Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 291956
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
356 PM EDT MON AUG 29 2016

.SYNOPSIS...
Tropical Depression 9 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. Refer to the latest advisories on Tropical
Depression 9 from the National Hurricane Center.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
This Evening and Tonight: A weak mid/upper lvl low currently
centered along the coast of Southeast South Carolina and Southeast
Georgia will slowly drift west over land this evening and tonight,
helping advect deep-layered moisture onshore. Moisture and heating
will continue to fuel scattered showers and/or thunderstorms into
late evening hours for most areas. Although severe weather is not
anticipated, we could see a stronger thunderstorm or two before
diurnal heating is lost this evening. Strong wind gusts and brief
heavy rain will be the main concern. Much like last night, most
shower and thunderstorm activity will diminish over land within an
hour or two of sunset, but we could see a few showers through the
overnight period along coastal locations. Greatest precip chances
should return late along coastal Georgia where a northeast flow
advects moisture onshore. Outside of showers, lows will remain mild
under some clouds associated with the western side of the mid/upper
low. In general, low temps should range in the low to mid 70s away
from the coast.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
Tuesday and Wednesday: Wet, unsettled weather is expected during
this time as a pronounced north-south oriented moisture channel
with origins off the Florida East Coast becomes directed into
Southeast South Carolina and Southeast Georgia. Guidance shows the
strongest 310-320K isentropic assent becoming aligned along the
Georgia coast by late Tuesday morning as the moisture channel
interacts with a weak coastal trough lurking just offshore then
spreads up the coast Tuesday afternoon into Wednesday. Pops have
been trended higher into the 50-70% category Tuesday afternoon
into Wednesday with the highest gridded pops confined to the
coastal counties. Refer to the hydrology section below for an
assessment of the flood risk. Extensive cloud cover and rain will
limit highs to around 90 or so inland with mid-upper 80s at the
coast. The record span of consecutive days of highs 90 degrees or
warmer at the Savannah/Hilton Head airport may finally come of an
end over the next few days. Lows will range from the mid 70s
inland to around 80 at the beaches.

Thursday: Thursday will be a transition day as influences from
Tropical Depression 9 (likely a tropical storm by this time)
begin to increase as it approaches northern Florida. The risk for
wet weather will continue with the greater rain chances shifting
from the coast to the far southern areas. Pops will range from
30-40% north to 50-60% south with highs lower 90s inland to the
mid 80s coast. Breezy conditions could develop along the Georgia
beaches, especially coastal portions of Liberty and McIntosh
counties.

&&

.LONG TERM /THURSDAY 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
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 GA coast Friday morning, especially in
squalls. Cooler and drier conditions should then prevail into
Monday, although a few showers ca not be ruled out mainly near
the coast.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
Brief MVFR cigs/vsbys will be possible at the CHS terminal due to
showers/thunderstorms through 22z. A few showers or thunderstorms
could impact either terminal into the evening, but the overall
trend supports prevailing VFR conditions late this evening
through 18z Tuesday. The likelihood of low ceilings tonight looks
fairly low at this point since there will probably be more mid and
high clouds in place.

Extended Aviation Outlook: Impacts from showers/tstms are possible
Tuesday into Wednesday at both KCHS and KSAV and into Thursday at
KSAV. Additional impacts are possible Friday depending on the
track of Tropical Depression 9.

&&

.MARINE...
This Evening and Tonight: A weak coastal trough will linger over the
coastal waters, supporting northeast to east winds no higher than 15
kt. Winds will likely be highest in Georgia waters where the
pressure gradient and 1000mb geostrophic flow is slightly
enhanced/stronger. Although conditions are expected to remain below
Small Craft Advisory levels through tonight, long-period swell will
build seas up to 3-5 ft, highest beyond 20 nm from the coast.

Tuesday: Solid east to northeast winds will persist as a weak
coastal trough remains anchored offshore. Winds will remain
elevated within the pinched gradient regime, averaging 15 kt or so
Charleston County waters and as high as 15-20 kt elsewhere.
Seas will build 3-4 ft nearshore waters and as high as 4-6 ft
offshore waters. Latest NWPS shows 6 ft seas reaching the Georgia
offshore waters by late Tuesday afternoon (2nd period) and
continuing through early Wednesday. WW3 output does not snow 6 ft
seas arriving until later Tuesday Night (3rd period), so given the
timing differences noted, this issuance of a Small Craft Advisory
for Hazardous will be deferred until after the 17/00z guidance
can be reviewed.

Wednesday: Per coordination chats with WPC, the GFS`s depiction
of weak cyclogenesis off the South Carolina coast is likely
overdone and will be discounted. This will keep an east/northeast
wind regime in tact with an enhanced pressure gradient holding in
place. Winds look to max out at 15-20 kt with seas 2-4 ft
nearshore waters and subsiding at bit to 4-5 ft offshore waters.

Thursday through Saturday: This period will primary dominated by
the timing, strength and location of Tropical Depression 9. The
latest NHC forecast shows the TD strengthening to a tropical
storm later today and moving north across the eastern Gulf of
Mexico over the next few days before emerging off the Southeast
U.S. coast Thursday Night/Friday Morning. At this time, the
Georgia coastal waters is the area mostly likely to see tropical
storm conditions with solid Small Craft Advisory conditions
elsewhere in the tight pressure gradient that develops between
between the tropical cyclone and inland high pressure. There
remains much uncertainty with TD 9 and further changes, possibly
significant, may be needed over the coming days. Generally used a
blend of the WW3 and NWPS to derive wave heights.

Rip Currents: 14-15 second swell have been impacting the beaches
today. Expect similar conditions into Tuesday with a slight
dampening of periods as swells approach the coast. This
necessitate extending the high risk for rip currents into Tuesday
evening.

&&

.HYDROLOGY...
There will be risk for locally heavy rains Tuesday into Thursday
as well defined moisture channel that originates off the Florida
east coast interacts with a coastal trough lurking just off the
coast. The strongest moisture transport is directed right into
coastal Southeast South Carolina and Southeast Georgia and this is
the area that will have the highest risk for flooding, especially
if heavy rains fall during already elevated high tides. The
overall flood risk is not high enough to justify a Flash Flood
Watch at this time, but trends are being monitored.

There is also a risk for heavy rains to develop along the coast
Friday as Tropical Depression 9 makes its closest approach. Again,
this will be highly dependent on its eventual track and
intensity.

&&

.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...
GA...High Risk for Rip Currents through Tuesday evening for GAZ117-
     119-139-141.
SC...High Risk for Rip Currents through Tuesday evening for
     SCZ048>051.
MARINE...None.

&&

$$
SYNOPSIS...
NEAR TERM...DPB
SHORT TERM...ST
LONG TERM...ST
AVIATION...DPB/ST
MARINE...DPB/ST
HYDROLOGY...
TIDES/COASTAL FLOODING...



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