Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 241835

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
235 PM EDT WED AUG 24 2016

High pressure will persist over the region for the next few days,
allowing temperatures to climb back above normal. Another weak
surface front will approach the Western Carolinas from the northwest
toward the end of the work week, with high pressure building back to
our north over the weekend. Rain chances will remain below normal
through the beginning of next week. Another front will approach the
area by the middle of the next work week.


As of 215 PM: A broken stratocu deck lingers over the wrn two-thirds
of the CWFA, evidently being maintained by weak westerly upglide
over the high pressure over the Eastern Seaboard. The deck
is becoming more cellular on visible satellite, but in some
places is being replaced with diurnal cumulus as fast as it is
dissolving. Isolated showers are also developing within the deck
as destabilization occurs. Models have backed off their depictions
of SHRA/TSRA coverage over the high terrain, given the delayed
warming, but there still is potential for widely scattered activity
to develop there.

The pattern remains largely the same tonight, with low level winds
veering in response to a trough moving thru the northeastern CONUS,
but still generating weak isentropic lift and bearing a fair amount
of moisture. Patchy midlevel clouds thus are expected to persist,
albeit not as uniform as what we experienced this morning. Guidance
consensus develops fog in the mountain valleys and portions of
the foothills before dawn, which in light of limited mixing today
is plausible as long as the cloud decks remain broken. Min temps
should be a tad above climo. For tomorrow, I was a little reluctant
to really clear skies out quickly seeing how it developed this
morning, seeing as the pattern will have changed so little. One
difference is that winds will be more downslope, which could
help speed erosion. Once again the mountains are expected to
destabilize sufficiently to expect scattered convection, with the
Piedmont still seeing the limiting effect of subsidence within
the ridge airmass. So PoPs are limited to the high terrain. Max
temps are expected to be near climo, a bit warmer than today with
the expectation of less cloudiness.


As of 215 PM EDT Wednesday...the late part of the week still looks
relatively quiet as a mid/upper anticyclone/ridge moves overhead on
Friday. This should keep some warmer air aloft and convective
inhibition that will help to put a damper on the amount of deep
convection. Thus, have limited the precip probability to the low end
of the chance range and only over the higher terrain. Temps will
climb back above normal. Starting on Saturday, uncertainty ramps up
with regard to the effect of the anticyclone center moving off to
the east and the position of a weak front drifting down from the N.
The operational GFS is more responsive than the other models,
perhaps because of a stronger SE flow at low levels which brings in
Atlantic moisture, while the operational NAM might be too dry
because it drives the surface boundary to our S and actually has a
wedge-like configuration by late afternoon. The official forecast
still favors the higher terrain with a slightly below climo chance
of precip, but allows for isolated storms to move out across the
I-40 corridor in the afternoon and the rest of the Piedmont/Upstate
in the evening. Not sure yet how this will trend, but still expect
precip chances to be diurnal in nature. Temps will depend on the
ability of the shallow surface boundary to slip southward. The
chance for severe storms looks low.


As of 230 PM EDT Wednesday...the extended forecast begins 12Z Sunday
with a large upper ridge over the southeast beginning to flatten.
Model guidance diverges almost immediately with the handling of this
ridge and subsequent features, so the forecast consists of a multi-
model blend that is weighted towards the ECMWF, which has displayed
better run-to-run consistency than the rest of the global model
suite over the past few days. The upper ridge does maintain a
warming and drying influence over the southern Appalachians into
early next week, though it begins to decay somewhat Sunday and
Monday. Pops were therefore kept below climo through early next
week, and maximum temperatures were kept a few degrees above climo.
A surface frontal boundary will begin to approach the area next
Tuesday, though its timing and the amount of moisture it has to work
with are dependent on the track and intensity of a potential
tropical system that is being investigated by the National Hurricane
Center. Most models now show some development of the system, and
though the tracks of the system in the major models are currently
quite different, it appears that no direct impact from it will occur
in our area through the next seven days. The ECMWF allows for more
moisture return into the southeast, so pops were elevated to
near/just above climo by the middle of next week. Overall, little
change was made to the previous medium range forecast, especially
considering the large amount of uncertainty in global model
solutions at the current time.


At KCLT and elsewhere: Offshore Atlantic high pressure will
generally maintain southerly surface flow over the area thru
Thursday. However, moist west-southwest flow atop this airmass
has brought a persistent area of stratocumulus to the southern
Appalachians and vicinity. These clouds are showing signs of
breaking up and lifting at 18z, but will persist in some places
thru evening. Sufficient destabilization is expected over the
mtns to prompt VCTS inclusion at KAVL. Winds aloft will veer and
bring in additional moisture overnight. Low VFR cigs are likely
to come and go throughout the period. Models depict only a small
amount of restrictive clouds overnight, generally favoring the
production of fog instead. With so much higher cloud cover present,
and uncertainty about how much mixing/drying will occur in the
low levels this aftn, fog mention is included only in TEMPO at
KAVL/KHKY; at KAND nearby soaking rains earlier today will play
a factor.

Outlook: A deep ridge will remain in place over the Southeast thru
the weekend, keeping afternoon SHRA/TSRA chances below normal each
day, and generally confined to the mountains. However, sfc moisture
will gradually return, and morning vsby restrictions will become
increasingly likely during this time.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High  97%     High  80%     High  91%
KAVL       High  98%     High  94%     High  84%     High  84%
KHKY       High 100%     High 100%     High 100%     High  91%
KGMU       High 100%     High  97%     Med   75%     High  91%
KAND       High 100%     High 100%     High  86%     High  88%

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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