Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 230744

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
344 AM EDT Sun Jul 23 2017

A cold front will approach the area from the northwest today and
settle across the region Monday through Wednesday. This should
result in a little cooler temperature and better chances for rain
over the next few days.


As of 315 AM EDT Sunday: After some skepticism about the mesoscale
model guidance bringing deep convection into the Nrn mtns before
daybreak, lo and behold some thunderstorms have developed over
extreme northeast TN and northwest NC.  For the time being, the
trajectory of the convection will carry it past the fcst area
just to the north, so a chance of precip will suffice for the
time being. The rest of the fcst remains the same, with valley
fog developing in the higher terrain and temps remaining quite
warm for the time of day.

Today might be an interesting day, in terms of thunderstorm coverage
and intensity. We lose the suppressing influence of the upper
anticyclone that helped to keep more of a lid on the deep convection
the past few days. Instead, the upper ridge will be replaced with
a more cyclonic flow as a trof dips down from the north, displacing
the potential MCS track a bit farther south. Speaking of which, we
already see the west-to-east progression of numerous storms across
the OH River Valley/Central Appalachians at this early hour. It is
not difficult to imagine this could leave behind an outflow boundary
or cloudy/clear boundary strung out near the northern part of the
fcst area later today that would act as a trigger/focus. Meanwhile,
the model guidance suggests our air mass will become moderately to
highly unstable today with sbCAPE ranging from 2500 J/kg in the GFS
to about 4000 J/kg in the NAM. It may take until early afternoon
for the cap to erode, which is the reason for undercutting the
guidance on the precip chance this morning, but once that cap
is gone, could we see some explosive storm growth? I have seen
events around the wrn Carolinas where we have a few very hot days
in a row with limited convection, but then a subtle change occurs
that unleashes pent up energy resulting in storms more numerous
and severe than expected.  This could be one of those days. Will
trend the fcst toward higher precip chances over the Piedmont late
this afternoon and evening. If that wasn`t enough, we will have
some potential for bringing an organized convective system in from
the WNW later tonight. Will keep a chance in the fcst overnight
to account for that. High temps will be above normal again, but
maybe a degree or so below yesterday if the atmosphere turns over
as much as it might, so heat advisory criteria should not be met.


As of 300 AM Sunday: a progressive upper trough will deamplify as it
crosses the Appalachians on Monday, then retreating to the New
England coast on Tuesday. At the surface, a cold front is progged to
be draped across the CWFA on Monday, sagging slightly south into the
lower SC Piedmont or the Midlands on Tuesday. Depending on how much
convective overturning occurs on Sunday into Sunday, Monday may end
up being a little less active than the model guidance suggests. The
00z Hires Windows and the NAMnest are now depicting most of the deep
convection off to the east of the area, with only isolated to
scattered weaker activity atop the CWFA Monday aftn. I have lowered
PoPs slightly to trend in this way. However, confidence in the
amount of instability is still low. Temps may also end up being a
tad cooler. However, the 00z MOS guidance has actually trended
warmer than previous runs. In any case, will continue to forecast
highs a category or so above normal.

On Tuesday, the latest operational models now all depict a weak
upper disturbance or possibly an MCV that develops over the lower MS
Valley. This may help enhance convection across the deep south and
to the north along the stalled front. With the overall weak flow
aloft and typical late-July heat and humidity, I expect typical
diurnal convection, expect perhaps a little greater than usual
coverage owing to the lingering front. A small pulse severe threat
can be expected both days.


As of 330 AM Sunday: The medium range starts out 12z Wednesday with
a weak closed 500 mb low trying to develop INVOF the Carolinas under
a building ridge. By the time the low starts to close off, a
northern stream trough will enter the Great Lakes and should kick
the energy out to the east. This trough will continue to dig along
the East Coast Friday into next weekend. At the surface, a weak low
briefly develops along the stalled frontal zone draped across the
Deep South. This wave be at its strongest during the day on
Wednesday, keeping a light easterly low-to-mid level flow across NC.
With a 1025 mb sfc high over New England and models showing plenty
of clouds and precip across the Carolinas, it almost looks like a
weak cold air damming setup. However, low-level thicknesses do not
suggest true damming. So temps will be slightly below normal, but
not as cold as would be with CAD. PoPs will feature basically a
shotgun CHC for now.

From there, with another upper trough digging in across the eastern
CONUS, expect temps to remain near normal to slightly below. PoPs
will be near climo on Thursday, then above climo for Friday with
another front pushing in from the NW. Depending on exactly where the
axis of the upper trough sets up, the unsettled weather may shift
just east of the area, resulting in a relatively dry weekend.
However, the 00z GFS and ECWMF both keep the trough just enough west
to continue at least a chance for diurnal convection.


At KCLT and elsewhere: VFR through the period except at KAVL and
other mountain valleys, where conditions will range from MVFR to
VLIFR through daybreak. The exception will be in thunderstorms,
which are expected to be more numerous this afternoon and evening.
Before we get to that, wind should be SW through daybreak and for
most of the day, until thunderstorm outflows impose their will.
After convective debris lingers across the area early this morning,
we should see another round of high based Cu forming around midday
as we reach the convective temp. The atmosphere this afternoon is
expected to become highly unstable with sbCAPE on the order of
3000-4000 J/kg. Expect scattered storms to be able to develop over
the mtns and then move outward across the Piedmont on outflow
boundaries. Thus, all sites get an extended PROB30 for the late
afternoon and evening. After storms die down this evening, we look
to the west for a possible organized convective syste late this

Outlook: Increasing chances of convection will continue into the
middle of next week ahead of a series of weakening cold fronts.
Overnight restrictions will continue in the mountain valleys with
chance increasing elsewhere.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  93%     High  96%     High 100%     High  90%
KHKY       High  96%     High  96%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  96%     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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