Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 221058

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
658 AM EDT Tue Aug 22 2017

Typical late summer conditions will persist ahead of an approaching
cold front, which will reach our area tomorrow. A large region of
surface high pressure will then gradually build into the area late
this week through the weekend, bringing unseasonably cool and drier


As of 620 AM EDT: conditions are calm over the fcst area with only
a few isolated low and mid lvl clouds lingering over the CWFA.
Patchy fog has developed over the mountain valleys and parts of
the Foothills and Upstate but should dissipate by 14z or so.
Otherwise, minimum temps will remain about 3 to 6 degrees above
climatology. Later today, the upper ridge will weaken a bit as a
broad upper trof moves over the Great Lakes and is slow to move
farther eastward. This should help promote a broad area of upper
lvl divergence to our north and favor convection across the higher
terrain based on a diurnal trend. Propagation into the foothills
and piedmont will be possible later in the day/evening as long as
ample instability persists. Temperatures should remain a few degrees
above normal thru the period.


As of 3:15 AM Tuesday: Frontal boundary and perhaps associated
convection should be moving into the forecast area at the beginning
of the period, before the front "jumps" into the lee trough position
by afternoon. Convective trends Wed afternoon will depend largely on
how much convection and attendant cloud cover is impacting the
forecast area Wed morning. Since that is the subject of much
uncertainty, model guidance (both the traditional operational
guidance and convection-allowing models) are at odds as to how
trends will evolve during the afternoon, with some guidance
suggesting that limited heating/destabilization will preclude much
in the way of initiation outside the mtns Wed afternoon.
Nevertheless, the Piedmont should see sufficient heating to realize
moderate instability during the afternoon, with the front/lee trough
acting as a focus for at least scattered convection, and most of the
forecast area receives 50-60 pops on Wed. Increasing height gradient
aloft will result in improving shear profiles (albeit still less
than 30 kts in the 0-6km layer). The combination of modest shear and
modest instability will yield a modest severe weather threat,
especially across the Piedmont, and the SPC Day2 marginal risk
across our far eastern areas appears well-placed.

Drier/more stable post-frontal air will gradually chase the
convection out of the area Wed night/early Thursday. After some
disagreement among yesterday`s model guidance regarding the
southward penetration of the front, and/or how quickly a moist
easterly flow would develop north of the boundary, there is now a
strong consensus that the period from Thu morning through Fri
morning will be dry, with temperatures averaging a few degrees below


As of 335 AM Tuesday: A long wave trough will remain the dominant
synoptic scale feature impacting the weather across much of the East
Coast through the medium range. Mid/upper level confluent flow into
the base of the trough will anchor expansive surface high pressure
across the northeast quadrant of the country through much of the
period, resulting in an increasingly moist low level easterly flow
across our area. This may allow some showers or light precip to
develop in the vicinity of the Blue Ridge over the weekend, although
that`s hardly a given, and only token small pops will be carried in
those areas. By far the greatest impact of the anomalously low
heights and easterly flow will be on temperatures, which are expected
to be around 5 or more degrees below normal through the period. By
the end of the forecast cycle, our attention will be turning to the
tropics, as it`s becoming clearer that the Southeast will be
sandwiched between tropical cyclone Harvey (likely somewhere in the
vicinity of the northwest Gulf of Mexico) and another tropical
cyclone off the Southeast Coast.


At KCLT and Elsewhere: expect VFR conditions thru the 12z taf
period with the exception of continuing restrictions at KAVL
and KHKY for the first few hours of the period this morning.
I expect that both sites should recover to VFR by 14z or so
with KHKY possibly coming up a bit sooner. In addition, KAVL
could see some thunderstorm activity later this afternoon/evening
with associated restrictions. I mentioned VCTS from 19 to 21z
and then a PROB30 for TSRA from roughly 21z to 02z overnight.
KAVL could also see another round of fog and/or low stratus
tomorrow morning during the last few hours of the taf period.
Otherwise, moisture will gradually increase over the region
later today/tonight ahead of an approaching cold front, with
surface winds turning southwest and slowly increasing. Expect
predominately VFR cumulus from daytime heating with bases in
the 4000 to 6000ft range.

Outlook: chances for diurnal convection and morning mtn valley
fog/low stratus gradually increase through midweek, with the best
chances expected on Wednesday as a cold front pushes into the area.
The probability of restrictions and diurnal convection should
diminish again Thursday and Friday.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High  94%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       Low   41%     High 100%     High 100%     High 100%
KHKY       High  89%     High 100%     High 100%     High 100%
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:




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