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Area Forecast Discussion

FXUS62 KCHS 271131

Area Forecast Discussion
National Weather Service Charleston SC
731 AM EDT Tue Sep 27 2016

Low pressure will move up the southeast coast today. A cold front
will then slowly approach the region through mid week before low
pressure forming over the Mid-Atlantic accelerates the front
through the area Thursday morning. High pressure builds into the
region for the end of the week.


We have made fairly significant reduction of pops for inland
areas today as the bulk of the moisture will remain involved with
the upper level disturbance and Atlc surface wave and this should
be pulling away from the upper South Carolina coast after early to
mid afternoon. After morning stratus and fog dissipate, locations
along and west of I-95 should see a good bit of sunshine and warm
temps reaching the upper 80s. A few spots could even touch 90
degrees given low level thickness values. Along immediate coastal
Georgia we maintained slight chance pops after mid morning with a
few showers perhaps lingering until mid/late morning and perhaps
some weak sea breeze convection developing this afternoon. Across
coastal South Carolina, we maintained chance pops and a good bit
of cloud cover until late morning, then trended toward only slight
chance pops for the afternoon as the effects from the coastal low
wane and drier deep layer air oozes in from the west. Most of the
tstms will remain offshore but coastal areas could hear a rumble
through mid morning. With some afternoon breaks in the clouds
anticipated, Charleston and surrounding areas should rise quickly
to the mid and upper 80s later today.

Tonight: A cold front approaching from the west is expected to get
hung up in the mountains as the upper pattern awaits a digging
upper jet in the upper plains to transition a large and deep cut-off
low southward. We do not think pre-frontal convection will make
much of a charge toward the forecast area overnight and have
backed off pops accordingly, especially with the lack of much
upper forcing and poor deep layered instability. We think most
areas will be rain-free overnight given the latest guidance
information. There could be some patchy fog inland with lows upper
60s to 70-72 along the coast.


A cutoff upper low over the Great Lakes Wednesday will drop down and
meander over the central Appalachians through the second half of the
workweek. At the surface we will spend Wednesday entrenched within
the warm sector of the system as the cold front slowly approaches
from the west. Chance of thunderstorms maintained for the entire
area Wednesday afternoon as moisture content should recover well
after a mostly dry Tuesday. PW values near to just above normal and
weak lapse rates/unimpressive instability will keep the severe
threat very low Wednesday. Convective activity should wane with the
loss of daytime heating Wednesday afternoon, but showers and
thunderstorms will remain possible along and just ahead of the cold
front, which will move through during the overnight and early
morning Thursday hours.

Thursday will bring about a significant airmass change as the cold
front moves off the coast and cooler continental high pressure fills
in behind it. Models are hinting that there could actually be two
frontal passages Thursday, the first in the morning will bring in
the drier air, while the second, possibly Thursday afternoon, will
bring more efficient cold air advection and more fall-like
temperatures. Some remnant showers may hang around into Thursday
evening mainly along the far northeastern portion of the CWA due to
its proximity to the upper level low, but most likely scenario is
everywhere stays dry behind the front, with cooler temps being the
most notable weather feature during this time. Thursday night lows
in the mid 50s to lower 60s will be the coolest that much of the
area has seen since spring.


Upper level cutoff low over the eastern U.S. will weaken and get
absorbed in the large scale, low amplitude trough developing across
much of the U.S. east of the Rockies. Surface high pressure
stretching from the Great Plains across the Southeast will remain in
control of our sensible weather locally. A stalled front just off
the coast could bring unsettled weather to the immediate coast over
the weekend, but confidence is low in any impacts at this point.


KCHS: Stratus is developing on the back edge of the cloud shield
across the Tri-County area. It looks as if IFR or lower cigs will
be confined to areas just west but cannot rule out some low cigs
in the next 2 hours. Otherwise, VFR with late night patchy fog
possible again early Wed.

KSAV: IFR and lower conditions developed shortly before dawn and
06Z NAMM soundings suggest most of the low clouds will be eroding
prior to 15Z if not a bit before. LIFR cigs/vsbys are likely prior
to 13Z. Conditions will become VFR after 15Z and likely remain VFR
until perhaps late night when some patchy fog is possible with
much lower risk for low clouds.

Extended Aviation Outlook: Showers and thunderstorms associated with
a cold front may bring brief flight restrictions Wednesday and
Wednesday night. VFR conditions expected Thursday through the end of
the week.


The pressure gradient has been weaker this morning as high
pressure weakens over the northwest Atlc. A wave of low pressure
is expected to ride northeast along the coastal surface trough
along the western wall of the Gulf stream. This will allow onshore
flow to back with time with the gradient likely falling flat
enough later today to suggest light and variable winds offshore
and weak sea breezes in the very near shore waters. Seas will
range from 2 to 4 ft this morning, highest offshore and mainly
around 2 ft near shore tonight.

Wednesday through Saturday: Unsettled weather will persist over the
waters Wednesday into Thursday as a cold front approaches from the
west. This front should move through the waters during the day
Thursday and stall east of our offshore zones late in the week,
allowing cool northwest flow to fill in Thursday night through the
weekend. Southerly pre-frontal, veering to northwesterly post-
frontal, winds will mainly remain 10 to 15 kt or less outside of
thunderstorms through the period. Seas of 2 to 3 ft can be expected,
with 4 ft at times for the western portion of the offshore Georgia





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