Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 170608

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
208 AM EDT Thu Aug 17 2017

High pressure will gradually erode over the next few days, though
warmer than normal conditions will remain. Moist and unsettled
weather will return late week and into the weekend. Some drying
is favored by Sunday with a return to moist conditions to start
the new work week.


As of 145 AM EDT: The latest HRRR runs have scattered weak
convection developing along and east of the I-77 corridor through
09Z this morning where LAPS sbCAPE values above 1000 J/kg linger.
Any activity surviving into our eastern piedmont should dissipate or
move east before sunrise. The potential for fog development looks a
bit better than Wednesday morning, with surface condensation
pressure deficits likely 5 mb or less by daybreak over the
mountains, foothills, and NW NC piedmont. Will advertise patchy to
areas of fog at this point, but any patchy dense fog should occur
mainly in the mountain river valleys through 13Z. Some slightly
better moisture may start arriving in the western NC mountains
around daybreak, but with any PoP confined to isolated coverage.

The convection-allowing models today have mountain showers and
thunderstorms developing from 18Z to 20Z this afternoon in a region
of better low level wind field convergence over the high terrain.
This activity should move east off the Blue Ridge around 20Z to 21Z.
A secondary axis of low level convergence should impact piedmont
sections east of the forecast area, so piedmont coverage will depend
on the mountain activity surviving east. 850 to 500 mb lapse rates
appear a slight tick higher this afternoon than yesterday, with
sbCAPE values at 2000+ J/kg, so an isolated severe storm cannot be
ruled out. Precipitable water values remain above climo, but storms
should move along steadily from west to east to keep any excessive
rain/flooding problems isolated in nature. Heat will be an issue
this afternoon with many piedmont temperatures in the low to mid
90s, with dewpoints in the lower 70s. Will likely post an SPS today
for piedmont Heat Index values in excess of 100 degrees at times.

The shortwave ridge over the region late today will move east
tonight, with heights falling once again from the west overnight.
Precipitation chances will increase along the spine of the southern
Appalachians during the early morning hours as deeper moisture
begins to push back in. Expect continued mild and muggy min temps
early Friday, with mountain valley fog and low stratus redeveloping.


As of 100 PM Wed: As low pressure crosses the northern Great Lakes
Thursday night, heights will begin to fall over our region. During
the evening, several guidance sources depict convective activity
continuing to propagate thru the Piedmont, mainly over the NC
zones, given the likelihood of persistent outflows still with
favorable mid-upper lapse rates. Meanwhile, low-level flow will
veer, becoming west-southwesterly into the Appalachians by Friday
morning. This could lead to some redevelopment near the Tennessee
border overnight.

The cold front associated with that low appears poised to enter
the WNC mountains Friday afternoon, subsequently progressing across
the CWFA thru the evening and overnight. Hot and humid conditions
will precede the front with dewpoints easily reaching the lower
70s in much of the Piedmont, and heat indices above 100 in the
warmer spots. Model QPF response generally features a local max
over the mountains during the afternoon, with a second max over
our eastern zones and central NC during the very late afternoon
or evening. 0-6km shear will be less than 20 kt and instability
in the 1000-2000 J/kg range, as might be expected this time of
year. Profiles are not excessively moist and it is reasonable to
expect a couple of severe wind producing cells will develop. For now
I will discount the pronounced bullseye in QPF traversing the area
on the NAM, which looks to have resulted from convective feedback.

Partial clearing is expected Friday night as a weak sfc high builds
into the area out of the Mississippi Valley. This most likely will
suppress convection through Saturday over the vast majority of the
CWFA. A small PoP is retained southeast of I-85 where the influence
of the high will be least, and because a few SREF members still show
convection firing there during the day. Max temps Saturday look
to be only slightly cooler than Friday, but with dewpoints having
dropped off a bit, apparent temps will be a tad more comfortable,
with heat indices remaining below 100.


As of 230 PM EDT Wednesday: The medium range picks up Saturday
night with an upper shortwave trough shifting eastward towards
the spine of the Appalachians as a surface frontal boundary lays
draped over the lowcountry of South Carolina, providing a focus
for shower and thunderstorm activity to our south. The shortwave
will clip our area Saturday night into Sunday morning before the
upper flow transitions to a more zonal pattern by Sunday afternoon
and evening. At the surface, a few showers and thunderstorms may
result overnight Saturday, with some slightly drier air able to
work into the area by Sunday afternoon as a subtropical ridge
begins to strengthen over the Atlantic. Global models are in
decent agreement at the this point that convection on Sunday stays
generally suppressed through the evening hours.

By Monday, persistent southerly surface flow will moisten much
of the area, bringing precipitable water values back to near
climatological levels. The GFS, having generally been the driest
global model thus far, has continued its trend towards a stronger
upper high over the southeast and likely slightly better convective
suppression during the day on Monday. In latest model runs, the
previously-discussed surface front therefore remains over the SC
coastal plain instead of retrograding back up towards the upstate.
As a result, cloud cover and pops around eclipse time were slightly
decreased from the last forecast package...though with continued
model disagreement over the strength and positioning of the upper
ridge and the location of the surface front, confidence in the
forecast remains rather low at this point. The current package
now features a forecast that is very close to climatology for
late August. The evening hours will continue to feature chances
for scattered showers and thunderstorms. High temperatures will
be in the mid 80s in the NC, SC, and GA mountains and close to 90
in our Piedmont and Foothills areas, with heat indices in the mid
90s across much of the area.

The airmass will continue to moisten in our area as southerly
flow persists through Tuesday. A more substantial upper trough
will arrive over the east coast late Tuesday as a better defined
surface front makes its way towards the Appalachians. The front`s
progress, though steady, will not be particularly fast, and enhanced
shower and thunderstorm activity will result over most of our area
both Tuesday and Wednesday afternoon. Much drier air will sweep
in behind the front on Thursday, resulting in what may be a dry
forecast through the day.


At KCLT: A few showers may linger in the vicinity of the I-77
corridor early this morning, but chances of precipitation at the
airfield look rather small. Cannot rule out some low cloud and fog
restrictions toward daybreak, but a brief window of MVFR should be
the best forecast. Anticipate slowly building cumulus today, with
late day chances of convection from the west as thunderstorms build
east off the Blue Ridge. Light winds will become steady SW through
the day.

Elsewhere: KAVL will see the best chance of low stratus and fog
problems this morning with very low condensation pressure deficits
over the mountains. The foothills may see patchy fog and low clouds
as well, with KHKY having the next best chance of daybreak
restrictions. Otherwise, cumulus will build today with mountain
showers and thunderstorms developing 18Z to 20Z, with TEMPO TSRA
most likely from KAVL to KHKY, and PROB30 after 19Z for the Upstate
SC TAFs. Expect light winds to toggle southwest through the day, but
remain less than 10 kt. Any late day thunderstorms should move east
this evening with just lingering debris clouds around.

Outlook: The transition to a more typical late summer pattern
continues Friday through early next week. Expect scattered afternoon
convection and morning low stratus and patchy fog each day,
especially in the mountain valleys and in locations seeing
appreciable rainfall.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  97%     High  92%     High 100%     High 100%
KGSP       High  89%     High  94%     High 100%     High 100%
KAVL       Low   24%     High  80%     High 100%     High 100%
KHKY       Med   65%     High  91%     High 100%     High 100%
KGMU       High  97%     High  94%     High 100%     High 100%
KAND       High  89%     High  94%     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:




SHORT TERM...Wimberley
LONG TERM...Carroll
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