Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 150534

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
134 AM EDT Sun Apr 15 2018

A vigorous cold front will move through the area Sunday bringing
showers and thunderstorms, some of which could be severe with
damaging wind, hail, and isolated tornadoes. Much cooler air will
overspread the region in the wake of the front on Monday with
temperatures warming again through midweek.


As of 1030 PM: Very little change to the existing forecast was
needed for the overnight hours, mainly a minor downward adjustment
to temperatures across the northern tier where the clouds have been
fairly thin this evening. Otherwise, water vapor imagery continues
to depict the center of the strong, closed upper system spinning
eastward over the MO/IA border late this evening, with the
associated vigorous surface cold front extending southward through
western TN into MS. Deep moisture ahead of the front, along with
lingering instability in the rich low level theta E air across the
Deep South, are regenerating convection across mainly AL and the FL
pandhandle late this evening, with this activity forecast to move
slowly east as the upper system increases the negative tilt.
Meanwhile, closer to home, a high-elevation Wind Advisory has been
posted overnight through Sunday morning across the southwestern NC
mountains. The 850 mb flow on the RAP continues to trend up with
plenty of 50 to 60 kt winds expected during the peak of the pre-
frontal warm advection, with some mixing/channeling of the better
winds as showers begin to arrive late tonight. Windy conditions will
also be possible down in the valleys, and farther north along the
mountain chain, but probably with sub-advisory gusts.

All attention will then turn to Sunday, as robust shear profiles and
gradually improving instability will cross the region just ahead of
the sharp cold front. The 18Z NAM has trended up on instability
along/near the I-77 corridor for Sunday afternoon, with sbCAPE
values now above 2000 J/kg, but this is an outlier compared to the
approximately 400-600 J/kg values on the ECM/GFS/GEFS mean for 18Z.
Regardless, shear will be tremendous with backed low-level flow, 50+
kt of sfc to 3 km bulk shear and 60+ kt in the sfc-6km layer. The
current Day 2 outlook Marginal west and Slight east still look well-
placed. Wind and tornadoes will be the primary threats, especially
for the eastern half of our area Sunday afternoon.

Still anticipate that localized flooding will be possible given the
high precipitable water values (2 to 3 standard deviations above
normal) and the efficient, heavy rain producing showers and
thunderstorms expected. However, given the limited duration of the
event on Sunday, training of cells will be the primary player in any
flash flooding and any Watches would necessarily have to cover the
entire region. Given the short duration and drier antecedent
conditions, will hold off on any Flash Flood Watches at present, but
short-fused hydro products remain possible.


As of 230 PM Saturday...The large upper low will drift east across
the Great Lakes, allowing heights to rise atop the CWFA thru the
short term. The associated sfc low will push its trailing cold front
east of the area overnight Sunday night, taking the convective into
central NC. Some wrap-around moisture will bump up against the TN/NC
border thru Monday aftn, producing scattered showers. Strong 850 mb
CAA will bring snow levels down to about 3500 ft by early Monday
morning. Snow accums should be light, with 1-2" possible in the
highest elevations before the showers taper off. Temps will drop
into the 30s in the mountains Sunday night and 40s piedmont. Then
with continued low-level CAA, temps warm only into the 40s to lower
50s in the mountains and upper 50s to lower 60s piedmont Monday with
breezy conditions. Monday night looks the coldest night of the week
with temps dipping into the 20s to mid 30s in the high terrain. A
freeze warning may be needed across the mountains where the growing
season has started (all but the Northern Mountains). Conditions
don`t look ideal for frost, but nevertheless some mid 30s will be
possible across the NC foothills and NW piedmont. Temps rebound
nicely on Tuesday as mild high pressure settles in from the west.
Highs slightly below normal in the mountains and near normal east.


As of 230 PM Saturday: The medium range forecast begins
Tuesday night as the forecast period is expected to be an overall
quiet one. With sfc high pressure in place across the Southeast and
an exiting upper trough well north of the FA, latest guidance
continues to prog the eastward progression of a low amplitude upper
ridge across the central US Tuesday night into Wednesday as the
system behind it tracks across the central Plains and into the OH
Valley Wednesday night. With nearly zonal flow aloft and a limited
supply of moisture, a trailing cold front from this system will push
through the FA from west to east Wednesday night into Thursday
morning. Expect the development of any isolated showers to remain
confined to areas along the TN border as the Upstate and NW Piedmont
will remain dry with passing clouds and breezy winds. In fact, ahead
of the front on Wednesday through into Thursday evening, do expect
breezy winds across the entire FA with a tightening pressure
gradient. Behind the front, the upper trough associated with this
system is expected to push eastward across New England Thursday
night into Friday with sfc high pressure returning to the region for
the end of the week. Another weak upper ridge is progged to build
across the central US ahead of the next developing system across the

Wednesday will be the warmest day of the rest of the week, with
temperatures in the upper 70s/around 80 degrees across the NW
Piedmont and Upstate, and slightly cooler back across the mountains.
The coldest night/morning will be Thursday night into Friday morning
behind the recently passed cold front with temperatures across the
mountains in the 30s/lower 40s - mid to upper 40s elsewhere.
Otherwise, the warmer trend will continue through into Saturday,
with temperatures at or just above normal.


At KCLT and elsewhere: VFR everywhere still at TAF time, but MVFR
cigs approaching CLT so have started them MVFR; otherwise
transitioned other sites down over the next couple of hours, with
IFR expected by 08-09z everywhere. Cold front pushes through today
with SHRA/TSRA at all sites (VCTS at KAVL and KHKY, but prevailing
TSRA at other sites). Best chance of strong/severe storms are at
KCLT but possible at any site; however, feel this is best handled
with at the earliest the 12z issuance where we can add a TEMPO for
convective winds. Winds out of the S this morning and gusty will
back slightly more SSE before veering back SW and eventually, at
KAVL after the front, NW. Other sites will likely veer NW after the
end of the period. Gusts 20-30kt expected just front the gradient
winds today and could be higher in TSRA. Torrential downpours
expected with the convection, and cannot rule out small hail and
especially within the SPC Slight Risk area, damaging winds and
isolated tornadoes. Improvement late in the period to at least MVFR
but likely VFR for Upstate TAFs and KCLT.

Outlook: Other than perhaps some lingering mountain showers, dry and
cooler conditions should return Monday and continue into mid week. A
drier cold front may arrive late in the week from the northwest.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       Med   71%     High 100%     Med   78%     Med   68%
KGSP       High  93%     High  94%     High  83%     Med   78%
KAVL       Med   64%     Med   72%     High 100%     High  93%
KHKY       High 100%     High  94%     High  89%     Med   64%
KGMU       High 100%     High  94%     High  89%     High  85%
KAND       High 100%     Med   78%     High  89%     Med   64%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:


NC...Wind Advisory until noon EDT today for NCZ051-052-058-059-


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