Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210017

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
817 PM EDT Sun Aug 20 2017

Relatively dry High pressure will give way to a warming and
moistening airmass early in the first half of the week. This will
give our region hot and humid conditions ahead of an approaching
cold front which will reach our area Wednesday night or early
Thursday. Expect cooler and drier air from Thursday through next


As of 800 PM EDT: No significant changes needed with this update. A
few isolated showers were seen still bubbling up along the TN/NC
border and across the lower Piedmont. This activity should wane
fairly quickly with loss of heating. Minor tweaks were made to the
temps to line up with latest obs.

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 wasn`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 2:30 PM EDT Sunday: We remain squeezed between weather systems
with the western extent of the Bermuda High holding sway early in
this part of the forecast cycle, but then shifting south and
weakening. Meanwhile we have moisture approaching from both the
south and north.

We will bring POPS down early Monday evening with loss of heating
and instability. We plan to re-introduce POPS in our southwest NC
mountains, with an upslope component developing, late Monday night.
Additionally we are trying to figure out what to do with the
moisture from the low pressure system off the east coast of Florida.
The models want to bring this north in time, and essentially getting
tangled up with other weather parameters coming our way. As a
result, we also have a small chance of precipitation late Monday
night in our northeast Georgia counties.

Tuesday appears fuzzy with ridge top convection early, shifting east
and southeast in a weak flow, or in cold pool driven entities.
Meanwhile moisture will continue to return from the south in some
fashion, which would warrant more POPS. At this point for Tuesday
most POPS remain in the chance category.

We will draw back POPS Tuesday night to near the NC/TN state line.
The next weather maker is part of a large complex upper trough
pivoting across the northern tier of states, and dragging a cold
front our way. Based on the upper level configuration, our Wednesday
forecast follows the slower GFS and European models, and throttled
back the fast paced NAM. This should lead to an active convective
day, if this scenario materializes, with plenty of moisture, a
surface convergence axis, instability, and 0-6KM bulk shear around
30, all in place.

Most spots we bring POPS into the likely category Wednesday. As far
as the temperatures are concerned we see little change, while heat
indices rise into the middle and upper 90s Tuesday and Wednesday
during the afternoon, primarily outside the foothills and


As of 155 PM EDT Sunday: For the medium range period beginning
Wednesday evening, operational models are still in good agreement
on cold front approaching from the NW at 00Z Thursday with good
coverage of convection across the western Carolinas and NE GA. At
00z Thursday, the greatest instability will be along and SE of I-85
where both the GFS and EC have 1400 to 1800 CAPE. The front is
expected to reach the Carolina coast on Thursday with drier, stable
air filtering over our area with fairly strong high pressure
centered west of the Great Lakes. The cold front slows down and
becomes stationary over the weekend from the northern Gulf to across
the Florida peninsula as the cool high becomes centered over New
England by late in the weekend. Once the drier air becomes
established Thursday night onward, dew points are forecast to be in
the 50s mountains and lower to mid 60s foothills and piedmont.
Expect some small increase in clouds by late in the weekend as the
center of the high shifts east of our longitude and some mid level
return flow starts.

Temperatures will be from 3 to 6 degrees below normal from Thursday
through the weekend.


At KCLT and elsewhere: A few lingering cu and isolated showers in
the western NC mountains and across the lower piedmont and midlands
as the sun sets. This activity is not expected to reach any of the
TAF sites as it should wane quickly. Overnight should then 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. Then on to
eclipse day. 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...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High 100%     High  97%     High 100%
KGSP       High 100%     High  97%     High  97%     High 100%
KAVL       High  97%     Med   64%     High  92%     High 100%
KHKY       High 100%     High  94%     High  97%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  97%     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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