Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201050

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
650 AM EDT Fri Oct 20 2017

High pressure will remain in place across our region into the start
of the upcoming weekend. Temperatures will gradually increase a few
degrees during that time. Meanwhile, a low pressure system will
develop over the lower Mississippi River Valley late in the weekend
and move across the southeast early next week. A cold front
associated with this system will cross the region Tuesday, and
cooler air will spill into the area through mid-week.


As of 630 AM EDT Friday:  Update discussion for 12z taf
issuance.  Otherwise, patchy fog has developed in some of the mtn
valleys and in the vicinity of the lakes as expected. Said fog
should burn off shortly after sunrise. The current near term portion
of the fcst remains on track therefore only minor tweaks to t/td
were made for this update.

As of 320 AM EDT Friday:  Upper ridging will continue to build
across the region through the near term fcst period with surface
high pressure setup atop the southern Appalachians.  Current
conditions across the region reflect optimal radiational cooling
with temperatures in the 30s across the high terrain and 40s
elsewhere.  Latest GOES16 fog products favor some patchy mtn valley
fog at this hour, with a bit of fog in the vicinity of the regional
lakes as well.  Think said lake fog will persist, with possible
further expansion of valley fog through daybreak.  Beyond that, am
expecting a rather sunny day amidst light ene/ese flow with an
occasional wisp of high level cirrus not to be ruled out.  As a
result of abundant insolation, temperatures look to continue
rebounding with maxes topping out nearly two categories above normal
levels.  As such, the above normal maxes will keep mins just above
normal tonight, despite clear skies yielding good radiational
cooling conditions.


As of 330 AM EDT Friday: An amplifying pattern will set up over the
CONUS on Saturday, with a prominent ridge axis along the eastern
seaboard and a developing full latitude trough emerging east from
the Rockies. The northern stream will remain fairly progressive
through Sunday, while southern stream energy cuts off over the
ArkLaTex region and deepens over the lower Mississippi River Valley
through Monday. This pattern will yield dry and fairly warm
conditions through Saturday and then increasing clouds but limited
PoPs through Sunday.

A period of much better southeasterly upslope and developing low
level moisture will arrive Sunday night. Although the parent surface
high pressure system to the northeast will be well offshore by this
time, cannot rule out some brief in-situ cold air damming just east
of the Blue Ridge as precipitation first develops. However, any
stable wedge layer should erode quickly through Monday. 850 mb flow
should be slightly backed toward the SSE by late Monday as the
system starts to open to the west, with the low level jet increasing
to 40 kt or better. Instability will be the main question mark, as
any lingering shallow CAD gradually gives way to the north and
conditions destabilize from the southeast. There should be at least
a few hundred J/kg of sbCAPE by PM Monday, so thunderstorms will be
possible and strong to severe activity cannot be ruled out for the
late day/evening hours. The best upper jet divergence will cross the
region late Monday, and the deepest moisture and forcing look timed
for Monday evening, perhaps into early Tuesday AM. This will be the
prime period for any heavy rain threat, but the dry antecedent
conditions will likely preclude flooding problems.


As of 330 AM EDT Friday: Considerable differences develop in the
handling of the southern stream cutoff system through Tuesday, with
the GFS remaining the slowest solution while the ECMWF absorbs the
low back into the northern stream more quickly. Will favor a blended
consensus that brings the associated cold front through the
Appalachians Tuesday morning, crossing the Piedmont Tuesday
afternoon. Either way, a deep trough will amplify from the Great
Lakes through the Deep South Tuesday night before migrating across
the region on Wednesday. A reinforcing cold front should cross the
region from the northwest Tuesday night, bringing about the best
period of low level moisture in the northwest flow, along with the
potential for subfreezing profiles along the higher terrain and
scattered snow showers. There is some potential for any showers to
extend a bit farther east than usual on Wednesday given the very
steep mid-level lapse rates in the passing upper trough. Still,
climatology suggests a mighty struggle to get any activity to break
containment east of the mountains Wednesday.

Winds will be up a bit for the Tuesday night to Wednesday period,
likely limiting any frost potential in the chillier high terrain.
Westerly downslope warming and drying should permit temperatures to
moderate somewhat through Thursday, but Thursday morning mins, along
with slackening winds, could bring the frost/freeze potential back
into play.


At KCLT and Elsewhere:  VFR conditions will prevail today at all
sites as high pressure dominates.  Winds will remain light, starting
up out of the ne this morning, then working to veer ese late in the
day before calming after sunset, all beneath skc or a few high
cirrus.  Overnight all sites look to remain VFR with the exception
of KAVL where early/mid morning mtn valley MVFR/IFR fog is expected.

Outlook:  Little/no restrictions expected through the weekend
with the possible exception of morning valley fog to affect KAVL.
A cold front will approach the region to start the new work week,
thus increasing chances for precipitation and restrictions.

Confidence Table...

            10-16Z        16-22Z        22-04Z        04-06Z
KCLT       High 100%     High 100%     High 100%     Low   56%
KGSP       High 100%     High 100%     High 100%     Low   56%
KAVL       Low   21%     High 100%     High 100%     Low   56%
KHKY       High 100%     High 100%     High 100%     Low   56%
KGMU       High 100%     High 100%     High 100%     Low   56%
KAND       High 100%     High 100%     High 100%     Low   56%

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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