Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 260744

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
344 AM EDT Wed Jul 26 2017

High pressure will move by to the north today with slightly cooler
temperatures expected. Another front will move in from the northwest
late in the week, with cooler and drier conditions arriving behind
it for the weekend.


As of 330 AM EDT Wednesday: Little change is expected in the upper
levels, with the eastern edge of a broad upper ridge nosing into the
Carolinas, while a flat trough digs slightly across the Great Lakes.
In the low-levels and at the sfc, however, there will be a light
easterly flow, which will support slightly lower thicknesses and
more cloud cover. There should be a pooling of decent SBCAPE by peak
heating along the eastern escarpment. So PoPs will be highest in
these areas, with low-end CHC to slight CHC elsewhere. As last few
days, severe threat will be low, but where CAPE pools to over 2000
J/kg, there may be one or two pulse severe storms that produce
damaging wind gusts. Temps will be a bit tricky. The NC piedmont and
eastern Upstate may start out with a stubborn stratus deck, which
will slow heating. However, the sfc high will be weak and exiting
off the Northeast coast. So little will be able to "lock" in the
clouds and they should scatter into a cu field by mid-AFTN. In any
case, max temps will be slightly below normal across most of the
area, and near normal in the SW, where there will be the most

Tonight, low-level flow will veer from E/SE to SW and guidance hints
at some very weak upglide. This may support stratocu redeveloping
across the Piedmont and also keep a lingering slight CHC of
nocturnal convection. So sky cover ramps up and PoPs linger,
especially in the southern escarpment with weak upslope flow. Temps
will be slightly above normal.


As of 345 AM EDT Wednesday: We are still looking at a pattern
change at the end of the week that will see an amplification of an
upper trof over the eastern part of the country that will bring
about a period of below normal temps for the weekend. Details
are still a bit murky as to how we will get there, though. Before
that happens, Thursday looks like it will be a rebound of sorts
with temps climbing a bit above normal even as the upper ridge
weakens and heights begin to fall. Low level moisture remains
sufficient to fuel the development of showers and storms over the
higher terrain with some heating. The strengthening westerly flow
should help to push them out across the foothills and wrn Piedmont,
so all but the easternmost zones get a chance of precip by late
afternoon/early evening. Cannot rule out some storm organization,
especially farther north closer to the approaching boundary, so the
Day 2 Outlook from SPC looks appropriate. From that point onward,
the uncertainty goes up with regard to the timing of the short wave
and the surface front. The models are converging on a solution that
takes the main vort center across the Mid-Atlantic region Friday
morning with weaker waves moving through in the afternoon. Shear
profiles increase nicely as this happens, with 0-3km bulk shear on
the order of 25-40 kt during the day. The problem might be a lack
of instability because of the early passage of the short wave and
clouds/precip. The GFS solution manages to eke out sbCAPE on the
order of 500 J/kg but lapse rates are putrid. The NAM has virtually
no sbCAPE across most of the fcst area. Hard to imagine much of a
severe weather threat with that scenario. Equally hard to imagine
such a lack of instability ahead of such a meaningful frontal
boundary, so the operational NAM/GFS seem highly suspicious. Of
note...the 03Z SREF gives the area east of the Blue Ridge a greater
than 50 pct chance of greater than 1000 J/kg CAPE. The new Day
3 convective Outlook has the entire fcst area in a slight risk
owing to potential for organized storms with upscale growth and
widespread damaging wind potential.  The course of least regret
will be to follow the lead of SPC on this one in spite of my
initial doubt. Precip chances ramp up to the likely range across
most of the area. Will undercut guidance for the time being and
not include any categorical. Temps will depend on the location of
the de facto frontal boundary across the Piedmont during afternoon
heating, so there is some bust potential there, but suffice to say
temps will be at or above normal to the south and possibly below
normal across the north. The changes will take place Friday night
as the front sinks southwest across the region. The fcst holds onto
a precip chance over the eastern zones into the evening hours and
only gradually tapers down the chance on the TN border where moist
upslope flow persists.


As of 330 AM EDT Wednesday: The medium range guidance continues to
show a fairly high amplitude upper trof along the Eastern Seaboard
this weekend with a sprawling surface high pressure air mass over
the upper Midwest/Gt Lakes region asserting its dry and relatively
cool influence over our region. The GFS/ECMWF support the idea
that we will be hard-pressed to generate even ridgetop convection
in this scenario. If the trend holds, we might just be looking
at the nicest weekend weather of the entire summer, with temps
topping out a category below normal each day. After we lose the
moist NW upslope early Saturday, will trim back the precip chances
a bit more Saturday and Sunday to emphasize increasing confidence
in the dry air mass. Expect some modification by Monday that will
allow for diurnal convection over the mtns.


At KCLT and elsewhere: Easterly low-level flow is setting up atop
the area during the pre-dawn hours, and will help support MVFR to
possibly some IFR stratus development across mainly the NC Piedmont
sites thru daybreak. With upslope flow, tho weak, may support
ongoing -SHRA near KHKY, so will add  VCSH for the next few hours.
Guidance generally doesn`t develop stratus west enough to affect the
Upstate sites. At KAVL, valley fog and stratus is expected to
develop. After sunrise, low clouds will be slow to lift and scatter
out due to the moist easterly flow, but should go to VFR by midday
at all sites. Slightly better coverage of afternoon convection is
expected across the area. So PROB30 for TSRA will be carried in all
the TAFs. Winds will be light thru the period, favoring a NE
direction thru midday, then SE this afternoon and evening.

Outlook: Scattered diurnal convection can be expected each day
thru Friday across the region. Then drier air will filter in from
the north behind a passing cold front this weekend. Chances for
morning fog and stratus will also be possible each day in the
mountain valleys.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High  93%     High  81%     High 100%     High 100%
KGSP       High  90%     High 100%     High 100%     High 100%
KAVL       Med   67%     High  94%     High 100%     High  95%
KHKY       High  94%     High  81%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  90%     High  96%     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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