Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210754

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
354 AM EDT Mon Aug 21 2017

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


As of 2am EDT Monday:  Going forecast looks pretty good, with only
some minor changes to temperatures and some adjustments for latest
radar trend.  Some light to moderate showers the last few hours have
finally taken the advice of the HRRR and died-out with only some
very light precip. still persisting in Mecklenburg and Cabarrus

Otherwise, an upper trough has pushed to the east, while Atlantic
ridging is building back into the area. High pressure stretching
down the Appalachians will slide east tonight. A shortwave in the
flow will pass to the north tomorrow morning, and a weakness in the
subtropical ridge just off the SC coast (leftover from the front),
with a bit of a surface reflection and attendant moisture, will be
pulled inland around the upper anticyclonic flow, increasing
moisture during the day tomorrow. Low-level moisture should pool up
against the escarpment and cannot rule out some patchy fog Monday
morning, with of course conditions a little more favorable for
mountain valley fog and low stratus.

For Monday, the pattern remains very similar to today, but with
increased moisture. NAM bufr soundings and other NAM-derived
parameters are a little more pessimistic on the cloud cover front
with near-saturation progged between 800-850mb. ECMWF and especially
GFS are much more optimistic with the higher RH airmass remaining
slightly to the southeast. The latter both have a 850-700mb winds
swinging around, with the GFS actually very weak NWLY around eclipse
time, leading to some very weak downsloping. I suppose all this is a
fancy way of saying that overall, there really hasn`t been much
change to the sky cover forecast, with most coverage remaining over
the mountains where convection will be again favored, and another
area of higher coverage across the southern Piedmont. Did actually
bump up percentages just a tad from previous forecasts along the I-
85 corridor but it will still be the favored area with least cloud
coverage. Convective coverage across the mountains should be greater
than this afternoon, and have kept trend of chance pops across the
higher elevations of the Balsams and Blacks, but generally slight
chance everywhere. Some hi-res guidance just trickling in is showing
that the escarpment (especially the NC/SC border area) would be
favored tomorrow rather than the higher ridges in the NC mountains,
but will have to continue to reevaluate that. Bottom line, eclipse
viewing is gonna be hit and miss with typical summertime afternoon
Cu development and possibly shower/thunderstorm development across
the mountains, but at least across the Piedmont and especially the I-
85 corridor, things look fairly optimistic for viewing in most
areas; since totality is about 2.5min that`s generally enough time
for any individual cloud to move enough that it wouldn`t completely
block any one spot, but with such weak steering flow that would be
the main concern.

As if a huge long paragraph about clouds and isolated-to-scattered
convection awns`t enough, temperatures will continue to be a concern
tomorrow.  Forecast highs haven`t changed much the past few days,
generally in the lower 90s across the Piedmont. Past data show a bit
of a lag in temperature reduction in areas of shadow - and while not
all in totality, most of our area will see significantly reduced
insolation during peak heating tomorrow, the warming up process
should be stunted somewhat. Nevertheless, with increasing low-level
moisture, heat indices will be pushing 100 degrees in our extreme
southern zones, and mid 90s across the I-85 corridor. These values
are certainly below our heat advisory criteria but as we`ve been
advertising for a while, residents and visitors across the area
should still take proper heat safety precautions, and most notably
stay hydrated.


As of 320 AM Monday: Heights will fall across the eastern Conus
through the short term, as a strong short wave trough sweeps across
the northern Great Lakes/northern New England. This will introduce a
cold front into the southern Appalachians and surrounding areas
during the Wed/Wed night time frame. In the interim, Tues looks to
be a standard late summer day across the forecast area, with
seasonal levels of heat/humidity/instability supporting scattered
coverage of deep convection, initially across the high terrain
during early/mid afternoon, but gradually expanding into the
valleys/foothills/Piedmont from mid-afternoon through early evening.
30-40 pops are advertised for the mountain and NC foothills zones,
with slight chances across the remainder of the area.

Pops will be on the incline Wednesday, as the cold front and
attendant deep moisture likely push into the forecast area by
afternoon. Mid-level lapse rates will remain weak, but steeper than
has been seen in recent days, and this should yield an afternoon of
at least moderate sbCAPE values. Meanwhile, the increasing height
gradient aloft will result in improving shear parameters, although
deep layer shear will likely peak at less than 30 kts. Nevertheless,
scattered to numerous coverage of convection, along with elevated
potential for some degree of organization will likely yield a
low-end severe thunderstorm threat Wed PM. Coverage of convection
should gradually diminish Wed night, as drier/cooler air gradually
oozes into the area in the wake of the cold front.


As of 335 AM Monday: The upper trough will be well-established
across the eastern Conus early in the medium range, and will be slow
to relinquish its influence through the period. Confluent mid-level
flow into the base of the trough will support sprawling surface high
pressure that will impact much of the East Coast through the period.
The only real controversy should exist early in the period regarding
how quickly to remove pops in the wake of the short term cold front.
Most of the global model guidance quickly develops an easterly flow
regime across our area, with lingering low level moisture possibly
resulting in considerable cloud cover as well as precipitation
potential late in the work week. This scenario is especially
prevalent in the latest GFS guidance, while other global model
guidance is quite muted with any qpf response after Thu morning. The
official forecast will indicate token small pops Thursday, tapering
down to less than slight chances by Friday.

Otherwise, conditions will dry/clear out into the weekend, with
anomalously low thickness values supporting temps 5-10 degrees below
climo, while increasingly dry air will act to limit the potential
for diurnal convection through the period.


At KCLT and elsewhere: A few isolated showers have died-out across
the region over the last couple hours.  Overnight should be mostly
clear with light winds. Winds are rather variable at KCLT this
evening, but guidance insists it should turn back out of the SE by
start of TAF period. Otherwise, mountain valley fog is expected, as
moisture has increased since yesterday. The Piedmont should be VFR,
but there may be some patchy MVFR BR or HZ near daybreak. It
looks like a cu field should develop around 16-17z, mainly FEW-SCT,
but possibly some BKN in the NC mountains. Isolated showers and
tstms possible in the AFTN, again mainly in the mtns. No mention
will be carried in the TAFs at this time, due to the low coverage.
Winds will start out VRB or favoring ENE, then switching out of the
SE by midday, picking up to around 5-8 kts.

Outlook: Chances for diurnal convection and morning mtn valley
fog/low stratus gradually increase during through midweek, with the
best chances expected Wednesday, as a cold front pushes into the
area. Chances for restrictions and diurnal convection may diminish
again during late week.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High  95%     High 100%     High 100%     High 100%
KGSP       High  95%     High 100%     High 100%     High 100%
KAVL       High  91%     High 100%     High 100%     High  93%
KHKY       High  87%     High 100%     High 100%     High 100%
KGMU       High  95%     High 100%     High 100%     High 100%
KAND       High  95%     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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