Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 170208

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1008 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 10 PM EDT: Isolated showers persist around the region, feeding
off the residual surface-based CAPE. Will have to make some minor
changes to account for this, mainly to extend the slight chance for
a few more hours. Still think that convective activity will diminish
in the early morning hours. Temp trends look fairly good. Otherwise,
guidance is showing some fog signals by morning as the debris
scatters out. However, have limited mention to the usual locations,
mountain valleys and NC Foothills and NW Piedmont for now. Lows will
up to 5 degrees above normal.

Expect another round of convection Thursday as a weak boundary moves
in from the west even with a slowly building upper ridge. Moderate
instability develops again with continued good low level moisture
and warm temps. An isolated severe storm cannot be ruled out with
moderate DCAPE values, but chance remains low. High PW values
continue keeping a threat of isolated heavy rainfall in place. Highs
will be up to 5 degrees above normal.


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: A few showers and storms still remain at this
hour, but the downward trend seen on radar is expected to continue
after sunset. The NAM guidance is most likely overdone with holding
onto scattered storms through the evening. All TAF sites will be
VFR through the evening and into the early morning hours. Only KAVL
and KHKY get a VCSH. Later tonite...guidance shows best fog chances
at KAVL/KHKY with lesser chances elsewhere, as per usual. Have
only included the fog at KAVL at this time, but will watch obs
trend at KHKY for amendment later this evening. After fog burns
off early Thursday, Cu develops again. Convection will probably
not develop until after 18Z in most places as a short wave ridge
moves overhead early in the day. Prefer a PROB30 at all sites to
handle this possibility, but storms are most likely over the mtns.

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

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High  93%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High  95%     High  91%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High  95%     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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