NWS Greenville-Spartanburg, SC wrote its last area forecast discussion before the first event at Apr 2, 8:36 PM EST, 3 hours 32 minutes earlier.
Area forecast discussion, Apr 2, 8:36 PM EST, 3 hours 32 minutes before
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
936 PM EDT Tue Apr 2 2024
.SYNOPSIS...
Showers and a few thunderstorms associated with a cold front will
cross the region tonight through early Wednesday. Breezy conditions
and northwest flow snow showers near the Tennessee border will be
possible as much cooler air returns for the latter half of the week.
Expect slowly warming temperatures by the start of next week.
&&
.NEAR TERM /THROUGH WEDNESDAY/...
As of 935 PM Tuesday: A few changes were made to the forecast for
this update. Storms out west are a tick slower than CAMs had in
previous runs. From the current trends, made adjustments to PoPs to
account for this small delay, but overall, still on track for
thunderstorms overnight. A look at the latest 00z sounding at FFC
shows steep lapse rates from sfc-1km and 300-400 J/kg of sbCAPE,
along with almost 200 sfc-1km SRH. SREF also shows an uptick in
sbCAPE overnight along and south of the I-85 corridor. Given the
latest obs and guidance, conditions still remain favorable for
severe weather overnight with all hazards possible.
With time, the aforementioned upper ridge will get shunted offshore
with upper height falls overspreading the Southern Appalachians.
Initial positive tilt of the trough across the Lower Mississippi
Valley will foster boundary parallel deep-layer shear vectors
indicative of training convection with updrafts rooted behind the
updraft/downdraft convergence zone. However, as the upper troughs
phase and the low rapidly deepens, the trough takes on more of a
negative tilt through the overnight which allows for reorientation
of the shear vectors to out of the west/southwest which would be
orthogonal to the initiating boundary. In addition, the development
of a pre-frontal surface trough across Alabama into Georgia will
allow for increased low-level convergence and locally backed surface
flow. Deep mixing this afternoon has brought down southwesterly flow
to the surface along with lower dewpoints, but that will change
drastically overnight as the surface trough arrives. The other
feature of note that will play an important role in convective
evolution is the development of a 50-70kt low-level jet across
Georgia into the Carolinas as a mass response to the deepening
surface low over Michigan. This will in turn advect a plume of deep
moisture into the area with dewpoints surging into the mid 60s.
Influx of rich boundary layer moisture will actually increase
instability overnight during the traditional nocturnal minimum with
MLCAPE values of 250-500 J/kg. Shear will not be hard to come by
with 30-40 kts of 0-1km shear and 250-500 m2/s2 of 0-1km SRH, which
will set the stage for a severe weather episode and all hazards.
Shear vector orientation will support a broken line of cellular
convection with hybrid linear segments and supercell structures. The
greatest overlap of severe weather ingredients appears to be most
likely along and south of the I-85 corridor on the northern end of
the moisture return/instability and just north of the significant
tornado parameter maximum of 2-4. All severe weather hazards will be
possible including damaging winds, a few tornadoes and isolated hail
up to quarter size. It is very important to note that the greatest
threat for tornadoes will be occurring during the middle of the
night, thus the reader is strongly encouraged to have multiple ways
to receive warnings in place. Farther north, the severe weather
threat remains more uncertain as convection encroaches on the
mountains from east Tennessee. A few severe storms may be possible
across the mountains and into the I-40 corridor, but will be
conditional as to how far north the better moisture and instability
makes it and how the storms pushing northeast along I-85 evolve.
Otherwise, convection will push east of the area by or shortly after
daybreak tomorrow morning with any lingering showers coming to an
end shortly thereafter. The front will also push through with a
somewhat cooler, but much drier airmass arriving in its wake. Deep
mixing in the dry post-frontal air will mix down frequent wind gusts
with strong downslope flow helping to keep temperatures in the upper
60s to low 70s. Bouts of showers rotating around the deep upper low
will also swing across the region with period of showers most likely
across the mountains, but a few isolated showers cannot be ruled out
across the foothills. The highest mountain peaks/ridgelines may also
be cold enough for snow showers and light accumulations.
&&
.SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/...
As of 114 PM EDT Tuesday: By Wednesday night, a well-established
cutoff low will be centered over the lower Great Lakes, and
the bulk of synoptic forcing will already be to our east. A few
ensemble members depict the accompanying surface frontal circulation
still lingering over the I-77 corridor, but even if this winds up
being the case it'll push east in short order. On its backside,
a slug of potent CAA will push across the forecast area as tight
pressure gradient develops at the leading edge of an advancing
cP air mass. Advisory-level gusts look increasingly likely at
the higher elevations, mainly above 3500ft where the mountains
penetrate into the stronger flow aloft. However, Wednesday night
and into the first part of Thursday will be breezy everywhere.
Expect lows on Thursday morning to fall into the low 40s most
locations...and well into the 30s for the mountain zones.
For locations along the NC-TN border, and into the northern NC
mountains, enough moisture will likely linger in valleys that as
NW flow gets going, we should be able to squeeze out some showers.
Given the temperature forecast, at least the higher elevations
may be under the gun for some light accumulation snowfall.
GFS and NAM forecast soundings depict saturation well into the
ice nucleation zone...and forecast SLRs support up to an inch and
a half of snow at the higher elevations. The GFS even depicts
some 50-75 J/kg sbCAPE...allowing for perhaps a rumble of thunder,
and some extremely localized higher accumulations. Nonetheless,
if current trends hold, we'll fall shy of advisory criteria for
any of our mountain counties.
On Thursday, as light snow continues along the highest elevations
of the NC-TN border, the rest of the CWA will continue to dry out.
So much so, in fact, that we may have borderline fire conditions
Thursday afternoon. Despite highs some 12-15 degrees below normal
on Thursday, diurnal mixing will be impressively deep - as high
as 700 or even 650mb per some of the more aggressive long-range
ensemble members - and will result in dewpoints crashing during
the afternoon. The overall trend is a little moister as air aloft
doesn't look quite as dry as it did in earlier model cycles...but
it's still well within possibility that we'll wind up with Red
Flag / Fire Danger criteria Thursday. Thursday night, the pressure
gradient will slacken a little, enough that although winds will stay
breezy, they'll be consistently below advisory criteria everywhere.
Lows will drop into the 30s everywhere.
&&
.LONG TERM /FRIDAY THROUGH TUESDAY/...
As of 206 PM EDT Tuesday: As the churning upper low finally passes
off the New England coast Friday and Friday night, a ridge of
high pressure will slide into the Ohio Valley for the weekend.
Conditions will, for the most part, quiet down. Skies will clear,
winds will - apart from some residual high-elevation gusts in the
mountains early Friday - slacken, and temperatures will exhibit a
wide diurnal range typical of clear and dry conditions. Highs will
climb into the 60s each day through Sunday, and lows will fall to
just above freezing. The main thing of note for the weekend will
be frost/freeze potential. Given light winds by Friday night,
frost appears likely along the Savannah River Valley, and then
more widespread frost will be possible on Saturday night.
The surface ridge will slide east by Sunday night, giving way to
a moist return flow, weak WAA, and a warming trend in low temps
that should inhibit any further frost formation. As a northern
stream cutoff low lifts across the lower Midwest, surface low
pressure over the Ozarks will begin to occlude. By the time its
attendant cold front arrives in the Carolinas, most guidance depicts
the circulation growing increasingly weak and moisture starved.
Should present trends hold, some drizzle may be possible for the
favored upslope zones of Appalachia, but little else.
&&
.AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: VFR conditions continue through the beginning
of the evening hours through at least midnight for all terminals. A
cold front out west brings scattered thunderstorms across all sites
between 04-10z, with temporary restrictions for VSBY/CIGS. For this,
a TEMPO at all sites for TSRA. Any lingering clouds behind the
storms will be slow to depart with a gradual improvement to CIGS.
Most sites are expected to return to VFR after daybreak, with the
eastern terminals KHKY and KCLT an hour or two later. Winds will
remain gusty through the TAF period but should remain SW/WSW at most
sites, with KAVL turning NW after the storms pass.
Outlook: Low clouds and scattered showers may linger over the
mountains Thursday into Friday with gusty NW winds at most
locations. Dry high pressure returns Saturday.
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.
&&
SYNOPSIS...DEO
NEAR TERM...CP/TW
SHORT TERM...MPR
LONG TERM...MPR
AVIATION...CP