Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 231136

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
736 AM EDT Fri Jun 23 2017

Widespread tropical showers will continue to impact our region
through Friday morning. Shower coverage decreases briefly before
a cold front approaches the mountains later tonight bringing
more showers and thunderstorms to the area. Light showers linger
on Saturday as the front slowly departs the area. Unseasonably
cool and dry conditions will spread over the forecast area in the
wake of the front and persist through the middle of next week.


As of 700 AM EDT Friday: Temperatures look good for now, but new
guidance is wanting to drop max temps just a tad; have adjusted a
few hourly trends but no major changes to afternoon highs. Made some
additional adjustments to pops based on CAM and other new guidance,
but overall trends remain similar. Other changes for the daybreak
update are mainly for aviation-related parameters.

Otherwise, the remnants of Cindy remain over the Mid-South, slowly
being absorbed by the broad longwave trough in place over the
northern tier of the country. And upper ridge over the South
Atlantic Bight will slowly flatten and be pushed south as the phased
trough pushes into the Appalachians tonight. In the meantime, we
remain in the warm sector and the upper ridge will give its last
gasp today with highs fairly close to seasonal normals (but lows
several degrees above with all the moisture in place). Most of the
day today should be a fairly "typical" muggy summer day with perhaps
some diurnally-driven convection across the mountains (though we`ll
be in sort of a stable "hole"), but as the wave and attendant front
approach tonight, a non-diurnal rampup of convection should start.
Increasing surface pressure gradient as the remnant Cindy surface
circulation approaches should lead to breezy south/southeast winds
which may provide at least some relief from the humidity.

As we move into this evening and the front approaches, convection
should begin with some upslope showers in the southwest mountains,
with a potential convective line moving into western zones sometime
after 00z. Surface-based instability will be limited, but should
still see CAPEs slowly increase to between 500-750ish J/kg ahead of
the front. Deep-layer shear will move in as well with a 50+kt 850mb
LLJ ahead of Cindy`s absorbed remnants, with resulting 0-6km bulk
shear generally 30-40kt, but could creep up toward 50kt. So with
this, despite the overall lack of widespread instability, cannot
rule out isolated severe convection, and as with any tropical
remnants, mini supercells and perhaps a brief tornado cannot be
ruled out. SPC`s Marginal Risk looks to cover this well, but the
problem is that anything that does occur would be nocturnal.

Other concern is rainfall, but it really looks like the mountains
will break up quite a bit of the widespread heavy rain. Day 1 QPF
isn`t all that impressive, with around 1.5" in extreme western
Graham County (highest in the CWA)...and though 1hr FFG is 1-3",
that 1.5" in 24h was enough to warrant a Slight Risk of excessive
rain. For now...just doesn`t seem like enough concern of
*widespread* heavy rain for a Flash Flood Watch. No doubt training
convective cells may lead to an isolated threat especially in the
mountains, and with soils fairly wet (especially after this
morning`s rain) it probably wouldn`t take much. But just not enough
confidence for a Watch at this time.


As of 305 AM EDT Friday: the short-term fcst picks up at 12z on
Saturday with longwave upper trofing digging down across the Great
Lakes and steep upper ridging over the West and East Coasts. The
upper lvl pattern is not expected to change much thru the period
with the trof axis remaining just to our NW and the ridge amplifying
to our west. At the sfc, a robust-looking cold front will be moving
thru the CWFA by early Sat, with widespread showers and thunderstorms
likely as it does. The models are hesitant to move the front south
of the CWFA on Saturday and keep the bndy just to our southeast thru
Sunday. By the end of the period early Monday, strengthening Canadian
high pressure to our NW finally pushes the bndy offshore. Precipitation
chances will steadily diminish Sat night thru early Sun with the rest
of Sunday expected to be dry and a bit cooler.


As of 255 AM EDT Friday: the extended fcst picks up at 12z on Monday
with a broad longwave upper trough centered over the Great Lakes to
our NW. A reinforcing upper shortwave digs southward on the backside
of the trof and moves over the fcst area on Tuesday. As the shortwave
lifts NE, heights begin to rebound as upper ridging begins to build
to our south. At the sfc, cooler and drier should be in place over
the CWFA by early Monday with broad high pressure centered to our NW
and a frontal bndy to our SE. Over the next few days, the high will
slide SE and over the fcst and eventually move off the Atlantic Coast
by the end of the period late Thursday/early Friday. As for the sensible
wx, the period should be mostly dry with only some slight chance PoPs
towards the end of the period when we get back under more moist southerly
flow. Temps and dewpts will remain below normal for late June with values
warming back to near normal by day 7.


At KCLT and elsewhere: MVFR to IFR cigs dominate the area this
morning, though VFR continues at KAVL. Should see rapid improvement
in the next hour or so, with VFR conditions through the day and
breezy S/SW winds as the pressure gradient increases ahead of the
approaching front. Front pushes in this evening and tonight, with
convection developing across the mountains ahead of it. Some
question as to the amount of instability that will lead to -TSRA, so
have included some PROB30s after about 06z for the Upstate TAFs and
KAVL, but only VCSH or PROB30 -SHRA for KHKY/KCLT. Will likely see a
wind shift by the end of the period for KAVL, with plentiful
moisture leading to another round of MVFR to possible IFR

Outlook: Abundant moisture will continue into Saturday with
associated restrictions and possible convection. The front will push
through on Sunday to usher in a period of drier weather through the
middle of next week.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High  89%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  94%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High  95%     High 100%     High 100%
KAND       High  89%     High  95%     High 100%     High 100%

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:




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