Area Forecast Discussion
Issued by NWS Greer, SC

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS62 KGSP 270229

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1029 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.


As of 1010 PM: Cloud cover along an outflow boundary stretches
from the Georgia mountains to near Lake Wylie attm. A couple of
showers have formed near this boundary since sunset, but have
been brief. Low-level flow has already veered to SE following
high pressure moving off the Northeast Coast and is expected to
continue veering overnight. Short-term guidance generally depicts
occasional/isolated development as this flow and lingering mesoscale
boundaries allow elevated instability to be realized. Consensus is
greatest on activity firing near the southern-facing Blue Ridge
Escarpment west of Hendersonville, and to a lesser extent along
the northern Blue Ridge. I have revised PoPs to initially focus
along the aforementioned outflow boundary but still transition to
a Blue Ridge focus by early morning. Dewpoints remain elevated
and did not mix out much today. Patchy fog is likely, but with
the southerly flow being associated with some upglide, expanding
cloudiness is expected to limit radiation and keep vsbys from
being problematic. Min temps have been raised a bit in light of
the juicy conditions.

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: Guidance generally has trended toward fewer
restrictions overnight, perhaps reflecting the lack of convection
today and drier soils as a consequence. However it is still
expected that low level winds will veer and pick up a bit overnight,
resulting in isentropic upglide and some VFR level cloudiness. This
should keep radiation fog at bay outside the deeper mountain
valleys. Near the Blue Ridge, several sources continue to depict
MVFR stratus forming with upslope enhancement toward dawn. Thus
a mention is still warranted at KAVL/KHKY. Isolated showers could
also regenerate overnight due to elevated buoyancy along and south
of the Blue Ridge; this is chiefly handled with VCSH though it was
included in the TEMPO at KAVL. Diurnal convection will develop
tomorrow afternoon, with chances close to climatology. Improved
shear could allow some clustering along outflow boundaries, which
could threaten all sites with wind shifts if not squalls.

Outlook: Diurnal convection and a threat of strong to severe
wind gusts will return 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...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High  90%     High 100%     High 100%
KAVL       High  95%     Low   45%     Med   62%     Med   60%
KHKY       High 100%     Low   58%     Med   65%     High 100%
KGMU       High 100%     High  94%     High 100%     High 100%
KAND       High 100%     High  91%     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:




NEAR TERM...JDL/Wimberley
LONG TERM...Carroll
AVIATION...Wimberley is the U.S. government's official web portal to all federal, state and local government web resources and services.