Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 161824

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
224 PM EDT Wed Aug 16 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 130 PM EDT: Convection developing along the northern Blue
Ridge at the moment. Latest CAM guidance still shows this convection
developing all along the mountains and moving SE through the
afternoon. Have updated PoP with the latest trends. Moderate CAPE
develops outside of the mountains with the best DCAPE values across
the I-77 corridor. Shear remains minimal, so no organized severe
storms are expected. However, as usual, cannot rule out an isolated
damaging downburst. Highs still look on track.

Otherwise, the southeastern ridge will become resurgent today
through tonight. Under the slowly building upper ridge, lapse rates
and afternoon sbCAPE values should remain relatively modest and the
convection-allowing models show limited coverage this afternoon and
early evening. Still feel that the best scattered shower and
thunderstorm coverage should be along the eastern slopes of the Blue
Ridge and the immediately downstream western foothills during the
mid to late afternoon hours. Max temps this afternoon should reach
the 90s in most areas east of the mountains, and heat index values
should reach 100 in the lower piedmont. Early evening convection
should wrap up quickly tonight. Expect another warm and muggy night
with min temps some 5 to 10 degrees above climo and another round of
patchy fog developing toward daybreak Thursday.


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 CLT and Elsewhere: Latest guidance shows convection developing
along the Blue Ridge and spreading SE across the area but
dissipating before reaching KCLT and KAND. Have followed these
trends for VCTS mention this afternoon. Otherwise, scattered to
broken VFR Cu this afternoon dissipates this evening. Guidance
showing best fog chances at KAVL/KHKY with lesser chances elsewhere.
After fog burns off early Thursday, Cu develops again. Convection
may form before the end of this TAF period, but have left our for
now as better chances just after this period.

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

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High  97%     High  94%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  97%     High  80%     High  89%
KHKY       High 100%     High  97%     High  91%     High  94%
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:




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