Area Forecast Discussion
Issued by NWS Columbia, SC

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FXUS62 KCAE 261023

Area Forecast Discussion
National Weather Service Columbia SC
623 AM EDT MON SEP 26 2016

An upper-level ridge will move off the coast today. A stalled
weak cold front over the northern Midlands will provide a focus
for scattered showers and thunderstorms. Another cold front will
cross the area Tuesday night into Wednesday. The air mass behind
this system will bring dry weather and more seasonable
temperatures for the latter half of the week into the weekend.


Weak backdoor front that was pushing through NC earlier seems to
have slowed or even stalled near the NC/SC border in the Pee Dee
region with much of the stratus clouds remaining north of Columbia
through 2 am. Some isolated showers have been ongoing across
Lancaster/Chesterfield counties but showing signs of diminishing.
KCAE VAD wind profile confirms winds in the lower 5000 ft are from
the southeast which will help advect moisture off the atlantic
into the forecast area today.

Weak upper ridge currently over the area will flatten and shift east
today as a digging upper trough/closed low moves into the Great
Lakes region this afternoon and tonight, driving a cold front into
the mountains this afternoon and slowing its push toward the
forecast area overnight.  A band of enhanced moisture with
precipitable water values around 1.7-1.8 inches will precede the
approaching front.  The increased moisture combined with surface
heating should support weak to moderate instability this afternoon
with scattered showers and thunderstorms expected along and south of
the stalled diffuse frontal boundary currently over the northern
Midlands.  This boundary  may serve as a focus for convection but
there is some uncertainty regarding the strength of convergence
given the broad southeasterly flow.  Some variability among the high
resolution models with timing and location of convection today but
it looks as if the northern Midlands may be more favored and later
in the day.

Will continue to advertise chance pops this afternoon into early
evening during peak heating and while the severe thunderstorm threat
appears low, cannot rule out an isolated strong to severe storm.
Temperatures today will again be well above normal in the mid
to upper 80s with overnight lows tonight in the upper 60s.


Tuesday looks to provide the best chance for precipitation across
the forecast area as precipitable water values maximize just ahead
of the cold front with values approaching 1.8-1.9 inches with upper
height falls and moderate instability forecast.  The cold front is
slow to push through the forecast area Tuesday afternoon and Tuesday
night as the 500mb flow back more southerly, nearly parallel to the
front as the upper trough amplifies.  The front eventually pushes
through on Wednesday as the upper level closed low drops southward
into the Ohio Valley and much drier air pushes into the area from
the northwest with precipitable water values dropping below an inch
by Wednesday evening.

Will carry high chance pops across the forecast area on Tuesday into
Tuesday evening with diminishing pops late Tuesday night as the
atmosphere stabilizes and continue higher pops on Wednesday across
the eastern half of the forecast area with lowering pops through the
afternoon from west to east as drier air filters into the region.

Temperatures will remain above normal with highs Tuesday in the mid
to upper 80s and slightly cooler on Wednesday in the lower to mid
80s.  Overnight lows Tuesday night will remain mild in the mid to
upper 60s while temperatures will drop a couple of categories for
Wednesday night with min temperatures in the upper 50s to lower 60s.


Medium range models are in reasonable agreement in the overall upper
pattern showing a closed low somewhere in the vicinity of the
central Appalachians or eastern Tennessee Valley Thursday which then
slowly migrates northward into the Ohio Valley over the weekend.  At
the surface, cooler and drier continental high pressure will build
over the forecast area with precipitable water values well below an
inch.  Despite very cold 500mb temperatures around -12C to -16C over
the area Thu/Fri with steep mid level lapse rates, the lack of
moisture should limit or preclude precipitation associated with the
upper low, although expect some cloud cover.

Temperatures during this period will feel much more seasonable with
near normal temperatures.  Highs will be in the upper 70s to lower
80s and overnight lows will be in the mid to upper 50s.


Back door front appears to have pushed south into the NE FA and
stalled. Even south of the front, moist low levels and an E to SE
boundary layer flow promoting development of stratus and some fog,
with IFR to LIFR conditions at our terminals. Expect this to
continue in the near term, with a gradual return to VFR expected
by late morning.

Back door front expected to remain NE of our terminals, and
eventually become diffuse or drift north as a warm front ahead of a
cold front approaching from the NW. Meanwhile, upper and surface low
to drift NW towards the lower SC coast today. Daytime heating and an
onshore flow to provide potential for scattered showers/isolated
thunderstorms this afternoon, mainly S FA. Will mention VCSH at
DNL/AGS/OGB. Other convective activity along or ahead of the
approaching front late today/this evening appears will remain north
of the terminals. Moist low levels suggests deteriorating conditions
on tap again late tonight.

EXTENDED AVIATION OUTLOOK...Stratus/fog possible late Monday
night/early Tuesday morning. Chance of showers and thunderstorms,
and associated restrictions, Tuesday/Tuesday night. No impacts to
aviation expected Wednesday afternoon through Friday.




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