Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Charleston SC
737 am EDT Monday Aug 29 2016

Synopsis...
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 late in the week.
Cooler and drier high pressure will then return from the north this
weekend.

&&

Near term /through tonight/...
deep layered east to NE flow will persist today due to a closed upper
low drifting west and an associated surface trough lingering over
our coastal waters. As of Sunday evening's sounding, the
precipitable water value shot up above 2 inches after several days
closer to 1.3 inches. Through daybreak only isolated showers will
continue over the coastal waters with a few moving onto the
beaches before dissipating. After sunrise we should see increasing
coverage of showers, with some tstms possible in the afternoon.
Given the continued upper height falls, deep moisture, increasing
low-level instability and a decent afternoon sea breeze, we are
carrying 30-40% rain chances in most locations. High temps will
reach the low to mid 90s inland while coastal locations top out in
the upper 80s.

Tonight, most showers or tstms over land areas will diminish after
the sun sets. Then, showers will become more numerous over the
coastal waters, especially the Georgia waters as the coastal trough
strengthens under the lingering upper low. Some showers may move
onshore especially late tonight but the brunt of the convection
will remain over the ocean.

&&

Short term /Tuesday through Thursday/...
weak low pressure system could develop off the southeast U.S. Coast
Tuesday night before tracking northeast into Thursday. However, the
main concern as we head toward the end of the period will be
Tropical Depression Nine which is forecast by the NHC to intensify
into a tropical storm in the eastern Gulf of Mexico Tuesday and then
track northeast across northern Florida. Still some uncertainty
regarding this system but confidence is pretty high that there will
be minimal impacts through this period. Otherwise, general troughing
will persist with the deepest tropical moisture likely to remain
south and east of the area. Thus, rain chances should not be much
more than typical for this time of year, which is about 30 percent.
Also, rain amounts will generally be limited to about an inch or
less most places through the period. Temperatures are likely to be
slightly above normal, mostly lower 90s for highs and mid 70s for
lows.

&&

Long term /Thursday night through Sunday/...
the latest forecast from the NHC indicates a tropical storm will be
moving off the northeast Florida coast Thursday night and then staying
offshore as it moves away from the Georgia/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
although the forecast does not show poor conditions at this time
that could change. The best chance of tropical storm force winds
looks to be along the Georgia coast Friday morning, especially in
squalls. Cooler and drier conditions should then prevail into
Monday, although a few showers can't be ruled out mainly near the
coast.

&&

Aviation /12z Monday through Friday/...
occasional IFR ceilings through mid-morning, lifting to a period
of MVFR before going VFR by midday. Otherwise we expect scattered
showers and tstms to develop and shift inland through the day,
especially in the afternoon. Maintained vicinity showers at both
sites though cannot rule out a thunderstorm at some point. The
brunt of the activity will taper off around sunset. The likelihood
of low ceilings later tonight looks fairly low at this point since
there will probably be more mid and high clouds in place.

Extended aviation outlook: no significant concerns, although much
depends on the track/strength of Tropical Depression Nine late
this week.

&&

Marine...
a weak coastal trough will linger through tonight with NE to east
winds generally 15 kt or less. Winds will strengthen to close to
15 kt along the coast this afternoon with the sea breeze. The
combination of several swell trains plus additional wind wave will
yield 3-5 ft seas during the period.

Tuesday through saturday: a weak tropical cyclone is forecast by the
NHC to be near the NC Outer Banks into while a weak low pressure
possibly develops off the southeast U.S. Coast Tuesday night and
tracks to the northeast into Thursday. Then, a tropical storm is
forecast by the NHC to move northeast across northern Florida
Thursday and then well off the Georgia/SC coasts through the rest of the
week as it moves farther into the Atlantic. Tropical storm force
winds are possible in the Georgia waters, especially beyond 20 nm,
beginning Thursday night. Farther north across the SC nearshore
waters only advisory conditions are anticipated. There is still some
uncertainty in this forecast so stay tuned.

Rip currents: since we had rip current reports both Saturday and
Sunday and the swell height/period continues to increase, a high
risk seems warranted again today. An enhanced rip current risk
will linger through much of the week due to continued long-period
swells.

&&

Tides/coastal flooding...
tides remain elevated due to ongoing east/northeast winds, long
period swell energy and influences from the approaching new moon.
The latest extratropical surge guidance suggests afternoon high
tides through mid-week will be very close to levels which will
produce shallow coastal flooding, especially along the lower South
Carolina coast. Coastal flood advisories may eventually be
needed.

&&

Chs watches/warnings/advisories...
Georgia...high risk for rip currents through this evening for gaz117-119-
139-141.
SC...high risk for rip currents through this evening for scz048>051.
Marine...none.

&&

$$
Near term...jrl
short term...rjb
long term...rjb
aviation...jrl/rjb
marine...jrl/rjb
tides/coastal flooding...

National Weather Service Glossary of Abbreviations