Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 300839
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
439 AM EDT Thu Mar 30 2017

.SYNOPSIS...
A stationary front across the region will lift north as a warm front
through tonight. A cold front will move through Friday night
followed by dry high pressure into Sunday. Unsettled weather should
return again next Monday, possibly lasting into Tuesday. Dry weather
should then return through Wednesday before rain chances return
Thursday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Pre-dawn: A band of low stratus clouds has been advecting inland
and slowly southward overnight with low clouds and patchy fog
recently reaching the Savannah area. A stationary front looks to
be across south GA early this morning with a weak mid level
short wave moving ne over the southern half of GA. Moisture is
too shallow for any pre-dawn rainfall. It is still uncertain if
the stratus will build down along and south of the Savannah
River enough to produce any dense fog and we maintained fog
mentions GA zones. Temps and RH were very mild and moist
respectively, unlikely to change much over the next few hours.

Today: The large scale synoptic features remain on track with
deeper layered ridging giving way to backing upper flow and
modest mid level height falls throughout the day. Low level flow
will gradually veer from east to southeast, while Atlantic
moisture slowly builds with time. The stalled front should
gradually lift north through South Carolina later today and
north of the forecast area tonight. The details in the forecast
revolve around a very complex mesoscale environment with many
moisture and instability gradients noted on model depictions
through the day, enhanced by regions of differential heating to
begin the day.

There will be increasing low level moisture convergence and
isentropic ascent along the SC coast, and recently models
show an increasing chance for scattered showers along coastal
areas as the sea breeze begins to develop by this afternoon.
The most interesting forecast region of concern this afternoon
and evening, is to the west of Interstate 95 where models
show fairly strong atmospheric destabilization. Deep layered
wind fields exhibit a significant uptick across east central GA
and the CSRA around 21Z and this coincides with broad synoptic
moisture convergence and upper difluence between complex jet
segments. High resolution models develop scattered deep
convection from the midlands to inland southeast GA this
afternoon. A few of these storms could be strong with isolated
strong/severe tstms possible as far east as our far nw tier of
zones from Metter to Allendale. Limiting factors include weak
mid level lapse rates and a lack of a significant mid level
feature during peak heating. At this time, we do not think this
convection will organize, but it will need to be watched closely
giving some model energy helicity indices in the 1 to 1.5
range.

Along with the increasing chances for convective rains today,
high temps will be quite warm once again, upper 70s far north
to at least the mid 80s south. Breezier conditions will be
limited to coastal SC this afternoon as the onshore flow is
enhanced by the sea breeze.

Tonight: Wind fields will continue to build above the surface
inversions with the flow becoming southerly overnight in the
wake of the warm front passage. Scattered showers and a few
thunderstorms overnight will tend to increase from the west
late with the approach of a strong upper wave which is expected
to both weaken a bit and slightly tilt negatively as it moves
through the lower Gulf Coast region. Overnight, the potential
for organized convection appears best to our west. Severe
potential will be marginal at best until late given the forecast
shear but weak elevated instability. We raised temps tonight,
in the mid to upper 60s most areas.

&&

.SHORT TERM /FRIDAY THROUGH SUNDAY/...
A cold front will approach Friday and then push through later Friday
and Friday night. Showers and some storms are expected Friday
morning, especially inland and across SC, as a shortwave moves
through. Conditions will then improve as deeper moisture shifts
offshore. Although wind fields will be strong Friday, instability
appears minimal and thus the threat for any severe storms is quite
low, mainly confined to SC. Either way it should be a breezy day
with gusts up to around 25-30 mph. Dry weather is on tap through the
weekend as high pressure builds in.

Despite the slight cool down behind the front for this weekend,
temperatures should remain above normal through the period.

&&

.LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
Dry high pressure to prevail through Sunday night although moisture
will be increasing Monday morning as low-level jetting ramps up
ahead of approaching low pressure from the west. Looks like a good
shot of showers and storms Monday and Monday night as low pressure
passes by to the northwest of the area. Could be another high
shear/low CAPE situation and a few severe storms might be possible.
Things should clear out by Tuesday but the ECMWF still indicates a
few showers possible, mainly across SC. High pressure will then move
in through Wednesday before another storm system likely moves across
the Deep South and brings rain back to the area Wednesday night into
Thursday. Temperatures should remain above normal through the
period.

&&

.AVIATION /08Z THURSDAY THROUGH MONDAY/...
KCHS: Low level winds were increasing and the stratus had lifted
to stratocumulus and MVFR should be persistent overnight. Fog
potential is unlikely given the latest trends and winds in the
forecast soundings. There is some uncertainty how quickly cigs
will rise above MVFR levels Thursday morning, but data generally
support a 14-15z timeframe. Convective rains are certainly later
in the day but until then, only shallow showers are possible.

KSAV: The primary concern is low-stratus and fog. Low-level
moisture will steadily increase from off the Atlantic tonight as
a stationary front lingers near the Altamaha River. The terminal
looks to become embedded within the corridor of highest surface
dewpoints so expect extensive low-stratus and possibly some fog
(stratus build down) to occur. Will go ahead and show prevailing
IFR cigs right at alternate minimums and MVFR vsbys 07-14z with
a tempo group 08-12z with LIFR cigs and IFR vsbys. There is a
potential for lower conditions, including conditions below
airfield minimums, but it still appears the boundary layer
winds will be too strong to support this. This will be monitored
carefully and amendments issued as needed. Similar to KCHS,
there is some uncertainty how quickly cigs will rise above MVFR
levels Thursday morning, but data generally support a 14-15z
timeframe.

EXTENDED AVIATION OUTLOOK: Restrictions likely due to
showers/thunderstorms/low clouds Friday morning and again Monday
and Monday night. Breezy conditions are also expected Friday.


&&

.MARINE...
Today: High pressure centered well to the north will move east
and maintain an onshore flow over the waters, strongest over
Charleston waters where the gradients remain tighter. The flow
should begin to veer more to the se later today while seas range
from 3-5 FT north to 2-4 FT south. The veering flow will remain
modest tonight a in warm air advection regime, increasing to
15-20 KT and 15 KT near shore GA waters. Seas should continue to
respond and SCA conditions may develop over Charleston Waters
by this evening or perhaps later tonight as seas touch 6 FT just
beyond the pilot buoy. SCA conditions are also possible over
outer GA waters where seas build to 6 FT out near the Gulf
Stream.

Friday through Tuesday: A cold front will move through Friday
night/early Saturday. Advisory conditions likely Friday from the
outer GA waters northward into the SC waters into Friday night
mainly for seas building to 6-7 feet, highest toward the Gulf
Stream. Cannot rule out some sea fog over the near shore waters
until the cold front passes later Friday/Friday night. Conditions
will go downhill again Monday as a storm system approaches from the
west and Advisories will be possible Monday into Tuesday, at least
for the offshore GA waters and SC waters.

Rip Currents: Moderate risk for rip currents along the SC coast
today. Onshore winds along with a 2-3 foot long period swell
may produce rip currents. The enhanced risk for rip currents
could persist into Friday.

&&

.TIDES/COASTAL FLOODING...
Onshore winds combined with a long period swell impacting the
coast will cause saltwater inundation, especially in vulnerable
coastal areas. The high tides this morning could reach advisory
levels, especially in SC. This evening`s high tide could
approach warning levels in CHS county, but moderate to high-end
shallow coastal flooding is more probable. There could be some
water on Highway 80 in Chatham County late this evening with
a good bit of flooding on roads in Downtown Charleston between
10 PM and Midnight. Coastal Flood headlines are expected with
each of these tide cycles today.

&&

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

&&

$$
NEAR TERM...
SHORT TERM...RJB
LONG TERM...RJB
AVIATION...RJB
MARINE...RJB
TIDES/COASTAL FLOODING...



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