Area Forecast Discussion
Issued by NWS Charleston, SC
FXUS62 KCHS 310525
Area Forecast Discussion
National Weather Service Charleston SC
125 AM EDT WED AUG 31 2016
Tropical Depression Nine is expected to intensify into a tropical
storm and track from the eastern Gulf of Mexico northeast across
northern Florida and off the Southeast U.S. coast through Friday.
Cooler and drier high pressure will then return from the north this
weekend and persist into early next week before another area of low
pressure possibly develops offshore.
.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Moisture convergence expected to increase overnight especially
along the GA coast ahead of a weak low pressure developing off the
northern FL moves north off the GA coast. This should lead to more
showers and some thunderstorms shifting onshore, possibly heavy at
times. Far inland areas should remain mostly dry. Lows mostly in
the lower to mid 70s inland and upper 70s at the coast.
.SHORT TERM /6 AM THIS MORNING THROUGH FRIDAY/...
A weak low pressure system off the Southeast U.S. coast will
deteriorate as it moves off toward the northeast through the day on
Wednesday. The strongest destabilization and forcing in the area of
the low will mainly remain offshore, though its proximity to the
coast will keep the potential for showers and thunderstorms in the
forecast for mainly Charleston County and the immediate coast of
Southeast South Carolina and Northeast Georgia. The highest moisture
values will remain off the coast with the low Wednesday, so the
flooding threat appears low other than low-lying areas at high tide.
Moving into Thursday and Friday, the primary challenge is nailing
down Tropical Depression 9`s impacts on the area. 12Z GFS and EURO
deterministic runs, as well as the majority of the GFS ensemble
runs, have trended the storm further north/closer to the coast as it
moves back into Atlantic waters Friday. Have increased QPF across
the area to account for this, with storm total rainfall amounts near
the coast now in the 4-6 inch range. While wind impacts from this
storm remain uncertain due to significant timing and strength
differences between models, there is growing confidence in flooding
being a threat with this storm (WPC has the immediate coast under a
marginal risk for excessive rainfall Thursday), especially if heavy
rainfall occurs around high tide in Downtown Charleston. Please
continue to monitor the latest forecasts as the situation will
become more clear in the coming days.
Northerly low and mid level flow will fill in as early as Friday
afternoon as the storm moves toward Cape Hatteras, bringing cooler
and drier conditions to the area.
.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
The latest forecast from the NHC indicates a tropical storm will be
moving off the northeast FL coast overnight Thursday and then
staying offshore as it moves away from the GA/SC coasts through late
week. This will be followed by high pressure building from the north
this weekend into early next week. There is still quite a bit of
uncertainty regarding the track/strength of the tropical cyclone so
it is best to stay tuned to the latest forecast. Cooler and drier
conditions should then prevail for the weekend into Monday, although
a few showers can`t be ruled out from time to time mainly near the
coast. By Tuesday another area of low pressure could develop
offshore and head toward the coast.
.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
Moderate confidence mainly through this afternoon as much depends
on shower/thunderstorm coverage associated with weak low pressure
off the FL coast tracking northward near the GA and SC coasts
later today. Think the best chances for rain will be after 09Z at
KSAV and after 12Z at KCHS and last through much of the
afternoon. Prefer to maintain VFR conditions for now due to the
Extended Aviation Outlook: Ceiling and visibility restrictions are
possible Wednesday and Wednesday night at both KCHS and KSAV as
showers and thunderstorms mainly offshore may approach the
terminals. Additional impacts from showers/thunderstorms and winds
are likely Thursday night into Friday, depending on the track of
Tropical Depression 9 which is forecast by the NHC to be a tropical
storm as it passes northeast off the GA/SC coasts.
Rest of tonight: Main concern is across the GA waters beyond 20 nm
where winds are gusting to 25 kt and thus we will maintain the Small
Craft Advisory there. Seas likely to build to 7 feet close to 60
Wednesday through Sunday: Weak low pressure off the Southeast coast
will bring easterly winds in the 10 to 15 kt range, with mainly 3 to
5 foot seas Wednesday and Wednesday evening. Conditions will
deteriorate rapidly early Thursday mornings as a tropical storm is
forecast by the NHC to move off the GA/SC coasts. Tropical Storm
force winds and seas as high as 5 to 10 feet are possible across the
local waters beginning Thursday night and lasting into Friday. There
is still some uncertainty in the strength/track of the tropical
cyclone so mariners should stay tuned to the latest forecasts.
Rip currents: High risk of rip currents continues for Wednesday
due to solid groundswell from Gaston, persistent onshore winds,
and high tidal ranges.
Long period swells and onshore flow are expected to persist, thus an
enhanced risk will continue through at least Friday.
Guidance still shows tides to remain elevated due to ongoing
east/northeast winds, long period swell and influences from the
approaching new moon through much of the work week. The
extratropical surge guidance suggests evening high tides through
Thursday will be very close to levels which would produce shallow
coastal flooding, especially along the lower South Carolina coast.
More Coastal Flood Advisories may eventually be needed.
GA...High Risk for Rip Currents through this afternoon for GAZ117-
SC...High Risk for Rip Currents through this afternoon for
MARINE...Small Craft Advisory until 6 PM EDT this evening for AMZ374.