Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 281036

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
636 AM EDT Fri Jul 28 2017

A strong cold front will cross the region from the west today
and tonight. Cooler and drier air will arrive across the western
Carolinas and northeast Georgia through the weekend and linger
through the early part of next week. More typical summertime
conditions will return about the middle of next week.


630 AM EDT Update...No sigfnt changes to the fcst. Temps and
dewpoints are remaining fairly steady state with upper clouds
persisting. Low end PoPs are in good shape with -shra periodically
developing or pushing across the nrn mtns. An increase in coverage
and a few tstms will move into NE GA and the SW NC mtns by 14z or

Area radar roundup shows a little bit better strength and coverage
of -shra and tstms to the west and north of the CWFA. Expect this
activity to continue advancing toward the area reaching the wrn mtns
arnd 08z and affecting NE GA arnd 14z or so. In the
meantime...expect patchy fg and stcu across the mtn valleys and some
lowering of vsby across the Upstate where low tdd/s are spotty due to
yday/s convec rainfall.

For late this morning and into the afternoon/evening...ulvl s/w
energy will increase in response to a deepening anomalous h5 low
pushing into the OH Valley. Pockets of h5 vort energy will interact
and enhance convec ahead of a broad/wavy cold front crossing the FA
later this afternoon. The op models are a little uncertain as to
the amount of destabilization within the pre/frontal
airmass...however they are coming in line with general timing and
possible areas of organized convec thru the period. With the h5 s/w
advancing...will anticipate deeper tstms to begin developing across
the SW mtns by 17z...advancing east then out of the FA by 23z or

The SPC has lowered the DY1 marginal risk to general most areas
except the nrn tier where the best sbCAPE and shear interaction are
likely to occur. This makes sense as bulk shear will reach arnd 30
kts north and only 15-20 kts south per the latest model consensus on
the synoptic pattern. There will still be some stg to possibly svr
storms crossing NE GA and the Upstate...yet the better organization
and svr activity should occur across the NC fthills then into the
Piedmont including CLT as the afternoon progresses. The convec mode
will likely remain linear with multi-cell cold pools and outflows
producing damaging winds with some large hail possible. The risk for
SCs and tors will be low as llvl shear remains rather unidirectional
and the llvl jet only reaching arnd 20 kts.

The overnight period may see some lingering -shra and a couple tstms
over the far srn and ern zones with perhaps a few upslope -shra over
the wrn mtns. Otherwise...winds will shift n/ly and lowering td/s will
begin mixing in across the nrn zones. With winds remaining elevated
and mixed beneath a likely stcu deck...the possibility for dense fg
will remain fairly low outside sheltered mtn valleys and fthill
areas. Mins will remain above climo by a cat or so in this setup.


As of 300 AM EDT Friday: A pattern change is in store during the
short term, and it should be a welcome change. At the start of the
period, the surface front and associated initial vort max will have
passed just east of the area, with perhaps some lingering precip in
southeast zones, but the upper trough axis and primary vort max will
still be in place over the Appalachians with some northwest flow
showers possible early in the day. This will all dry out through the
day as much, much lower dewpoints advect into the region - at least
10 degrees lower Saturday afternoon than what we should see today.
This will be very noticeable with overnight lows Saturday night at
least 5 degrees below normal; mid 60s at best across the Piedmont
and 50s in the mountains. A nice, nice day expected on Sunday as the
trough digs through, with highs several (5-8) degrees below normal
on the backside of the upper trough with the CAA dominating.
Dewpoints in the 50s and really crazy pleasant for the last day in
July. There is some difference in placement of the upper trough
between the faster ECMWF and slower GFS, so if the latter works out
we could see a few more clouds.

Behind the trough axis, the Canadian surface high pressure will
begin building in more Sunday night into Monday, and though full sun
will allow temps to climb a couple of degrees higher than those on
Sunday, dewpoints will drop even further, with 50s in most areas and
even some 40s dewpoints showing up in the mountains during max
mixing. The upper wave will continue to broaden Monday night into
Tuesday as the surface high slides east across the mountains and
toward the Atlantic, with the airmass finally beginning to modify.
Still a couple of degrees below seasonal normals for Tuesday and
Tuesday night, but continued dry.


As of 315 AM EDT Friday: During the dry short term, a shortwave will
dive out of the Rockies into the Southern Plains, reinforcing and
sharpening the broad trough already in place over the eastern third
of the country. Moisture will lift up from the Gulf, and the high
pressure in place over the western Carolinas and NE GA will continue
to modify, with dewpoints recovering into the 60s everywhere (though
still really not all that bad). A vort max sliding around the flow
at 500mb will push into the mountains, and with slowly increasing
moisture, should be enough for a return of some pops to the higher
terrain Wednesday afternoon. Flow aloft will transition to
southwesterly as the trough digs down the Mississippi Valley,
lifting more moisture out of the Gulf for Thursday. GFS is a little
slower with this return and again keeps convection mainly in the
mountains, but with the uncertainty, have trended back more toward
diurnal pops, but still slightly below seasonal normals for temps
(not too bad of a start to August!).


At KCLT and elsewhere: Morning -shra shud be limited to KHKY and
possibly KAVL and KCLT thru daybreak. Not expecting much lowering of
cigs or vsby thru the early period outside oh KAVL and KGSP due to
upper clouds relatively high sfc tdd/s. KAVL and the Upstate will
see lowering vsby and some stcu this morning due to saturated llvls
and a stagnant airmass. The rest of the TAF period will focus on the
eastward advancement and tstm activity associated with a passing
cold front. With upper Ci thickening and holding on thru much of the
period...think the best chance for restrictions will be with the
storms themselves. Thus...have MVFR vsby included in
afternoon/evening Prob30s all sites. A lowering of cigs is expected
behind the front late in the period to MVFR possibly IFR aft 08z.

Outlook: 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 mainly in the mountain valleys. A typical
summer-time pattern is expected to return by the middle of next week.

Confidence Table...

            10-16Z        16-22Z        22-04Z        04-06Z
KCLT       High 100%     High 100%     High 100%     Med   64%
KGSP       Low   38%     High 100%     Med   77%     Med   61%
KAVL       Med   79%     High 100%     High 100%     Low   45%
KHKY       High  81%     High 100%     High  97%     High  94%
KGMU       Low   55%     High 100%     High  80%     Med   65%
KAND       Low   46%     High 100%     Low   57%     Med   67%

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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