Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 231441
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1041 AM EDT Wed Aug 23 2017

.SYNOPSIS...
High pressure will give way to an approaching cold front
through tonight. The cold front is expected to pass over the
region on Thursday. A wave of low pressure could form along the
front offshore Friday, as high pressure builds from the north
next weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today: No significant changes were made to the previous
forecast. The mid/upper level ridge is forecast to gradually
erode as troughing builds to our north. A piedmont surface
trough is forecast to become better defined over the CSRA and SC
Midlands while a cold front gets hung up in the mountains later
today. The morning hours should mainly be dry with an offshore
component in the boundary layer flow. This initial pattern with
morning sunshine and weak downslope suggests it will warm 2-3
degrees from Tuesday`s afternoon readings at many locations.
Soundings indicate a 2C-3C cap which is forecast to deteriorate
by 18Z at most locations. Since the sea breeze will be a slow
mover this afternoon, most of the low level moisture convergence
and dew point pooling looks to arrive along and to the west of
I-95 around 21Z. This time frame is when most models paint the
higher axis of 850 MB theta-e across the forecast area. There is
a slight uptick of mid/upper flow to the north of I-16 later
today and tstms should have an E or ESE translation. Convective
boundaries could collide with the sea breeze late, resulting in
isolated stronger storms with enhanced updrafts due to steep low
level lapse rates and DCAPEs near 1000 J/kg. We maintained 30
POPs after mid afternoon into the late day hours for just about
all zones. Highs in the mid 90s inland/around 90 near the coast.
Heat indices will top out about 100-105 most locales.

Tonight: The cold front will sag into the inland Carolinas and north
GA overnight. Slightly deeper moisture will pass through the region
while models indicate increasing upper difluence across the area
in the rear entrance region of a 50-60 knot 250 MB jet arcing
from the Mid Atlantic to the western Carolinas. Scattered convective
rains are possible with this setup, despite the loss of diurnal
heating and persistent poor mid level lapse rates. We maintained
lower end chance POPs for the overnight, especially SC zones where
we show some 50 percent POPs at times.

&&

.SHORT TERM /THURSDAY THROUGH SATURDAY/...
Thursday and Friday: The cold front will slowly cross through the
forecast area before crossing offshore Thursday night. Higher
coverage of convection is anticipated Thursday relative to previous
days given the focusing mechanism provided by the front itself.
PWats will exceed 2 inches with SBCAPEs climbing above 2000 J/kg,
suggesting locally heavy rainfall as a primary hazard. Greatest
precipitation chances will generally shift east and ultimately into
the coastal waters along with the front as surface pressures rise
behind. Near-normal temperatures are expected.

Saturday: The front will stall well offshore late Saturday. Model
guidance continues to indicate low pressure may begin to form on the
tail end of the stalled front by late Saturday or early Sunday.
Locally, a less active day looks likely, with greatest potential for
precipitation primarily confined to the coastal waters, though some
shower/thunderstorm activity near the coast is possible.
Temperatures will likely be just below normal values.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
Surface high pressure will continue to reach into the area from
Canada. Fairly reasonable model agreement depicts a coastal low
developing off the GA/SC coast by late this weekend. In addition,
guidance suggests that the remnants of Harvey will track towards the
forecast area from south Texas. At least some degree of wedging of
the high pressure over the Carolinas seems probable with low
pressure to the east and west, resulting in a cloudy and potentially
unsettled period. Daytime warming would be tempered in this regime,
with highs averaging in the mid-80s appearing reasonable at this
time. By Tuesday, deeper moisture may provide greater coverage of
convection, but model solutions diverge considerably by this time,
increasing forecast uncertainty.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
VFR. Risk for late afternoon showers/tstms impacting either KCHS
or KSAV Wednesday remains too low to justify a mention at either
terminal, especially since the potential window for timing remains
uncertain and latest high resolution reflectivity guidance continuing
to back off on the overall coverage. A chance of nocturnal convective
rains for later tonight ahead of a cold front, again confidence
in both timing and areal coverage is low at this time.

Extended Aviation Outlook: Thunderstorms may result in short periods
of flight restrictions, mainly each afternoon/evening. Numerous
thunderstorms are expected along and ahead of a cold front Thursday.

&&

.MARINE...
Through Tonight: Winds will mainly be from the SSW and SW
between high pressure offshore and an approaching cold front.
The gradient will continue to tighten through this evening and
wind speeds will peak between 15 and 20 knots for a period from
late today into the early morning, unless any convection or
convective boundaries moving offshore disrupt the synoptic wind
flow. Winds will gradually diminish late tonight as the cold
front nears from the NW and N. Seas will range from 2 to 4 ft
offshore to 4-5 ft out near the Gulf Stream.

Thursday through Monday: South to southwest flow generally 10-15
knots expected through Thursday night as a weak cold front crosses
through the area. Flow will veer northeast on Friday and strengthen
through early next week as high pressure builds while low pressure
appears poised to develop along the offshore cold front. Winds and
seas will steadily increase from Friday night through Monday, with
Small Craft Advisory conditions increasingly likely by Saturday
afternoon or night. The low pressure system is currently forecast to
make its closest approach next Monday into Tuesday, potentially as a
tropical cyclone.

Rip Currents: A 2-3 ft swell every 9-10 seconds will continue
today. This combined with fairly decent onshore flow in the
afternoon will yield a Moderate Risk for Rip Currents along the
Charleston County beaches today.

&&

.EQUIPMENT...
The KCLX radar is down for repairs and data will be sporadic.

The temperature and dew point sensors at the Downtown Charleston
observation site (KCXM) could periodically fail. Parts are on
order and technicians will attempt to resolve the problem once
they arrive.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$
NEAR TERM...RJB
SHORT TERM...JMC
LONG TERM...JMC
AVIATION...RJB
MARINE...RJB
EQUIPMENT...


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