Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 180530

Area Forecast Discussion
National Weather Service Charleston SC
130 AM EDT Sun Mar 18 2018

A cold front will drop in from the north on Sunday and then
stall just to the south Sunday night. The front will then lift
back to the north as a warm front on Monday as an area of low
pressure moves through the Tennessee Valley. Another area of
low pressure will cross the area by Tuesday night, followed by
cooler high pressure for the latter half of the week.


Low pressure sliding east through NC will move into the nearby
Atlantic by daybreak, allowing for a cold front trailing from
the low to start pivoting toward the local area and reaches
near the Santee River by 12Z. There isn`t much forcing with the
front, so we have no mention of PoP`s in the forecast. Instead
we show broken high level cloud cover, before late night stratus
either develops or advects into locations near and south of
I-16 in GEorgia toward sunrise. There is too much wind to show
fog, so it has been removed from the overnight forecast.

Higher dew points, mixing within the boundary layer and the
cloudiness will allow for our warmest night of March so far,
outside of March 1st.


Sunday and Sunday night: Aloft, mid/upper level ridging will
translate eastward as a vigorous trough passes through the Four
Corners region. At the surface, an area of low pressure will pass by
to the north in the morning and move offshore, leaving the forecast
area in a weakly forced region of high pressure between the
departing low and the developing low along the Front Range. Sunday
should be a quiet day within a slot of mid level dry air and ridging
aloft. Highs will rise into the mid to upper 70s across southeast
South Carolina and low 80s across southeast Georgia. Overnight, the
setup will begin to turn more active as shortwave energy ripples
through ahead of the main trough over the central CONUS. Models are
in good agreement that a large area of precipitation will blossom
upstream within some isentropic ascent along a warm front, and pass
through during the late night hours. Model soundings show a nice
area of steepening mid level lapse rates and showalter indices
beginning to decrease to near zero and even negative. Given the
presence of elevated instability, have added thunder to the forecast
after midnight and increased PoP`s into the categorical range.

Monday is the most interesting part of the short term period and the
primary forecast concern is the potential for severe weather. The
day will begin with an ongoing area of precipitation that will be
pushing off the coast during the morning along a warm front. Aloft,
the primary upper low/shortwave will move into the mid Mississippi
Valley and the primary surface low will slide eastward across the
Tennessee Valley. As the low draws closer, the warm front will lift
northward across the area, placing the forecast area solidly within
the warm sector. The focus then shifts to the severe weather
potential for the afternoon and evening hours. It looks like yet
another classic question about destabilization and recovery from
preceding rainfall. Will there be any breaks in cloud cover to
allow for direct insolation? Still hard to say at this point.
Regardless, forecast models show a large area of 1500-2000 J/kg of
CAPE across south and southeast Georgia, coincident with deep layer
shear increasing to around 50 knots as the nose of the 500 mb jet
pushes in during the afternoon and evening hours. Model soundings
show excellent wind fields combined with plentiful surface based
instability, especially across southeast Georgia. This matches up
well with the SPC day 3 slight risk area. The question will be what
takes place upstream and if an MCS can develop and move into the
forecast area. Regardless, the environment looks favorable for
damaging wind gusts, possibly large hail, as well as tornadoes. As
you go further north into southeast South Carolina the threat is
less certain with the northward progression of the warm front and
the degree of destabilization. Based on current model runs the
favored time appears to have shifted earlier to the afternoon and
evening hours, which is also a more climatologically favorable time.
It is certainly something to watch but a lot of the details will be
unknown until Monday morning comes and the setup is in place.

Monday night: The surface low will pass by to the north and showers
and thunderstorms will remain a possibility in the wake of the
departing convection from the evening. Likely PoP`s continue through
the night, but overall confidence isn`t very high in exactly where
the best rain chances will be. Expect a mild night with lows failing
to fall below 60s in many areas.

Tuesday: Within the progressive flow aloft another vigorous
shortwave will approach the area in the morning and pass through
later in the day. At the surface, the cold front from the previous
low will become stationary and another low will develop along it in
the afternoon hours across Upstate South Carolina. There is good
model agreement that precipitation will ramp back up along the coast
as the surface low deepens and draws closer to the coast. After a
lull in precipitation in the morning, PoP`s ramp back up to likely
for the afternoon and evening. Expect another warm day before the
cooler air arrives behind the surface low, and we could even see
some thunder along the coast and over the coastal waters late.


Low pressure will lift northeast away from the area on Wednesday,
with a secondary cold front set to drop through the area. Cool high
pressure will then build in from the northwest and prevail through
the end of the week. Some precip could linger on the backside of the
surface low early Wednesday, mainly across SC zones, with the
remainder of the period expected to be dry. Temperatures will
largely fall below normal.


KSAV: VFR conditions will prevail through most if not all of the
06Z TAF cycle, although there is a moderate potential for a few
hours of MVFR ceilings from 11-14Z Sunday morning due to the
formation of low stratus. Convective rains will be nearby late
in the period, but the majority looks to occur later on, and
will be accounted for in later TAF issuances. Offshore winds
will prevail at the terminal until around sunset Sunday, when a
cold front will shift winds to the E-NE.

KCHS: There is only a low end risk for low stratus early Sunday
morning, so VFR weather will prevail through 06Z Monday. Winds
will clock around from the W-SW to the N and eventually the E-NE
behind a cold front that moves through during Sunday.

Extended Aviation Outlook: Flight restrictions likely Monday
into Tuesday as showers/tstms associated with a cold front
impact the region.


Tonight: High pressure to the far southeast will move away as a
cold front approaches from the north. The interaction between
these two features will create an enhanced pressure gradient
across the coastal waters, leading to elevated winds and seas.
But not quite high enough for any Small Craft Advisory, as SW
and W winds average 15-20 kt with seas up to 3 or 4 ft.

Sunday through Thursday: A subtle backdoor cold front will drop
southward on Sunday and Sunday night, before lifting back northward
as a warm front on Monday. In the warm advection flow behind the
warm front on Monday, winds will become more southerly and increase
becoming up to 15-20 knots at times. Southwest flow will then
prevail on Tuesday before a surface low develops and passes offshore
Tuesday night. The flow will then turn more northwesterly Wednesday
through Thursday and increase due to pressure rises in the wake of
the departing low. Small Craft Advisories will be possible Wednesday
and Wednesday night, but should stay just shy of these thresholds
otherwise. Strong thunderstorms will likely move into the waters
late Monday and Monday night and could bring strong wind gusts and
perhaps even a waterspout.




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