Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 271045
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
645 AM EDT Tue Oct 27 2020

.SYNOPSIS...
Warm and partly sunny conditions should return Tuesday under weak
high pressure. Clouds return for Wednesday as tropical cyclone
Zeta approaches the central Gulf coast. Rain chances increase
Wednesday night as Zeta makes landfall and moves through the Deep
South. Periods of heavy rainfall and gusty winds are expected
overnight Wednesday through Thursday as the remnants of Zeta
pass over the region.  Conditions improve for Friday with cool,
dry weather expected through early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 645 AM EDT Tuesday: A Dense Fog Advisory and Special Weather
Statement for patchy dense fog are in effect for the eastern
portions of the FA through 10 AM. Otherwise, main update for the 12Z
TAF issuance along with minor adjustments made to temps to coincide
with latest trends as the rest of the near term forecast remains on
track.

With weak high pressure dominating the current weather pattern, fog
coverage has become quite widespread across the area this morning as
expected with plenty of low level moisture and favorable dewpoint
depressions, and nearly calm winds. At this time, dense fog
continues to be patchy in nature across much of the area, though
within the past few hours it has filled in across portions of the NC
Piedmont, thus warranting a DFA in and around the I-77 corridor.
Current temperatures are in the mid 50s to lower 60s.

As the fog dissipates and lingering low stratus lifts within a few
hours after sunrise, expect a pleasant day with a mixture of
sunshine and clouds across the western Carolinas and northeast
Georgia as the wedge continues to dissipate and upper ridging builds
in overhead. Downsloping flow will allow for quite the warm up
today, with max temps progged to climb above normal and into the mid
70s, cooler across the mountains. Late overnight, cloud cover will
begin to increase in coverage as deep tropical moisture infiltrates
in from the GOM ahead of Zeta. Min temps will remain above normal.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As of 330 AM Tuesday: As tropical cyclone Zeta lifts north over
the central GoMex, a weak stationary boundary over the southern
Appalachians, extending southwest into the Lower Mississippi
River Valley, will begin to lift north as a warm front Wednesday
morning. Tropical moisture will begin to advect into Georgia around
the periphery of high pressure moving offshore. PoPs will ramp up
during the day on Wednesday in response to the warm front, though
it appears the best forcing and rainfall amounts will set up to
our west.  Therefore, Wednesday rainfall should pose no concerns.
With the warm front lifting north and increasing thicknesses,
high temperatures should remain 5-7 degrees above normal despite
increasing cloud cover.

Zeta makes landfall over the central Gulf coast early Wednesday
night yet there remains some disagreement within the guidance
about the orientation and strength of low-level upslope flow over
our area.  Overall, we lack the classic signal of strong low-level
southerly flow necessary to support significant precip accumulations
ahead of a tropical cyclone.  The best upper diffluence is due north
of Zeta in response to the anomalous upper low over the southern
Plains and as such the best rainfall response is also to our west.
This pattern would thankfully work to reduce the priming effect
we often see in the favored areas of the southern mountains in
advance of a Gulf Coast cyclone and limit storm-total accumulations.

As Zeta moves across Alabama overnight Wednesday and approaches
Georgia by Thursday morning, a brief period of better upslope
forcing may set up ahead of the low across the southern mountains,
though precip rates should be of minimal concern. The NHC
track is well placed within a tightening spread of guidance,
increasing the likelihood that Zeta will pass over or just to
the west of the southern Appalachians Thursday morning into the
early afternoon as it begins to undergo extratropical transition.
Such a track keeps us within the favorable sector of the storm for
heavy rainfall as it passes, as well as maintains a non-zero risk
for a few storms capable of damaging winds Thursday afternoon.
As Zeta passes by, a slug of tropical moisture (2"+ PWs, or near
climo maxes) will accompany the storm.  As we`ve previously noted,
antecedent conditions and above-normal streamflows support excessive
runoff and near bankfull stream response, even though the speed of
the storm will work to limit accumulations.  Isolated to widely
scattered nuisance flooding by Thursday afternoon appears to be
the main hydro threat from this storm especially along and near
the Blue Ridge Escarpment, with one or two instances of more
notable flash flooding possible.  Storm-total accumulations of
1-3", especially from the Foothills west, appear to be on track,
with isolated 3-4" still possible along the favored ridgetops of
the southern mountains.  Expect lower accums, more on the order
of 0.50-2", for the Piedmont.

Model guidance is coming into some agreement that a wedge of 300-700
J/kg of SBCAPE will accompany the eastern quadrants of Zeta as it
surges NE and combined with 40-60kts of bulk shear we will continue
to advertise a non-zero damaging wind threat from isolated storms,
primarily over the lower Piedmont south of I-85.  The NAM, with its
slower storm motion, may best synchronize Zeta`s shear and forcing
with peak heating.  Furthermore, the NAM tries to develop showers
ahead of the cold front which is quickly following behind Zeta.
Uncertainty remains regarding peak wind gusts associated with Zeta
for Thursday as well.  While the storm should be undergoing rapid
weakening below tropical-storm strength it may also be undergoing
extratropical transition.  Therefore, we will have to monitor for a
gradient wind concern, especially over the mountains for Thursday,
which may warrant an Advisory if our confidence increases in
subsequent forecast packages.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 330 AM Tuesday: By Friday morning, Zeta will be a distant
memory with its remnants well offshore.  The once anomalous upper
low will be passing overhead as it is absorbed into a potent
shortwave trough.  Forcing and instability associated with the
trough axis and steep lapse rates aloft will work with residual
moisture to maintain a chance PoP across western North Carolina into
the morning hours, but this will quickly dissipate as the trough
axis slides east.  As high pressure builds over the Great Lakes,
the resultant pressure gradient with the offshore low will result
in a brief period of northwest-flow showers over the central and
northern mountains, which should fully dissipate by late Friday
afternoon as strong high pressure builds into the Northeast.
CAA will be stronger over the mountains on Friday, so expect highs
to be a few degrees below normal there, but near normal to the east.

By Friday night CAA is in full force over the forecast area with
low temperatures returning to near normal after being well-above
normal Friday morning.  The high pressure source will likely be
transient and quickly slide offshore on Saturday as a developing
longwave trough digs into the northern Plains.  Nevertheless, the
pattern supports high temperatures 6-10 degrees below normal on
Saturday and Sunday and light winds will support patchy frost in the
mountains both mornings as temps dip into the middle to upper 30s.

The progressive pattern brings a cold front to our doorstep on
Sunday with enough moisture to possibly support a slight chance
of showers, though it`s too far out to have confidence in this.
Another strong (>1030mb) high approaches from the west for Monday
behind the trough and settles over the Tennessee Valley by Tuesday
morning, keeping us dry and cool with temperatures 3-5 degrees
below normal.  With high pressure overhead, expect a more widespread
frost threat over western North Carolina by Tuesday morning.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: A mixture of VFR/MVFR/IFR/LIFR prevails at
the TAF sites this morning due to fog and low cigs. A dense fog
advisory which includes KCLT will continue through 13Z. With fog
expected to lift and dissipate within a few hours of daybreak,
expect improving conditions to VFR between 14Z and 16Z giving way to
mid to high level clouds SCT to BKN in nature for today and tonight.
Overnight, low cigs and fog will allow for flight restrictions to
return as moisture begins to infiltrate in ahead of Zeta. Winds will
remain light through the valid TAF period.

Outlook: Flight restrictions return Wednesday as the remnants of
Zeta bring widespread rain low cigs/vsbys to the area with the most
significant impacts anticipated on Thursday. Conditions will improve
at the end of the week.

Confidence Table...

            10-16Z        16-22Z        22-04Z        04-06Z
KCLT       Med   72%     High  90%     High  92%     High  89%
KGSP       Low   52%     High  97%     High 100%     High  89%
KAVL       High  80%     High 100%     High  97%     High  89%
KHKY       Med   77%     High  92%     High  95%     High  89%
KGMU       Med   67%     High  95%     High 100%     High  94%
KAND       Med   72%     High  90%     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 are available at
the following link:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...Dense Fog Advisory until 10 AM EDT this morning for NCZ036-037-
     057-071-072-082.
SC...None.

&&

$$
SYNOPSIS...JMP
NEAR TERM...SGL
SHORT TERM...JMP
LONG TERM...JMP
AVIATION...SGL


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