Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211728

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
128 PM EDT Fri Jul 21 2017

The upper level ridge that has brought such hot weather to our
region of the nation will gradually break down over the weekend as
an upper level trough develops over the eastern part of the country.
Expect temperatures to be a little cooler next week with better
chances for rain.


As of 125 PM EDT Friday: Convection developing across the NC
mountains ahead of outflow boundaries from OH valley MCS. Cells also
forming south of the CWFA near weak boundary. This is in line with
going forecast, so no significant changes.

Otherwise...the main concern today will be the heat as the large
upper anticyclone over the middle of the country noses eastward this
afternoon. Temps are expected to get a degree or two warmer compared
to Thursday. However, the dewpoint is still expected to mix out
during the afternoon, just enough to keep the RH and apparent temp
below Heat Advisory levels. So, in spite of high temps almost ten
degrees above normal, and isolated spots seeing the heat index max
out close to 105, a Heat Advisory will not be issued. Might be a
better shower and thunderstorm coverage this afternoon, mainly over
the higher terrain, but the fcst was kept in check with a chance at
the higher elevations and a slight chance elsewhere. Will never
completely rule an isolated severe storm at this time of year, but
warmer air aloft will yield unimpressive lapse rates and buoyancy.
Expect any storms that form to weaken and dissipate later this
evening. However, a weak front dropping down into the region from
the north might provide some focus/trigger for the convection to
redevelop across the mtns/foothills late Friday and early Saturday.
Min temps will remain seasonally warm.


As of 300 AM Friday: The large upper level anticyclone will begin to
weaken as we head into the weekend. This will allow gradual height
falls atop the region, increasing the chances/coverage of diurnal
convection for Saturday and even more for Sunday. Temps will remain
hot, with guidance keeping dewpoints in the upper 60s to lower 70s
Saturday afternoon. This combined with low to mid 90 temps will
result in widespread heat index values in the 100 to 104 range
across the Piedmont. With a front still well to the north and a
notable lee trough depicted in the guidance, I think dewpts should
mix out enough to preclude a need for a heat advisory. Temps look a
deg or two cooler on Sunday, with earlier convective initiation
expected. So max heat index values should be slightly lower than
Saturday. As for severe threat, afternoon CAPE values should be
easily in the 1500-2000 J/kg range with possibly higher values.
Shear will remain weak. So expect typical pulse severe threat, with
microbursts being the main threat.


As of 315 AM Friday: An upper trough will dig across the Great Lakes
to the Carolinas to start the workweek, then slides off the East
Coast by Wednesday. This will allow a cold front to lay over the
Mid-South to the Mid-Atlantic on Monday, then stall just to our
south on Tuesday. The front then gradually washes out over the
Southeast Wednesday and Thursday. So Monday looks like another
active day for convection, with PoPs in the mid-chc to likely range.
From there, we keep slightly above climo chc PoPs, due to the front in
the vicinity. However, if enough dry air can filter in from the
north from weak high pressure over the eastern Great Lakes,
convective coverage may end up being lower. Max temps will be near
normal for Monday and Tuesday, then slightly below normal Wednesday
and Thursday. Min temps will be slightly above normal Monday and
Tuesday nights, and near normal Wednesday and Thursday nights.


At KCLT and elsewhere: Latest CAM guidance shows KAVL and KHKY have
the best chance of seeing any convection from the cells developing
this afternoon. Have included VCTS in those locations. Will monitor
the other sites for possible AMD. Should be VFR through the evening
unless a cell moves through. There will be mountain valley fog again
overnight, but per persistence will leave out of KAVL. KHKY could
see fog, especially if a storm moves through. VFR for the rest of
the sites. Cumulus should develop earlier Saturday with winds
picking up as a lee trough develops. May be a better chance of
convection area wide, but will only include a PROB30 for now at KAVL
where chance is better earlier. Light winds early this afternoon
become light S to SW, then light and variable overnight. SW wind for

Outlook: Trends show increasing chances of convection  continuing
into early next week ahead of an approaching cold front on Monday.
Overnight restrictions will continue in the mountain valleys with
chance increasing elsewhere.

Confidence Table...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     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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