Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 221956
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
356 PM EDT Mon May 22 2017

.SYNOPSIS...
A series of disturbances will track across the Southeast through
midweek. A cold front will push offshore Wednesday night into
Thursday. High pressure will then prevail this weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Numerous showers and tstms have developed this afternoon across
portions of southern SC and GA, mostly propagating NNE. PWs of
1.8 to 1.9" supporting locally heavy rainfall with the
convection. The seabreeze has begun to develop based on coastal
wind directions and the erosion of cumulus right at the coast.
This, plus the concentration of upper vorticity energy over
inland areas will tend to push the greatest convection coverage
away from the coast and toward inland areas. Thus we have the
likely pops over inland areas through early this evening.

A lull is expected from mid to late evening before speed
convergence in low-level jetting over the waters induces new
shower and tstm development offshore. Additional showers and
tstms should develop later tonight across inland areas as
greater PVA shifts over the area ahead of more shortwave
activity. Instability will remain fairly weak so severe wx is
not much of a concern, though localized flooding is possible
given the high PWs.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
On Tuesday, a cold front is expected to run across the Piedmont
of the Carolinas and GA. Short range guidance indicates that a
front wave will develop along the cold front across GA or SC.
The front wave is forecast to track NE along the front during
the daylight hours on Tuesday. Model forecast soundings suggest
that LFC levels will remain below 2 kft with PW values around
1.9 inches. In addition, a wide field of 1500-2000 J/kg CAPE
will develop along and east of the front/wave. Convection should
easily form near the frontal wave, supporting categorical PoPs
for showers and thunderstorms. Shear between 0-6 km will likely
range around 40 kts, yielding loosely organized clusters or
bands of thunderstorms. It is possible to see a few damaging
wind events with the stronger storms. Perhaps more likely,
rainfall rates may become quite heavy during the passage of
thunderstorm clusters, see tide section below. High temperatures
should verify within a degree or two of 80 degrees on Tuesday.
Based on the timing of the frontal wave, coverage of deep
convection should shift NE during the evening hours. It appears
that a noteworthy decrease in coverage of showers and
thunderstorms will occur across forecast area Tuesday night,
highlight with PoPs decreasing to CHC. Lows Tuesday night are
forecast to range in the upper 60s to low 70s.

Wednesday, deep sfc low pressure should organize across the Ohio
River Valley. Short range guidance indicates that a cold front
will approach from the west during the daylight hours. At the
mid level, H5 trough will swing across the deep south during the
afternoon, GFS1deg shows a wide field of H5 Q-vector convergence
will spread west to east across the CWA. The environment ahead
of the cold front is forecast to see CAPE values pool to 1500
J/kg with SWEAT peaking near 400. I would expect that a band of
deep convection will form along and ahead of the cold front.
Given the synoptic scale forcing, instability, and shear, the
severe risk could peak Wed afternoon and evening. High temps
should range very close to values expected for Tuesday. Dry air
will spread east across the region Wednesday night.

Thursday, large low pressure will circulate across the Mid West
and Great Lakes regions as high pressure builds over the Gulf
of Mexico. The sfc pressure gradient is forecast to range from
2-3 mbs across the forecast area. WSW winds should remain gusty
across forecast area, especially during the morning.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
A ridge building across the East Coast combined with surface high
pressure will provide dry weather in the long term along with a
gradual warming trend each day.

&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...
The greatest chance for convection will gradually shift west of
the terminals as the sea breeze pushes inland. However, scattered
coverage across southeast GA may affect KSAV before 6 pm.
Scattered showers possible overnight at KCHS as some activity
develops ahead of another shortwave.

Another round of convection expected during the day Tuesday but
with the best coverage inland we did not mention convection at
either terminal.

Extended Aviation Outlook: An unsettled weather pattern will bring
periods of flight restrictions through Wednesday night.

&&

.MARINE...
Low-level jet strengthens later tonight as shortwave energy
approaches the area. Sustained winds will increase to 15-20 kt
later tonight with some gusts approaching 25 kt over outer
portions.

Tuesday through Wednesday night: Weak low pressure will track NE
along a sfc front across during the daylight hours Tuesday. SW winds
across the marine zones will increase through day. Gusts across
AMZ350 and 374 are forecast to exceed 25 kts by the afternoon. Wave
heights are forecast to build to 3-4 feet within 20 nm and around 5
feet across AMZ374. Small Craft Advisories may be needed for
Tuesday. In addition, rounds of thunderstorms are forecast across
the marine zones on Tuesday. Winds may decrease slight Tuesday
night, however, wave heights are forecast to gradually build through
the night.

Wednesday through Wednesday night, SW winds will increase as a cold
front approaches from the west. SCA conditions are expected. Marine
zones should see common gusts to 25 kts within 20 nm and around 30
kts beyond 20 nm. Wave heights are forecast to peak Wednesday night,
with 6 feet possible within 20 nm and up to 9 feet across eastern
AMZ374. Then, winds/seas should relax significantly by Friday.

&&

.TIDES/COASTAL FLOODING...
Evening high tide levels will trend higher each day approach
the new Moon perigee this Friday. The Mean Lower Low Water
(MLLW) levels are forecast to remain below flood stage. However,
it is possible that thunderstorms could pass over urban coast
areas during the evening high tide cycle. Rainfall rates could
exceed 0.75/hr during any thunderstorm. The combination of
elevated MLLW water levels and heavy downpours could result in
deep street and sidewalk water.

&&

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

&&

$$
NEAR TERM...JRL
SHORT TERM...NED
LONG TERM...MS
AVIATION...JRL/NED
MARINE...JRL/NED
TIDES/COASTAL FLOODING...NED



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