Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 201502

Area Forecast Discussion
National Weather Service Charleston SC
1102 AM EDT Tue Mar 20 2018

A strong cold front will approach from the west today, then
cross the area this afternoon. Cool high pressure will build in
for the latter half of the week. Another cold front could impact
the area late next weekend.


Today: An "Enhanced Risk" of severe weather is forecast across parts
of Southeast South Carolina and Southeast Georgia, especially along
the coastal corridor.

Convection experienced from an ongoing MCS late last night/early
this morning has shifted offshore and dissipated. Questions remain
as to how well the boundary layer destabilizes over the area in wake
of this activity, especially as cloud cover begins to develop
late morning. However, the general consensus among models is
that low-lvl warm advection along with sfc heating experienced
late this morning and early afternoon along with cooler air
arriving with a mid/upper lvl trough currently depicted on water
vapor to the west, will allow the boundary layer to recover in
time for additional thunderstorms this afternoon, some of which
could be strong/severe along and in advance of a sfc cold front
approaching from the west.

Given strong unidirectional wind fields associated with the passing
system, the main severe weather threat should come in the form of
damaging straight-line winds. Mid-lvl lapse rates around 7-7.5 C/km,
lifted index values upwards to -6/-7 C, along with 0-6 km bulk shear
of 50-60 kt could also produce small-moderate size hail in deepest
convection and perhaps splitting thunderstorms. Although low-lvl
lapse rates and veering are not particularly strong over the area,
the orientation of convection seen earlier this morning suggests an
east-west outflow boundary capable of being ingested by storms
shifting over Southeast Georgia and Southern Southeast South
Carolina. For this reason along with a fairly large amount of 0-3km
CAPE and low LCLs, an isolated tornado or two is possible, mainly
in Southeast Georgia and perhaps even more so along coastal
Georgia where a seabreeze circulation develops and instability
is greatest. Elsewhere, dewpts in the mid/upper 60s, PWATs
between 1.25-1.5 inches and SBCAPE approaching 1500-2500 J/kg
could support strong/severe thunderstorms across the area, with
a better chance along coastal regions as temps warm into the
upper 70s to around 80 degrees while strong dynamical forcing
occurs with the approaching h5 shortwave and sfc cold front. At
this time, the suggested development/arrival time of stronger
thunderstorms and/or severe potential should be between noon to
5 PM in Southeast Georgia and 2 to 6 PM in Southeast South
Carolina. However, developing cloud cover over the region could
delay the start-up time of stronger convection early this afternoon.

Tonight: There is a consolidation of the cold fronts into one
front that moves into the Atlantic this evening, followed by
drier and cooler air advecting into the area. This will lead to
a much cooler night, and with large pressure climbs and a packed
gradient with the CAA to occur, gusty W/NW breezes will persist
the entire night. It`ll be enough where we have raised a Lake
Wind Advisory for Lake Moultrie overnight.


Massive low pressure system centered off the mid-Atlantic coast will
begin to lift northeast away from the area on Wednesday. Deep
westerly flow will usher much drier air into the area. Skies will
remain mostly clear across most locations, with the exception being
over extreme northern areas in closer proximity to the departing
low, where some clouds may linger longer. The biggest concern of the
day will the winds. A tight pressure gradient will result in breezy
west winds through the day. Gusts of 30-35 mph will be common,
especially across southeast Georgia. Conditions will also remain
hazardous over Lake Moultrie, and a Lake Wind Advisory will be
ongoing through 22Z. Temperatures will be drastically cooler than
previous days, with highs well below normal in the upper 50s to low
60s. Wednesday night lows will range from the mid 30s inland to the
low 40s at the immediate coast. Think winds will remain too strong
to support any frost development.

High pressure will build in from the northwest Thursday and Friday.
Aloft, an amplified longwave trough will remain situated just off
the east coast. Quiet conditions are expected with mostly clear
skies. Temperatures will continue to fall below normal with highs
topping out in the 60s. Lows Thursday night will again fall into the
mid 30s inland, with upper 30s closer to the coast. Winds do lighten
up some, so think there is some potential for frost.


High pressure is expected to prevail through much of the weekend.
There are some model discrepancies on Sunday, but general consensus
is that a cold front will approach from the north and will then drop
through the forecast area Sunday night. A wedge-like setup looks to
develop on Monday. Rain chances will accompany the front, although
it looks that rainfall totals will not amount to too much.
Temperatures will be near normal.


VFR conditions will likely prevail at both CHS and SAV for the next
couple hours. However, MVFR/IFR conditions are possible as another
round of convection develops this afternoon along outflow boundaries
and the sea breeze in advance of a cold front. Numerous SHRA/TSRA,
which will likely impact both KCHS and KSAV. For now we have VCTS
and CB clouds with VFR ceilings from 18-23Z. But MVFR/IFR conditions
are likely from any direct impact, along with strong and potentially
damaging winds, heavy rains and frequent lightning. As the cold
front moves offshore this evening, drier and more stable air will

Extended Aviation Outlook: VFR. Breezy conditions expected on


Today and Tonight: Hazardous marine conditions will develop over all
coastal waters this afternoon as low pressure passes through the
Southeast and Mid-Atlantic region, followed by a trailing cold front
that moves through this evening. Warm air advection will limit how
much mixing early today, but some mixing into strong low-lvl wind
should support Small Craft Advisories starting this afternoon on all
waters. Tonight, we have colder air arriving in tandem with a packed
gradient and large and steady isallobaric pressure climbs. This will
lead to likely Gales developing, and we have hoisted a Gale Watch
for all but Charleston Harbor. Although they too could see Gales.

In addition, strong to severe t-storms will occur early this
morning on the SC waters, plus again this afternoon and maybe
early evening all waters, until the cold front moves through.
Mariners should be prepared for damaging wind gusts, isolated
waterspouts, frequent lightning and heavy rains.

Wednesday through Sunday: Strong pressure rises in wake of a
departing low will result in hazardous marine conditions. West winds
could gust to gale force through much of the day, and thus a Gale
Watch has been issued for all waters with the exception of the
Charleston Harbor. Confidence is fairly high that the watch will be
upgraded to a warning for most zones with future forecast updates.
Winds and seas will greatly improve Wednesday night, with all waters
expected to be headline-free Thursday morning. Northwest winds will
continue Thursday and Friday, before veering to south and eventually
southwest on Sunday ahead of an approaching cold front.


SC...Lake Wind Advisory from 2 AM to 6 PM EDT Wednesday for SCZ045.
MARINE...Gale Watch from late tonight through Wednesday afternoon for
     Small Craft Advisory from 4 PM this afternoon to 2 AM EDT
     Wednesday for AMZ352-354.
     Small Craft Advisory until 2 AM EDT Wednesday for AMZ350.
     Gale Watch from 10 PM EDT this evening through Wednesday
     evening for AMZ374.
     Small Craft Advisory until 10 PM EDT this evening for AMZ374.
     Small Craft Advisory until 8 PM EDT Wednesday for AMZ330.


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