger for convection.
We increased pops a bit during the afternoon. Convective
parameters looks similar to Monday except the cap is weaker in
the morning. There would be enough 0-6 km shear to support some
multicellular organization, and the dry air aloft will be
conducive for damaging winds.

Slightly cooler temps expected Wednesday as the front remains
stalled over the area and more cloud cover is present. There
will be enough forcing for isolated to scattered convection
mainly in the afternoon, though instability and shear will be a
little lower than Mon-Tue.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
A fairly active period is in store as a quasi-zonal flow exists
aloft and weak shortwave perturbations continually traverse the
area. With increasing moisture through late in the week and
daytime temps in the upper 80s or low 90s, scattered to numerous
showers and tstms expected to develop mainly during the
afternoon and evenings.

&&

.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
VFR conditions expected to prevail through 06z Monday. One
thing to keep an eye on will be the slight chance of
thunderstorms near KCHS tomorrow. An outflow boundary from
ongoing thunderstorm activity could settle over the area during
peak heating tomorrow and serve as an initiating feature.
Coverage should be limited, if indeed there is any, so there is
no mention in the TAF at this time.

Extended Aviation Outlook: Mainly VFR conditions. Brief flight
restrictions possible in mainly afternoon/evening showers/tstms
starting Monday.

&&

.MARINE...
Today through tonight: Overall the setup is quite similar to
the last couple of afternoons. Morning westerly flow will shift
to more southwesterly in the afternoon as the sea breeze
develops. We will see enhanced flow mainly along the land/sea
interface with a solid 15-20 knots along the Charleston County
coast. Elsewhere 10-15 or a solid 15 knots is expected. The
thinking is again that conditions in Charleston Harbor will stay
just below Small Craft Advisory thresholds. Seas are expected
to be 2-3 feet.

A slightly stronger gradient will persist into Monday with a
decent 15 kt sea breeze possible in the afternoon. Thereafter,
fairly weak winds and small seas are expected as a front drops
into the area and lingers through the week.

&&

.TIDES/COASTAL FLOODING...
High tide will again be elevated this evening due to the new
moon and perigee. Fortunately the wind directions are not very
conducive for surge, but we could see very marginal coastal
flooding along the South Carolina coast during the evening high
tides Sunday.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$

NEAR TERM...BSH
SHORT TERM...JRL
LONG TERM...JRL
AVIATION...BSH/JRL
MARINE...BSH/JRL
TIDES/COASTAL FLOODING...


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