Area Forecast Discussion
Issued by NWS Charleston, SC-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KCHS 280011
Area Forecast Discussion
National Weather Service Charleston SC
811 PM EDT Thu Apr 27 2017
A weak disturbance is expected to affect the area tonight, followed
by high pressure Friday into the weekend. A cold front will move
through Monday night, then a stronger storm system will affect
the area mid to late week.
.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
-- Changed Discussion --We`ve recently trimmed back the PoP`s along our NW tier, and
also adjusted sky cover grids based on recent trends. The
general forecast though does not require any significant
Satellite pictures and radar trends clearly shows the initial
large cluster of convection has dwindled, but a second cluster
is developing over central and southwest GA where cooling cloud
tops are noted. This appears to be in a region where the
increasing CINH has not yet encompassed the area, unlike it is
over our CWFA. The collision of the sea breeze, already through
all of the SC zones, and passing near and west of US-301 in GA
will occur after dark. That should limit the strength of
convection, and with a steering flow from SW to NE, the bulk of
the showers and t-storms will remain outside the local area.
Still, isolated-scattered showers and t-storms will remain in
the forecast along our extreme NW tier, through about midnight.
There is still little if any risk for severe storms.
Overnight we do find isentropic ascent and low level coastal
convergence, maybe enough to generate a few showers or t-storms
brushing Charleston County, but nil PoP`s elsewhere.
Turbulent mixing within the boundary layer will negate the need
for any mention of fog, although there is considerable moisture
beneath a well pronounced nocturnal inversion, which will lead
to a deck of stratus.
These same conditions, plus plenty of convective debris clouds
through at least the first half of the night will easily prevent
temps from dropping any lower than the mid or upper 60s, which
is about 10-12F above normal.
-- End Changed Discussion --
.SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/...
The Bermuda High will continue to expand into the forecast area
through Sunday, with onshore flow around the High maintaining low-
level moisture. This will promote a primarily rain-free but humid
pattern over the weekend. Prognosed moisture convergence near the
coast under varying levels of capping suggests a nonzero probability
for an isolated shower or two to develop dependent on the strength
of the seabreeze, but occurrence appears unlikely at this time with
the most probable outcome being a diurnal cumulus field. Under this
regime, temperatures will remain several degrees above normals, with
maximum temperatures reaching the upper 80s and lower 90s away from
the coast with minimum temperatures only in the lower 70s or upper
60s. Some uncertainty is introduced Sunday as high level clouds
ahead of a developing low pressure system appear poised to move over
the forecast area, but some model soundings indicate potential for
low-level stratus-type development early Sunday morning. Prevalent
cirrus cover would assist in mitigating this occurrence. While the
net effect on sky cover is moot, this would serve to keep maximum
temperatures a degree or two cooler than previous days.
.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
A mature surface cyclone will lift northeast into the Great
Lakes region Sunday night and Monday, pushing a cold front
through the local area Monday night. A decent channel of
atmospheric moisture ahead of the front will allow scattered to
numerous showers and thunderstorms to move through Monday
afternoon and Monday night. A deeper upper trough moving through
the central United States late week will bring a stronger low
pressure system through the area, accompanied by numerous
showers and tstms.
.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Mainly VFR, although there are indications of flight
restrictions for a few hours late tonight into Friday morning
due to a low stratus deck. South winds will gradually fade this
evening, then increase again Friday afternoon due to max heating
and enhancement from the sea breeze circulation.
Extended Aviation Outlook: Flight restrictions due to low ceilings
will be possible at either terminal Friday and Saturday nights,
possibly persisting into early the following morning. Ceiling and/or
vsby restrictions will again be possible Monday into Monday night
due to scattered showers and/or thunderstorms.
A cold front will stretch across the Appalachians this evening
and overnight. The pressure gradient will equate to southerly
winds increasing to 15-20 kt across most of the coastal waters
this evening, then easing a bit after midnight. As a result,
wave heights will build to 4-5 ft this evening before subsiding
about a foot after midnight. Some 6 footers are possible beyond
20 nm, but confidence was not high enough to issue a Small Craft
Advisory for AMZ374.
Generally onshore flow averaging 10-15 kts will persist through the
weekend, though winds nearest the coast may veer southwest or even
west-southwest each night as a land breeze develops after sunset.
Seas will generally remain 2-4 feet through the period. A cold front
will approach the region Sunday into Monday, and flow will increase
in advance of its arrival, building some 5-6 feet seas offshore as
early as Sunday, with Small Craft Advisory conditions arriving over
most of the waters Sunday night, persisting into Monday night.
Rip Currents...The combination of swell impacting the beaches, an
afternoon sea breeze, and astronomical influences will generate
a Moderate Risk of Rip Currents along southeast South Carolina
beaches Friday, with a low risk for our Georgia beaches.
Coastal Flood Advisories are in effect this evening, thanks to
previously elevated tides, moderate onshore winds and the recent
Perigean Spring Tide cycle.
Elevated tides are expected to continue through Saturday, so
more advisories will likely be needed with each evening high
GA...Coastal Flood Advisory until 11 PM EDT this evening for GAZ117-
SC...Coastal Flood Advisory until 11 PM EDT this evening for