Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191830

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
230 PM EDT Tue Sep 19 2017

A weak trough will cross the forecast area from the west tonight
and tomorrow. Broad high pressure will then return over the region
for Thursday and Friday, with more moisture in the easterly flow.
Drier and stronger high pressure will settle over the region during
the weekend as both Hurricane Jose and Maria remain well offshore
in the western Atlantic.


As of 220 PM: Water vapor imagery indicates a shortwave trough
over the middle Ohio Valley this aftn, with a small surface
reflection and cold front driving showers south from there into
Alabama. Closer to home, weak high pressure remains dominant over
the southern Appalachian region. Abundant shallow cumulus exist
over the Piedmont with more congested clouds appearing over the
mountain ridges. Though profiles initially feature only minor
sfc based instability, as the shortwave advects east and temps
continue to warm at the sfc, slightly better convective conditions
exist later this aftn. Hi-res models indicate convection remaining
isolated, except over the Blue Ridge. A couple rumbles of thunder
are possible there. A few sprinkles are possible over the cloudier
areas of the Piedmont even if accumulating showers remain confined
to the Blue Ridge and adjacent foothills.

Tonight, as heights continue to fall ahead of the advancing
shortwave, expect patchy midlevel cloud cover. Though some
model profiles reflect small amounts of elevated instability,
particularly over the northwestern zones, the tiny amount of DPVA
in play warrants only a carry-over of the aftn convection into
the evening. For the most part it looks to be a dry night. Min
temps should be a bit warmer than last night given more cloud
cover. Patchy fog again is expected in the mtn valleys and

The axis of the shortwave will arrive in the area tomorrow morning,
maintaining fairly good lapse rates aloft and perhaps enhanced
lift via DPVA. Model consensus indicates SBCAPE values will near
1000 J/kg in parts of the area. Deep layer shear is not indicative
of severe threat, though showers and storms may cluster similar
to how they are behaving in the Ohio Valley today. Slight-chance
to chance range PoPs are included for the whole CWFA. Max temps
will remain quite warm, in the mid to upper 80s in the Piedmont
and lower 80s mountain valleys.


As of 220 PM EDT Tuesday: A weak upper trof over the area Thursday
closes off into a weak upper low Friday. Weak short waves rotate
through the trof and around the upper low both days. At the surface,
weak high pressure remains centered over the central Appalachians
through the period. A weak lee trof develops Thursday then
dissipates Friday. Low level moisture remains relatively high
Thursday then increases slightly Friday on developing easterly flow.
Mid level lapse rates remain relatively steep as well with cooler
mid level temps from the trof and upper low. The result of all this
is increasing chances of generally diurnal convection each day. Even
with overall better chances Friday, the mountains have the best
chance both days where LFC values are lower and low level
convergence better. Highs 5 to 10 degrees above normal Thursday drop
a few degrees Friday. Lows remain nearly steady around 5 degrees
above normal.


As of 205 PM EDT Tuesday: the extended fcst picks up at 00z on
Saturday with impressive upper ridging in place over the eastern
2/3 of the CONUS and steep upper trofing over the West Coast.
The trof will gradually slide eastward thru the period, however
the ridge will maintain its dominance over our region thru early
next week. At the sfc, very broad high pressure will be in place
to start the period. The remnants of TS Jose will still be
lingering off the New England Coast while TS Maria is forecast
to approach the Bahamas by early Sat. Maria is expected to remain
far enough off the east coast to not have any significant impacts
on our fcst area. We will remain under broad high pressure thru
day 7 with predominately dry, N to NELY low-lvl flow expected to
persist. As such, the sensible forecast is mostly dry with
temperatures about a category above normal through the period.


At KCLT and elsewhere: This afternoon a low VFR cu field will
be present over all sites. Any showers that develop will remain
isolated so no precip mention was made. A shortwave trough will
move overhead tonight and is expected to bring increasing amounts
of midlevel clouds. Winds will trend toward SW in the late aftn,
possibly going VRB overnight, though the shortwave should reinforce
the SW flow. Mountain valley fog will develop once again, but
with crossover temps being higher and guidance being in better
agreement than for last night, a restriction was included at KAVL.

Outlook: Chances for diurnally favored showers and possibly a
thunderstorm will increase each day this week.  The best chances
look to be Thursday and Friday as flow veers easterly and marginal
moisture advection returns across the region.  That said, the best
chances for restrictions will be from early morning fog across the
northern NC Piedmont and foothills, as well as in the mtn valleys.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High  97%     High  94%
KAVL       High 100%     High  97%     High  84%     High  92%
KHKY       High 100%     High 100%     High  97%     High  91%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  97%     High  94%

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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