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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KGSP 271948
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
248 PM EST Sat Feb 27 2021

.SYNOPSIS...
A front stalled south of our region will gradually lift
northward as a warm front through Sunday. A cold front will approach
the southern Appalachians from the northwest Sunday night and cross
the area on Monday. Dry high pressure moves from the Great Lakes to
the mid Atlantic coast Monday night into Tuesday before unsettled
weather returns from the west Tuesday night into Wednesday. A
stronger storm system could arrive by Friday.

&&

.NEAR TERM /THROUGH SUNDAY/...
-- Changed Discussion --
As of 210 PM: The CWA remains under a decaying CAD setup, with
parent high pretty much out of the picture and low-level winds
veering such that low-level upglide is pretty much history. However,
tell that to the low cloud trapped over the area beneath subsidence
inversion, which has kept most of the CWA under low stratus or
stratocu so far today, which has kept a number of automated sites
from rising out of the 40s. Strong insolation and a remarkably warm
subtropical airmass are gradually doing their thing, gradually
thinning/narrowing the cloud layer, but we effectively remain
under the cooler wedge airmass at the sfc. Obs at the sunny
southern Upstate sites have now surged into the 60s--though much
more stubborn to warm compared to areas just a little further
south on the other side of the wedge boundary. The NC mtn valley
sites are in the upper 60s to near 70 with the additional aid of
downsloping SW winds. It`s difficult to accurately say how much of
the cloud cover will be able to erode before sunset, but some of
it likely will remain in place. The max temp fcst this aftn and
potentially overnight min fcst will depend considerably on that,
which limits our confidence thru tonight.

Meanwhile, a convergence zone over the upper TN Valley and near
the Appalachians to our north will maintain a small chance of
light precip for our northern mtns and vicinity for the remainder
of the aftn, probably as sprinkles or weakly convective showers;
thunder no longer expected.

While the areas that see the low cloud deck scatter should have a
few hours of mostly clear sky, the subtropical ridge will maintain
WAA thru tonight, which will amplify as sfc cyclogenesis occurs
over the Midwest. This is expected to lead to abundant cloud cover
once again, likely joined by fog in the NC Piedmont and possibly
the eastern part of the Upstate, where the shallow fragment of the
wedge should remain. Dense fog is not out of the question given
the pattern and guidance consensus. A small PoP exists overnight
over much of the mountains and northern NC Piedmont via the same
mechanisms; upslope forcing along the southern Escarpment and
adjacent mtn ridges will further enhance chances there.

The low stratus looks likely to erode late Sunday morning,
though moisture higher aloft is expected to maintain mostly cloudy
conditions across the board. With the wedge finally dissolving in
the morning, that should allow deeper mixing; midlevel winds will
be fairly strong in the gradient between the Midwest low and the
Atlantic ridge. This should result in a breezy/gusty afternoon,
with temps reaching well into the 70s in many locations, a few
degrees shy of records at our climate sites. RH will not be low
enough for this to pose any fire danger concerns.
-- End Changed Discussion --

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
-- Changed Discussion --
As of 205 pm Saturday: Frontal zone will be approaching the
forecast area at the start of the period, then will sag southeast
across the area throughout Monday. A rather vigorous precip band is
expected to accompany the front across the TN Valley into the
southern Appalachians, with likely-to-categorical pops forecast
across the NC mtns by daybreak Monday, with high pops overspreading
the remainder of the forecast area throughout the morning. Sort term
model guidance continues to reveal a strong signal toward weakening
of the precip region as it move across our area, as low level
convergence diminishes due to disruption of the flow pattern by the
high terrain, and as deeper forcing remains north of the forecast
area. In fact, by early afternoon, it appears likely that there
won`t be a whole lot left of the frontal band as it`s pushing east
of the area. All this being the case, any excessive rainfall
concerns Sun night/Monday should be confined to the areas closer to
the TN border, and even there the threat appears marginal at best.
Guidance continues to depict scant instability (i.e., sbCAPE < 100
J/kg) in the warm sector ahead of the front. However, the weakening
forcing profiles do not support much of a high shear/low CAPE severe
storm threat, nor do the shear profiles, which are rather inadequate
given the paltry buoyancy.

Otherwise, high temps will generally occur during the morning across
the mtns, with temps steady (valleys) or slowly falling (peaks/ridge
tops) throughout the day. As usual, downslope warming will offset
any late morning/afternoon CAA across the Piedmont/foothills,
allowing max temps to again warm to above normal...by about 5
degrees or so. The remainder of the short term will see a return to
normal...if not a slightly below normal temps and dry weather.
-- End Changed Discussion --

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
-- Changed Discussion --
As of 220 pm Saturday: A split flow regime will dominate the pattern
across much of the Conus, with an active southern stream likely
bringing two rounds of precipitation during the period. Fortunately,
there is more of a consensus among deterministic global model
guidance regarding these two events, particularly an event that is
expected to unfold in the Tue night/Wed time frame, when a compact,
but rather potent upper low is forecast to move across the TN and OH
Valleys into the Appalachians. The GFS continues to be weaker/more
suppressed with this feature, but much less so than in yesterday`s
runs, giving us the confidence to bump pops into the
likely/categorical range for much of the forecast area after
midnight Tue night, with pops quickly tapering off from the west Wed
morning.  Current indications are that sufficient cool air will be
left in place by retreating surface high to create a wintry precip
concern across the mtns and perhaps the I-40 corridor in the
foothills/Piedmont. However, the absence of any signal of a strong
cold air damming event/mech to lock in cold air suggests such an
event would likely be of the nuisance variety.

Guidance is also in better agreement regarding potential impacts of
a southern stream system later in the week, although differences
remain regarding the details. Nevertheless, the GFS and
ECMWF/Canadian camps identified yesterday are definitely trending
toward one another, with a slightly more potent/wetter system
depicted in the ECMWF/Canadian, and a much less potent/not nearly as
wet and more phased flow solution per the GFS. As such, pops are
warranted Friday through early Saturday, but these generally remain
only in the 30-40% range. Temps are forecast to remain around, if
not slightly below climo through the period, although a brief return
to above-normal conditions may return Thursday ahead of the late
week storm system.
-- End Changed Discussion --

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
-- Changed Discussion --
At KCLT and elsewhere: Low stratus/stratocu deck associated with
decaying CAD regime will make for a tricky forecast this aftn
at all sites except KAVL. The initially widespread LIFR deck has
begun to scatter/lift in some spots but has remained essentially
unchanged since daybreak elsewhere. LAMP guidance is having trouble
keeping up with the trends, it seems. Have leaned pessimistic in
the first few hrs, but gradual improvement does look likely this
aftn across the SC sites and KCLT. Erosion of the deck should
allow VFR to persist into evening. A weak front will stall across
the area late today, being reactivated as a warm front tonight by
low pressure over the mid-MS Valley. Combined with a moist PBL,
that looks likely to bring back IFR-LIFR cigs and fog (possibly
dense) by early morning to most of the area. This likely won`t
scatter/lift until nearly midday Sunday. Winds will be fairly
light and mainly southwesterly today/tonight; gusts will develop
Sunday, especially following the erosion of the low clouds, with
deep mixing in the presence of fairly strong midlevel winds in the
warm sector of the aforementioned low. Precip chances are too low
to mention this period.

Outlook: Flight restrictions and chances for precip are expected
to return Sunday and linger into early next week. Drier hipres
will build back into the area by Tuesday.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High  82%     High  87%     Low   59%     Med   71%
KGSP       High  88%     High  94%     Med   77%     Med   77%
KAVL       Med   72%     High 100%     High  87%     High  93%
KHKY       Med   79%     Med   76%     Med   62%     Med   66%
KGMU       High  87%     High  95%     High  81%     High  80%
KAND       High  85%     High  95%     High  83%     High  87%

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
-- End Changed Discussion --

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

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