Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 232152

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
552 PM EDT Tue May 23 2017

A train of gulf low pressure systems will track over the area along
a stalled frontal boundary through late tonight. As the front moves
southeast, a broad area of showers and thunderstorms will cross our
area Wednesday ahead of a closed upper low. A drying trend will begin
on Thursday and continue into the weekend.


As of 540 PM EDT: Seeing the effects of a dry slot of sorts moving
in from the west late this afternoon. Thicker blanket of cloud cover
continues to slide east with the steadier precip shield now confined
to the eastern zones generally along/east of I-77. To the west,
where water vapor imagery shows the mid-level drying, we continue to
develop modest CAPE as convective instability is realized. That has
allowed numerous showers to develop, mainly over the mtns and
foothills. The trend should continue into the early evening hours
with the categorical precip prob over the east being replaced with a
chance/likely precip prob to acct for the new convection. This
should diminish somewhat with the loss of heating in the mid/late
evening. Temps look ok.

Otherwise...Water vapor satellite imagery shows a closed low
pressure system drifting southward over Iowa this afternoon with the
associated trough amplifying from the southern plains to the lower
MS River valley. Coupled upper jet divergence east of the trough
axis will remain over our foothill and piedmont zones through the
late afternoon hours before lifting off to the northeast later this
evening. Regional radars also depict the best surface wave and
associated stronger convection moving through central GA and this
should brush our piedmont through early evening before moving off to
the northeast. Limited drying is expected to develop from the
southwest behind the wave later this evening. This should herald a
lull in the better QPF across the region for the nighttime hours,
with mainly light rain continuing in the western mountains as 850 mb
flow veers more westerly tonight before backing southerly by
daybreak Wednesday.

A more potent 120 to 130 kt upper jet will move over the region on
Wednesday as the trough sharpens up to the west. In addition,
southerly flow at low levels will improve upglide and coverage.
Locations along and southeast of I-85 will see the best chance of
having an uncovered warm sector airmass Wednesday afternoon with
instability bubbling sbCAPE to near 2000 J/kg. In addition, Piedmont
850 mb wind speeds should reach 25 to 35 kt by late afternoon during
peak heating to briefly enhance bulk shear values and drive the best
severe thunderstorm threat. Model QPF is not terribly impressive,
but the convective rates could once again raise the hydro threat.
See the Hydro discussion section for details on the Flash
Flood/Flood Watch thought process for the ongoing hydro threat.


As of 200 PM EDT Tuesday:  The short term fcst period kicks off
on Wednesday evening amidst amplified upper flow thanks to a slow
moving closed H5 cyclone centered atop the MidSouth region, while
a series of shortwave impulses pass through the mean flow aloft.
At the surface, guidance favors an approaching cold front from
the west as a warm front lifts well to the north.  With that,
mesolow development beneath one of the aforementioned upper
shortwaves is possible through late evening just ahead of the
cold front.  As such, broad scale upper divergence ahead of the
primary H5 cyclone will provide ample synoptic scale lift to support
remnant convection into the evening hours therefore warranting the
continuation of elevated pops in the likely/categorical ranges,
before lowering through the night into Thursday morning.

For Thursday, chances for showers and thunderstorms will remain
elevated as the upper low moves overhead providing continued large
scale ascent atop the southern Appalachians.  With that, fcst
soundings favor cooling aloft that will effectively steepen lapse
rates in the mid levels, which would yield increased chances for
convection across the region if not for lowering surface dewpoints
behind the front.  As such, models suggest a relatively stable
environment across the southern tier of the fcst area where the
drier air will have its first impacts, while profiles are slightly
more unstable to the north where upper 50s dewpoints are progged
through mid day.  Therefore, although not likely, think chances for
deep/strong convection on Thursday should remain confined to the
NC zones, specifically along/north of I40.  Otherwise, the heavy
rain threat will diminish early in the period, likely by early
morning Thursday as llv flow becomes more veered.  Friday looks
to be a rather quiet day weather wise as the upper low ejects to
the northeast allowing heights to rise as a ridge builds in from
the west.  As a result, expecting dry conditions with mostly sunny
skies allowing for abundant warming.  Temperatures behind the front
will suffer somewhat as highs top out a few degrees below normal for
Thursday, before returning rather sharply to above normal by Friday.


As of 235 pm EDT Tuesday: the extended fcst picks up at 00z on
Saturday with low amplitude upper ridging beginning to build over
the region as broad upper trofing lifts off the New England Coast.
The ridge is expected to flatten out as we move into Sunday and
another broad upper trof digs down over the Northern Plains. It
appears that some lobes of mid to upper lvl shortwave energy will
pass just to our north on Sunday, however the main trof will be
slow to move eastward and won`t approach the fcst area until later
in the period. At the sfc, high pressure will be centered to our
SE with warm SLY flow in place over the region. A cold front will
approach the CWFA on Sunday but likely won`t move thru the area
until Monday. It appears to become stationary just to our SE by
early Tues and lingers over the area thru the remainder of the
period. As for the sensible fcst, no significant changes were made.
We still have slight to solid chances for showers and ts each day
with the highest POPs over the higher terrain. Temps will be above
climatology thru the period with little change from day to day.


At KCLT and elsewhere: Very moist profiles continue around the
region but with ceilings quite variable in most areas. Still expect
ceilings to settle down into the MVFR to tempo IFR range through
late afternoon and become IFR to possibly LIFR in most areas
overnight. Any lingering thunder threat appears to have shifted
mainly east of the terminal forecast area so will carry only showers
going forward, with shower coverage diminishing from the southwest
through early evening. Winds are another tricky aspect to the
forecast. They should be light away from the heaviest showers, but
with mainly ENE at KCLT/KHKY and southeast at the Upstate sites and
KAVL. Convective chances return through the day on Wednesday with
the approach of the upper trough and continued surface front in the

Outlook: Scattered showers and thunderstorms will continue on
Thursday as the upper trough passes. Drying is expected to finally
occur Thursday night through Saturday.

Confidence Table...

            22-04Z        04-10Z        10-16Z        16-18Z
KCLT       Med   65%     High  86%     Med   63%     High  86%
KGSP       Med   65%     High  92%     High  85%     Med   65%
KAVL       Med   77%     High  82%     Med   69%     High  91%
KHKY       Med   64%     High 100%     High  95%     Med   65%
KGMU       Med   72%     High  90%     High  85%     Med   68%
KAND       High  80%     High  90%     High 100%     High  94%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:


The anticipated lull in the precipitation for tonight is vastly
complicating Watch issuance going forward. We have an existing Flash
Flood Watch through 06Z tonight for the foothills/piedmont that can
probably be pared away from the southwest as the surface wave and
associated heavier precip moves by. Any watches for Wednesday onward
would likely be needed for the entire forecast area with good
coverage west over the mountains but better convective rates east
over the piedmont. Will thus stay the course with the current FFA
and allow later issuance to fine tune (in space and time) a new
watch when and if needed.


GA...Flash Flood Watch until 2 AM EDT Wednesday for GAZ018-026-028-
NC...Flash Flood Watch until 2 AM EDT Wednesday for NCZ035>037-056-
SC...Flash Flood Watch until 2 AM EDT Wednesday for SCZ001>014-019.


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