Area Forecast Discussion
Issued by NWS Charleston, SC
FXUS62 KCHS 301134
Area Forecast Discussion
National Weather Service Charleston SC
734 AM 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.
.NEAR TERM /THROUGH TONIGHT/...
The weather will continue to be dominated by a weak closed upper
low spinning over the area and a strengthening surface trough just
off our coast. Onshore flow over a deep layer will maintain a
tropical airmass with PWs hovering around 2", likely rising a bit
this afternoon behind the sea breeze. Through daybreak, isolated
showers and tstms will move into coastal areas while better
coverage resides over the Atlantic. Once the sun comes up and
partly to mostly sunny skies prevail, 1 to 2 thousand Joules of
CAPE will develop. Scattered showers and tstms will again develop
and spread slowly inland. The best coverage is likely this
afternoon as the sea breeze interacts with existing convection and
moves inland. Given the very moist profile, the potential for
severe weather is low. High tide is around 730 pm and levels will
probably be close to Coastal Flood Advisory criteria. Any rainfall
that occurs around this time would cause additional problems but
the brunt of the convection should be pushing inland and
dissipating around this time.
Tonight, most of the activity over land will dissipate while
convection over the Atlantic waters lights up in association with
the coastal trough. This trough is expected to strengthen tonight,
with fairly strong moisture convergence resulting in good coverage
of showers and tstms mainly over the waters. However, scattered
showers and storms will likely affect mainly coastal areas through
the night, with a few showers drifting farther inland.
.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
A weak low pressure system could develop off the Southeast U.S.
coast early Wednesday 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 and track northeast across northern
Florida and then off the GA/SC coasts through Friday. Still some
uncertainty regarding this system so everyone is urged to stay
tuned. Worst case scenario would likely be a storm track a bit
closer to the coast as this would mean a higher risk for tropical
storm force winds, heavy rainfall/localized flooding, and possibly
tornadoes, especially near coastal areas. For now we increased rain
chances/amounts with coastal areas likely to see several inches
through the period. If heavy rain happens to coincide with high tide
cycles there will be a higher risk of flash flooding, especially in
downtown Charleston. High temperatures are likely to be near normal
with low temperatures a bit above normal.
.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
The latest forecast from the NHC indicates a tropical storm will be
moving off the northeast FL coast Thursday night 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. The best chance of
tropical storm force winds looks to be along the GA coast Thursday
night/early Friday morning, especially in squalls. Cooler and drier
conditions should then prevail 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 /12Z TUESDAY THROUGH SATURDAY/...
The main concern is pinning down the best chance for showers and
thunderstorms at either terminal. The deeper moisture axis is
setting up across southeast GA which is where scattered showers
are beginning to move farther inland than they did overnight.
Since showers will likely impact KSAV this morning we included
prevailing SHRA through late morning, then added vicinity TS for
the late morning into early afternoon at which point better
instability will be present. Only included vicinity SHRA at KCHS
for late morning into the afternoon since coverage looks a bit
less. Most of the activity will dissipate late this afternoon or
early evening. Low end chances for MVFR ceilings developing at
KSAV late tonight.
Extended Aviation Outlook: Restrictions from showers/thunderstorms
are possible into Wednesday at both KCHS and KSAV and into
Thursday mainly at KSAV. 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.
The coastal trough will strengthen today and tonight with
northeast to east winds increasing to as high as 15 kt over
nearshore waters and 15-20 kt over outer GA waters. The increasing
wind wave combined with increasing easterly swell energy will
likely push seas to 6 ft over the offshore GA waters this
afternoon, persisting through tonight. Therefore we issued a Small
Craft Advisory for this zone starting at noon.
Wednesday through Sunday: A weak low pressure could develop off the
Southeast U.S. coast close to the local waters and track 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 GA/SC coasts through the rest of the week as it moves
farther into the Atlantic. Tropical storm force winds are possible
across the local waters, especially the GA waters beyond 20 nm,
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 continues through today due to solid
onshore flow, wider tide cycles from the new moon, and 2-3 ft
swell every 14 seconds.
Long period swells and onshore flow are expected to persist, thus
an enhanced risk will continue through at least Thursday.
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
GA...High Risk for Rip Currents through this evening for GAZ117-119-
SC...High Risk for Rip Currents through this evening for SCZ048>051.
MARINE...Small Craft Advisory from noon today to noon EDT Wednesday for