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Area Forecast Discussion

FXUS62 KCHS 310023

Area Forecast Discussion
National Weather Service Charleston SC
823 PM EDT TUE AUG 30 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.


The latest radar indicates that most of the convective activity
has ended over the land and only isolated showers and
thunderstorms currently going over the Atlantic waters. Models
continue to show precip chances increasing overnight over the
coastal waters within a coastal trough of low pressure. These
showers may spread into the coastal counties later tonight and
into the pre-dawn hours. Have maintained the highest pops over the
waters with chance pops over the coastal counties, mainly after
midnight. Lows in the lower 70s most areas.


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.


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.


Tonight: Expect VFR to prevail much of the night. Increasing
chances for showers and isolated thunderstorms after 06z...but
have kept VFR through the night for now. Mesoscale models indicate better
chances for showers and some thunderstorms closer to KSAV

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.


Tonight: Have made only cosmetic changes to the forecast. Still
expect Small Craft Advisory conditions over the offshore GA waters
to continue, mainly for seas of 5 to 7 feet. Wind speeds expected
to increase later tonight, especially south of the Savannah River,
where the pressure gradient looks a little stronger on the
west/northwest side of the coastal trough.

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: Numerous rip currents were reported at area beaches
today. 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.


The latest tide observations for Charleston Harbor indicate that
the water levels will stay just below Coastal Flood Advisory
levels this evening. Have decided to go ahead and cancel the
Coastal Flood Advisory for this evening.

Still expect 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 Wednesday afternoon for
SC...High Risk for Rip Currents through Wednesday afternoon for
MARINE...Small Craft Advisory until noon EDT Wednesday for



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