Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 261832

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
232 PM EDT Wed Jul 26 2017

A front will move in from the northwest late this week,
increasing chances of showers and thunderstorms across the area.
Cooler and drier conditions will arrive behind it for the weekend.


Isolated convection continues to fire along the southwest Blue Ridge
within a moderately unstable air mass, while additional very
isolated cells are developing across the Upstate. Migrant outflow
boundaries associated with the stronger Blue Ridge activity should
develop and push south in light of moderate/strong dCAPE of around
1000 J/kg. Therefore, coverage should expand across the upper
Savannah River Valley with time, where minimal cloud cover has
allowed temps to soar into the 90s and CAPE to increase to over 2000
J/kg. Convective coverage should be considerably less, but not
necessarily 0 across the remainder of the forecast area, where
lingering low clouds have suppressed temps and buoyancy. Moderate
CAPE and high values of dCAPE will yield the potential for a
microburst or two from the strongest storms, mainly across the
western Upstate and northeast GA.

While the typical downward trend in convective coverage is
anticipated this evening, the flow in the H9-H8 layer is forecast to
veer from SE to S/SW and increase. In fact, ensemble guidance
depicts an anomalously strong southerly component to the H9 flow
late tonight. Meanwhile, a surge of low level Atlantic moisture is
depicted by a consensus of guidance between 03-09Z. The combination
of this increasing moisture, weak upslope/upglide flow, and elevated
buoyancy appears to be the reason for the uptick in qpf response
depicted in most model guidance late tonight. Slight/low chance pops
will therefore be maintained through the overnight, mainly across
the western half of the area, with the highest pops favored for the
southern escarpment.

Moderate to strong instability is forecast to develop again Thu
afternoon, especially across the western half of the area. Morning
clouds and perhaps showers may again delay destabilization, but
probably not to the extent that they did today, owing to the return
of SW low level flow. Meanwhile, higher resolution guidance depicts
an area of height falls approaching the southern Appalachians by
early evening. The result should be an enhancement of the typical
high terrain convective coverage, with 50-60 pops warranted across
the Blue Ridge zones by late afternoon. Steering currents and
outflow boundaries should take this convection at least as far east
as the NC foothills during the late afternoon/early evening, with
pops tapering down to a slight chance across the southeast quarter
or so of the CWA. A few pulse severe storms appear much more likely
on Thursday. In fact, increasing shear profiles could give storms a
little more longevity by the end of the day. Otherwise, max Temps
are expected to rebound to normal levels across the entire area.


As of 215 PM EDT Wednesday: Forecast confidence remains relatively
low with regard to the severe weather potential on Friday. As for
the details, a broad, cyclonically-curved upper jetlet will stretch
out from the mid-Mississippi River Valley through the central and
southern Appalachians Thursday night through Friday as heights fall
upstream. Associated upper level divergence will rapidly improve the
upper-level forcing during this period, while a cold front
approaches from the northwest. A mid-level shortwave will likely
cross the region from the west-northwest Thursday evening into early
Friday morning, with the best evening PoPs stretched out along the
I-40 corridor into the piedmont. Overnight convective chances will
then ramp up in the western mountains given the potential for any
middle Tennessee convection to make a run into the southern
Appalachians before daybreak Friday.

Deep layer forcing will then likely be the strongest just ahead of
the cold front Friday afternoon and evening, but instability remains
the primary question mark as cloud cover ahead of the upstream
convection should thicken up in many locations. The best chance of
moderate or better CAPE with decent insolation should be in the
lower piedmont - generally southeast of the I-85 corridor and east
of I-77 through Friday afternoon/evening. Westerly shear will also
improve through the day Friday, but with surface to 6 km bulk shear
values generally remaining less than 30 kt throughout.  Precipitable
water values will surge above 2 inches in many locales across the
region on Friday, but improving westerly steering flow should help
minimize hydro issues - except where any training occurs. Whether we
end up with a Marginal or a Slight risk of severe thunderstorms
going forward for Friday, the HWO mention still seems warranted
given the potential.

The surface cold front will cross the region from the northwest
Friday night and settle along the southeast piedmont Saturday
morning. Northwest flow moisture will likely linger in the mountain
upslope areas through Saturday morning, where a few showers could
continue, but deep layer drying will steadily work into the region
from the northwest through the day. Isolated to scattered showers,
with a few thunderstorms, could also linger in the lower piedmont
Saturday until the drier air builds in in earnest there late day.
Below climo temperatures are expected each afternoon, with
pre-frontal clouds expected Friday and then cold advection on
Saturday as a deepening east coast trough sets up over the region.


As of 200 PM EDT Wednesday: The medium range is still marked by a
relatively high-amplitude pattern, with persistent troughing over
the eastern CONUS to start the period. The trough will continue to
amplify through the weekend and into early next week, resulting in
temperatures five or more degrees below average.  At the surface, a
surface cold front will have cleared the area by the time the medium
range picks up Saturday night, and confidence is increasing that a
drier airmass will work its way into the southeast over the weekend
and through early next week. With a surface ridge extending its
influence over much of our area as well, convection should remain
almost entirely suppressed Sunday, Monday, and possibly Tuesday as
well. Temperatures will begin to moderate back to normal levels as
the longwave upper trough flattens a bit, with pops making a return
to near seasonal normals by the end of next week.


At KCLT and elsewhere: MVFR cigs, mainly in the 020-030 range will
remain possible at all terminals through 20Z, warranting a tempo at
most sites. Convection remains confined to the high terrain SW of
KAVL as of 1730Z. The extensive cloud cover will continue to limit
the overall coverage of convection into the evening, but a gradual
expansion is expected through 00Z, mainly near KAVL and the Upstate
SC terminals, where VCSH/VCTS is advertised during the late
afternoon/evening. After a brief lull late this evening,
isolated/scattered showers and storms may redevelop after midnight,
mainly across the mtns and foothills of SC and southern NC, as a
southerly flow develops above the surface. A VCSH therefore returns
to KAVL by 09Z. Otherwise, the chances of low cigs/restrictions
appear to be lower Thu morning in comparison with this morning, due
to the loss of easterly upslope flow. Nevertheless, the low levels
will remain quite moist, and MVFR conditions are forecast at most
sites during the pre-dawn hours. Otherwise, NE winds should
gradually weaken and veer to the SE with time.

Outlook: Scattered diurnal convection can be expected again
Thursday, and especially Friday. 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, mainly in
the mountain valleys.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  99%     High 100%     High  99%     High  95%
KGSP       High  99%     High 100%     High  97%     High  94%
KAVL       High  86%     High  89%     High  82%     High  86%
KHKY       High  95%     High 100%     Med   75%     High  87%
KGMU       High 100%     High 100%     High  99%     High  96%
KAND       High 100%     High 100%     High  97%     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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