Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211434

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1034 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 1030 AM EDT Friday: Going forecast generally on track. Morning
fog/stratus has burned off and temps are heating fast. Still expect
most locations to remain below heat index values of 105 even with
mid 90 temps as dew points follow a nearly normal diurnal mixing
trend. MCS currently crossing the OH valley is dissipating, but the
associated outflow boundaries or MCV will drop into the area later
this afternoon and evening. Latest CAM guidance shows convection
developing across the NC mountains then dropping S and SW into the
NE GA and Upstate mountains before dissipating. Cannot rule out some
of the convection moving SE into the foothills and Piedmont of all 3
states. However, the general consensus is that the convection
dissipates before moving into the NC Piedmont or remains to the
west. Therefore, will retain the current PoP trend of scattered
convection mountains and isolated elsewhere. Moderate instability
and very high DCAPE values are expected again today. Freezing levels
are still quite warm, 15k ft, but a little lower than Thu with a
continued large difference between them and wet bulb zero levels.
Mid level lapse rates remain weak. Therefore, isolated severe storms
are more likely today given the increased coverage, but storms will
have to become quite tall with large icy cores to overcome the
hostile conditions.

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: VFR through the rest of the period as morning
fog/stratus has burned off.Generally 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, with
brief veering to NW early this 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...

            14-20Z        20-02Z        02-08Z        08-12Z
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  95%
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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