Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 270231

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1031 PM EDT Mon Oct 26 2020

Warm and sunny conditions should return Tuesday under weak high
pressure. Tropical Cyclone Zeta will reach the central Gulf
Coast late Wednesday. Rainfall may be heavy at times Wednesday
night and Thursday as Zeta`s remnants pass near the southern
Appalachians. Conditions improve for Friday with cool, clear
weather expected through Saturday.


As of 1015 PM EDT: The main concern with this update is the
potential for dense fog across portions of the forecast area
overnight thru early Tuesday morning. The latest sat imagery shows
much of the Piedmont is clear, except for the Lakelands and a few
patches in the NW NC Piedmont. Dewpts are running in the upper 50s
to lower 60s, while temps are forecast to drop into the mid to upper
50s...a few degrees below cross-over temps. This is thanks to good
radiational cooling conditions outside any patches of stratocu, as
weak sfc high pressure from residual wedge lingers under a building
upper ridge. Guidance is still not hitting fog that hard, but
confidence in dense fog is increasing, especially along the I-77
corridor by daybreak Tuesday. Neighbors to the east have already
issued Dense Fog Advisories, where fog is already forming. If there
wasn`t so much uncertainty on what the stratocu is going to do later
tonight, I`d think about issuing one on this update. However, my
previous attempts to preemptively issue DFAs have not worked out. So
will allow the next shift to monitor trends and issue one, if and
where needed.

The lack of mid to high clouds Tuesday should allow low clouds and
fog to burn off fairly quickly with any dense fog dissipating by mid
morning. The effects of the wedge should just be just a memory by
Tuesday afternoon as the building ridge and weak downslope flow
support a return to well above normal highs into the 70s.


As of 130 PM Mon: Weak sfc high pressure will remain over the area
through Wednesday, with moist southwesterly flow aloft driven by an
offshore upper ridge and the deep cutoff low over the Plains. An
effective warm front will work its way north from the Gulf Coast
Tuesday night, preceding TC Zeta. Subtropical moisture will spread
over the CWA. Although this will warrant PoPs starting to ramp up,
the better warm-frontal QPF is depicted to our west with that
feature, so any precip that early looks relatively low-impact. A lull
could even occur some of Wednesday.

The models overall have narrowed their spread in terms of tracking
Zeta near the southern Appalachians, but some solutions take the
circulation west of the Escarpment and others east, and this leads to
a relatively large spread in the ensemble QPF via the opposing
effects of the terrain among those tracks. Additionally, the storm is
generally shown to spend more time over the Gulf and near the coast,
which suggests it will reach our area a bit later than originally
thought, though certainly no slower in terms of forward speed.
Strengthening low-level flow and upper divergence warrant a rapid
increase in PoPs/QPF early Thursday; the pass of Zeta across (or
nearest) our area now looks to occur morning to early afternoon
Thursday. PWATs should easily exceed 2 SD above normal, perhaps
closer to 3 SD. QPF numbers hug the WPC values, which reflect a track
in the middle of the NHC cone, just south of the TN/NC border. This
gives a quick hit of terrain-enhanced precip; while the rates will be
impressive, it won`t be that long-lived of an event. Storm-total
values are above 4 inches along the ridges near the Balsams and
southern Escarpment, with 1 to 3 for the surrounding mountains and
foothills. Still looking like the flood threat will be primarily in
the more flashy spots in these areas, but certainly can`t rule out
some minor issues elsewhere in the NW half of the CWA.

SBCAPE progs have trended down overall, seemingly reflecting the
consensus for Zeta to take a more easterly track, and possibly the
later expected arrival. A warm sector still generally will exist
ahead of the "other" low, which should track across the lower Ohio
Valley Thursday. Thus, a small amount of instability potentially will
remain over our area, post-Zeta. As shear parameters strengthen ahead
of the front, our risk for isolated damaging wind events may continue
to increase until it passes Thursday night or early Friday; on a
positive note, however, veered near-sfc winds in the wake of Zeta
would tend to limit the tornado threat.


As of 215 PM Mon: Precip should end for the most part early Friday
behind the front; some lingering northwest-flow precip is possible
later in the day, but that should taper off with additional drying.
Furthermore, precip likely will end before temps cool enough to
support any change to snow. Gusty winds will taper off Friday
evening; dry and rather settled conditions will continue through
Saturday under sfc high pressure. That high may prove transient,
being associated with a relatively flat upper ridge that some models
depict moving offshore Sunday ahead of a fast-moving trough. A small
PoP could end up returning to the forecast by Sunday, but confidence
is too low for that on this cycle. Max temps will be several degrees
below normal over the weekend, with mins near normal. With light
winds, a few areas likely will see frost.


At KCLT and elsewhere: Guidance is mixed on the development of fog
and low stratus across the area tonight. It looks a little more
favorable for fog (which could become dense) across the NC Piedmont
and the mountain valleys. But the Upstate, it looks more like a
stratus event. Either way, there will likely be a period of MVFR to
LIFR cigs, with areas of IFR or lower vsby). Lower confidence in the
timing for return to VFR which is pegged between 14Z and 16Z for
now. Light winds through the period.

Outlook: Flight restrictions return Wednesday as the remnants of
Zeta bring widespread rain low cigs/vsbys to the area. The most
significant impacts will probably be Thursday. Conds will likely
improve to VFR Friday.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High  96%     Med   71%     High  81%     High  91%
KGSP       High  86%     High  80%     High  88%     High 100%
KAVL       Med   77%     Med   70%     High  90%     High  98%
KHKY       High  87%     Med   68%     High  83%     High  94%
KGMU       High  88%     High  84%     High  85%     High 100%
KAND       High  81%     Med   60%     High  85%     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 are available at
the following link:




SHORT TERM...Wimberley
LONG TERM...Wimberley
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