Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211043

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
643 AM EDT Fri Jul 21 2017

An upper level ridge over the center of the country will continue to
bring hot temperatures and isolated showers and thunderstorms
through today. A broad upper level trough then develops over the
weekend and into early next week and is expected to bring cooler
temperatures along with greater chances for rain.


As of 630 AM EDT Friday: Relatively quiet at daybreak. Satellite
imagery shows some valley fog again this morning. It should burn
off by 14Z at the latest. Temp trends look ok.

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 bouyancy. 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: VFR through the period at all sites except
KAVL, where at least some MVFR fog around sunrise can be expected
as the mountain valleys see another round of fog/low stratus. Only
cirrus expected until midday when the convective temp is reached
and scattered high based stratocu develop. The models are a bit
more interested in convection across the higher terrain for the
afternoon, so a PROB30 was employed at KAVL. Weak pressure pattern
has led to VRB winds, but should generally favor the S side tonight,
with brief veering to NW Friday morning, and back SW Friday midday.

Outlook: Chances for diurnal convection begin a steady increase on
Saturday, with this trend 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...

            10-16Z        16-22Z        22-04Z        04-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  83%     High 100%     High 100%     High  91%
KHKY       High  91%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  91%     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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