Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 240254

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1054 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 1040 PM EDT: Seeing the effects of a dry slot of sorts still
moving east across the wrn Piedmont zones, with a new area of
showers/storms following right behind and moving into the wrn zones
as of 0230Z. This will require some reshuffling of the precip probs,
in order to acct for what should be a fairly high chance as the next
area of precip moves in from the west. This will be gradually
tapered off as it crosses the fcst area, if only because the
guidance handles it poorly so as to suggest it will fall apart.
Temps look ok.

Otherwise...still expecting another lull in action during the early
morning 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: Tough situation for aviation concerns over
the next 12-24 hours or so. Very moist boundary layer will continue
overnight and any slight forcing could result in a low cloud ceiling
or the development of more precip. Restrictions will be variable,
mainly due to the ceiling which will fluctuate from IFR to VFR at
most locations through the evening at least. A large area of rain
should move out of the wrn Piedmont that should bring at least a few
hours of improvement, but MVFR/IFR cloud patches lurk about and will
likely return by late evening. Additional showers and storms could
move in from the west to bring a visibility restriction as well.
Best that we can hope for is what model guidance suggests, that is,
ceiling/vis will deteriorate in the early morning hours to IFR/LIFR
at all locations. Of course, this could be for naught if precip
redevelops or not. Conditions should be slow to improve in the
morning, with IFR/MVFR a strong possibility into the middle part of
the day. Model guidance is at odds as to when the next best chance
for precip will be, so will lean on the 4km WRF and climo meaning
from early afternoon onward. Uncertainty has me going with a PROB30,
although am fairly certain that all sites will get at least a
passing shower/thunderstorm at some point, maybe even after the end
of the TAF period. Wind should stay some version of SW to SE through
the period.

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...

            03-09Z        09-15Z        15-21Z        21-00Z
KCLT       Med   71%     Med   62%     High  80%     High  80%
KGSP       Med   67%     Med   65%     Med   73%     High  96%
KAVL       Med   67%     Low   37%     High  93%     High  92%
KHKY       High  85%     Med   78%     Med   72%     Med   78%
KGMU       Med   77%     Low   56%     Med   75%     High  87%
KAND       High  86%     Med   73%     High  82%     High  95%

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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