National Weather Service National Headquarters

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KGSP 292000
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
300 PM EST Sun Nov 29 2020

.SYNOPSIS...
A strong low pressure system approaching from the southwest will
support widespread showers and isolated thunderstorms from this
evening through Monday morning. In the storm`s wake, Snow is
expected to develop in the mountains near the North Carolina and
Tennessee border from late Monday through Tuesday. Another strong
low may cross the region at the end of the week.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 1250 PM: Still expecting mainly just increasing cirrus over
the area until this aftn. A relatively dry sfc airmass remains
over our CWA, associated with high pressure off the mid-Atlantic
coast. Deepening moisture and upglide atop this airmass, along with
increasing upper divergence, will develop ahead of a shortwave and
sfc low pressure now spinning up near New Orleans. Cloud bases are
already lowering over the southern CWA, and we are beginning to see
some light showers develop over the northern SC Midlands...actually
a bit faster than any guidance suggested. This warranted a minor
adjustment to the aftn PoPs.

Sfc low will strengthen as it moves NE toward the southern
Appalachians this evening, with the associated shortwave being
absorbed into a deeper trough digging into the mid-Mississippi
valley. Consensus suggests the low will skirt along the NC/TN
border. As precip falls into the sfc airmass mainly ahead of and
along it, weak CAD is expected to develop. This wedge will remain
entrenched across the upper Piedmont and Foothills until the cold
front scours it out later in the morning. S to SE 925-850mb winds
will be favorable for upslope enhancement, and event totals above
2 inches appear likely along the southern Blue Ridge Escarpment
and other south-facing ridges. Isolated flooding could occur in
those upslope areas, but overall QPF is not expected to be much
of a concern.

LLJ crossing the area ahead of the cold front will produce very
strong low-level shear. Minuscule sfc-based instability, even south
of the wedge boundary, remains the limiting factor. Modified SHERB
(MOSH) values from both NAM and GFS remain low enough ahead of
the cold front that any storms that fire along it most likely
will not produce damaging winds or tornadoes, although any cells
developing via SBCAPE will need to be monitored closely. We do
retain slight-chance thunder south of the wedge boundary and
in some spots where there is MUCAPE atop the wedge. Given WAA,
the LLJ is not expected to produce advisory-level gusts except
perhaps at very high elevations.

Some degree of dry-slotting is expected in the very late overnight
or around daybreak; precip should come to an end for the Piedmont
from SW to NE during the morning. Often precip ends earlier
than synoptic models depict in these kinds of events, but the
lack of strongly organized convection along the front makes me
buy their consensus. The low clouds may take some time to erode,
nonetheless. Winds will become gusty following the fropa but high
elevations still don`t look to mix high enough to get advisory-level
gusts. Max temps will end up a tad above normal in the Piedmont.

For the mountains, the morning passage of the cold front will
initiate falling temps, which along with moisture wrapping
around the system on NW winds means precip will transition to
snow. Profiles also will become increasingly favorable for snow
crystal growth, so initially low SLRs will become pretty good by
late afternoon. Snow accums will begin in the morning at higher
elevations, falling to lower elevations with time. This event will
continue into the short term, and significant accums are likely
along the TN border. We are issuing a Winter Storm Watch with this
package; an elevation split between Warning and Advisory may be
needed tonight or on tomorrow`s day shift.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/...
As of 220 PM Sunday: Let Winter begin! We should be in the throes
of a NW flow snow event at the transition to the short range at
00Z Tuesday. Precip-type is not in question, with deep cold air in
place wrapping around the slowly departing upper low that moves up
the OH Valley Monday night. Moisture should be relatively deep on
Monday evening...with high RH well up into the dendritic growth
zone. Backward trajectories from the NAM Nest suggest at least a
few hour period with a Great Lakes moisture connection. The 850mb
flow is more WNW, but that is typical. What we lack is strong upper
support, but we will make that up with the potential for higher
amounts because of the moisture supply in the dendritic growth
zone. The upshot is that we have good potential for significant
high elevation snow near the TN border, and with that in mind,
precip probs have been raised in the likely/cat range on the TN
border, and QPF/Snow has also been raised according to the latest
model guidance. NOTE: We often like to err on the side of
caution with the first event of the season, thus the Winter
Storm Watch that extends to midday Tuesday along the TN border. The
deeper moisture starts to pull out around daybreak Tuesday, but
the low level moisture departs more slowly, such that there is
a high chance that snow shower production will continue past
sunrise and into the middle part of the day along the TN border.
The snow should taper off completely by the end of the daytime
period. East of the mtns, it should remain dry for the most
part. There is some indication that a small area of low level
convergence downwind of the Balsams could result in some snow
flurries along the NC/SC border, mainly over the nrn Upstate, on
Monday evening. Both operational models have some small
response east of the GSP metro area, but for now, it bears
watching but not including in the official forecast. As for
temps, needless to say, they will be the coldest of the season
and very much like winter across the entire region.

As for the rest of the period into the middle part of the week,
after the upper low moves away Tuesday night, we should be in for
a lull with cool sfc high pressure over most of the Southeast. A
slowly dampening upper ridge in-between the old upper low and
another upper low moving out over the central/srn Plains will move
overhead Wednesday. The ridge axis should move past Wednesday night
and we begin to come under the influence of the cyclonic flow/warm
advection ahead of the next upper system. However, the clouds and
precipitation associated with the next one are not expected to
arrive from the west before daybreak Thursday.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 240 PM Sunday: Forecast confidence deteriorates rapidly as
the end of the week approaches. At issue is the organization and
track of the next upper low, and what, if any, cyclogenesis takes
place to the lee of the Appalachians on Friday or Saturday. In
spite of timing differences, there is some agreement that the upper
low will move slowly east and begin to shear out over the Midwest
on Thursday and Thursday night, or perhaps Friday if the ECMWF
timing is correct. Precip chances will ramp back up during this
period but remain in the chance range, while overnight temps remain
borderline for rain/snow. The main difference comes along Saturday
with the new ECMWF being slower and deeper with one upper low moving
along the VA/NC border inducing lee cyclogenesis over NC. If that
scenario plays out, we could have another rain/snow event over the
mtns at least on Saturday. For now, the fcst will remain a blend
between the two solutions which comes up decidedly vague with an
unmentionable precip chance. At least we are reasonably confident
that temps will remain below normal.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Cirrus will be seen thru this afternoon, with
low VFR then MVFR cigs pushing in generally S to N during this time
as low pressure approaches from the SW. Deeper moisture will follow,
and IFR to LIFR cigs will set in soon after steady precip arrives,
as CAD forms. Mainly LIFR is expected overnight within this pattern;
chances for SHRA peak overnight as low tracks across the southern
Appalachians. Can`t rule out isolated TSRA forming either above the
wedge or along its southeast periphery, but likely too isolated to
mention in TAFs. Likewise, LLWS could occur atop the wedge but too
hard to pin down to warrant mention. There remains some question as
to how fast cold front will scour out low clouds and precip in the
early part of the day. Even if precip ends as early as suggested by
convection-allowing models, it may be hard to erode the restrictive
cigs. Have included PROB30 at all sites except KAND to reflect the
possibly lingering restrictions at that time. Scattering and VFR
should return by midday. KAVL will begin to see clouds associated
with NW flow over the mountains by midday; periodic restrictions
from these may develop later in the day. Brisk wind gusts will
develop in the late overnight around the region, and these will
continue/strengthen as winds flip to W or NW behind the front.

Outlook: Conditions improve Monday as the cold front crosses the
area, but windy conditions develop. Behind this system, wintry
precipitation will be possible in the mountains later Monday into
Tuesday, with VFR elsewhere. VFR all locations by midweek.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High  95%     Med   63%     Med   79%     High  81%
KGSP       High  92%     High  87%     Med   75%     High  84%
KAVL       High  85%     High  85%     High  92%     High  80%
KHKY       High 100%     High  82%     High  95%     Med   74%
KGMU       High  82%     High  89%     Med   68%     Med   79%
KAND       High  90%     Med   72%     Med   71%     High  82%

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...Winter Storm Watch from Monday morning through Tuesday morning
     for NCZ033-048>052-058.
SC...None.

&&

$$
SYNOPSIS...DEO
NEAR TERM...Wimberley
SHORT TERM...PM
LONG TERM...PM
AVIATION...Wimberley