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Area Forecast Discussion

339
FXUS62 KCHS 290018
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
818 PM EDT SUN AUG 28 2016

.SYNOPSIS...
High pressure centered to the north will prevail into Monday. Then
through mid week, Tropical Depression Nine is expected to track
northward through the Gulf of Mexico. By late week, the system is
expected to move across Florida and off the Southeast coast as a
Tropical Storm.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
This evening: Showers and thunderstorms have diminished
considerably in coverage within the prevailing northeast flow. Any
lingering activity has become concentrated across the southeast
Georgia coast. This activity seems to be mainly supported by a
weak upper level disturbance that is situated near the lower
Georgia coast. With this disturbance around, and other noted
upstream, we will likely see isolated showers and thunderstorms
continue to pop up through the night mainly along the coast and
over the coastal waters. Models also show some subtle low level
convergence along the coast where inland flow will be more
northeasterly and flow over the waters will be more east-
northeasterly. This could account for the coastal redevelopment of
showers and storms that some of the models show late tonight. The
only real impact will be brief periods of moderate to heavy rain.
Clouds will likely dissipate for a time before increasing in
coverage toward day break. Look for lows in the low to mid 70s
inland, increasing to mid to upper 70s at the coast.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
Tropical Depression 8 is forecast to near the North Carolina Outer
Banks Tuesday before ejecting out to sea in response to the
development of weakness in the upper ridge. No direct impacts are
expected this far south.

Upper shear axis/upper low will meander along the Southeast U.S.
coast through the period. This will promote scattered showers and
possibly a few tstms each afternoon with the low-level flow regime
supporting a risk for isolated to perhaps scattered nocturnal
showers impacting coastal locations. The convective pattern should
be similar each day; initiating along the coast then moving inland
as the afternoon progresses. Pops will be capped at 30-40% each
afternoon as confidence on the placement and timing of the greater
rain chances is just not high enough to higher pops attm. The
28/12z GFS spins up weak low pressure off the Georgia coast
Wednesday along a weak surface trough. This weak cyclogenesis
scenario is not really supported by the ECMWF nor most of the GFS
ensemble members and will be discounted, for now. Highs will range
from the lower 90s inland to the mid-upper 80s at the coast with
overnight lows ranging from the mid 70s inland to around 80 at the
beaches.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
NHC indicates that what is currently Tropical Depression Nine will
move northward through the Gulf of Mexico and strengthen into a
Tropical Storm. The forecast then calls for the Tropical Storm to
cross north Florida and move northeastward off the Southeast
coast. Have had make some adjustments to much of the extended
period to take into account the expected track of the low per the
NHC forecast. There is still much uncertainty on the track and
intensity of this system, so confidence is low on any direct
impacts. Additional changes are likely over the coming days.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
VFR conditions are likely to prevail through the TAF period, with
the main issue to consider being the possible development of MVFR
ceilings late tonight. Overall confidence is low but the forecast
does reflect a low end VFR ceiling at both sites closer to
sunrise. There will also likely be showers and storms developing
along the coast late tonight within a subtle convergence zone.
Models show the bulk of the near sunrise activity occurring along
the Georgia coast, so have added a VCSH at KSAV. Then for Monday,
another round of scattered showers and storms for the area.
Confidence is low in the probability of direct impacts, so no
restrictions or thunderstorms have been introduced into the
forecast.

Extended Aviation Outlook: Primarily VFR.

&&

.MARINE...
This Evening and Tonight: East winds will prevail over most waters
this evening and early tonight while an inverted trough is in place
over coastal waters. Winds should then tip back to northeast late
tonight over northern SC waters. Outside convective activity,
conditions are expected to remain below Small Craft Advisory
levels with wind speeds no higher than 15 kt and seas between 2-4
ft, mainly due to long period swell from the east.

Monday through Wednesday: Tropical Depression 8 will approach the
Outer Banks Tuesday and stay well northeast of the local marine
area. An east/northeast flow will persist through the period as a
weak trough remains just off the coast. Winds will remain 15 kt
or less with combined seas holding 2-4 ft nearshore and 4-5 ft
offshore. Long period swells that were generated by Hurricane
Gaston will continue to produce rough conditions at the entrances
to rivers, bays, harbors and inlets along the coast.

Thursday and Friday: Recently formed Tropical Depression Nine is
expected to become a Tropical Storm as it moves northward through
the Gulf of Mexico through mid week. By late week, it is expected
to cross north Florida and then track northeastward off the
Southeast coast based on the current forecast. There is
considerable uncertainty surrounding the track and intensity of
this system during this time so confidence in exact impacts is
low. Certainly based on the current forecast, strong gusty winds
and increasing seas will be possible.

Rip Currents: Given the reports received of rip currents today
and Saturday, it seems another high rip current risk is warranted
for all beaches Monday. An enhanced rip current risk will linger
through at least mid-week, possibly longer depending on the
evolution of Tropical Depression Nine across central/eastern Gulf
of Mexico.

&&

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

&&

$$

NEAR TERM...BSH
SHORT TERM...DPB/ST
LONG TERM...ST/BSH
AVIATION...DPB/ST
MARINE...DPB/ST/BSH
TIDES/COASTAL FLOODING...



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