Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 221841

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
241 PM EDT Sun Apr 22 2018

Dry high pressure will weaken and shift east ahead of a strong and
moist frontal system pushing into the forecast area early Monday.
This system will bring high amounts of rainfall into Tuesday. The
atmosphere will generally dry out Wednesday before another moist
area of low pressure crosses the region Thursday through Friday.


As of 200 PM EDT Sunday: Rainfall is spreading west to east
gradually and will likely favor normal upslope areas through the
evening, which is reflected in the current pop forecast. CAMs
continue to push back precip arrival, though the current forecast
trend has been holding for a few hi-res model runs now. Rain will
spread into Upstate SC/western NC over the late afternoon, with
rainfall arriving in the NC Piedmont around 00Z.

Plentiful high and mid-level clouds have overspread the FA as the
closed upper low and surface reflection are continuing to strengthen
as they dive into the southeastern states/Gulf coast. A surface high
still sits off the Mid-Atlantic coast, encouraging easterly flow at
the surface across much of our area. Strong SEly/Sly 850mb flow is
providing plentiful GOM/Atlantic moisture transport into the area
and will continue to do so through the entire near term period. A
wedge of surface high pressure is beginning to spread into the
southern Appalachians, and will continue to develop into a true cold-
air damming event as precip falling into the wedge increases in
coverage and intensity, providing diabatic enhancement of the
pattern. The temperature gradient at 850mb will also continue to
tighten through this period, enhancing rainfall potential overnight
and early into tomorrow morning as substantial isentropic upglide

The QPF forecast leaned heavily on WPC, and though
guidance continues to change the placement of the heaviest precip,
feel good about the normal upslope areas receiving the heaviest
rainfall considering the CAD pattern taking shape. A second precip
max does look to develop along the SC coast tonight and tomorrow,
however, which could result in some moisture-robbing and lower QPF
than forecast. Still, though uncertainty precludes very high
confidence in the current storm total QPF, the Flood Watch that will
go into effect later tonight still looks well-placed. 6-hrly rates
do not support much of a flash flood risk in the near term, but with
the volumes of rainfall expected over the next 48 hours, some main
stem river flooding and other areal flood issues are possible.
Overnight lows will be around normal, but with heavy precip and
substantial cloud cover tomorrow along with the expected cold air
damming pattern, temperatures will rise very little during the day,
remaining 15 or more degrees below average.


As of 325 AM Sunday: The timing of heavy rainfall and associated
flood problems appears to be coming a bit more into focus early this
morning. The numerical models continue to feature excellent
agreement in locating the closed, strong 500 mb low center over the
TN/MS/AL border region around daybreak on Monday. The band of
deepest moisture and forcing east of this feature should pivot
northeast across the forecast area on Monday with the associated
heaviest rainfall rates of the event. Meanwhile, hybrid or in-situ
cold air damming will continue across the region, which will promote
continued upglide forcing as the southeasterly low level jet
continues at 40 to 50 kt above the wedge layer. The east coast upper
blocking ridge will permit only slow eastward movement of the system
toward the Appalachians through Monday night, with the system
finally phasing and lifting over the central Appalachians Tuesday
through Tuesday night. Some mid level drying will wrap in from the
southwest on Monday night, but strong upglide and upslope forcing
with the robust southeasterly low level flow will continue to wring
out rainfall. All told, the storm total QPF will now feature some 3
to 4 inches across the foothills and the eastern upslope areas, with
localized 4 to 6 inches possible in the most upslope preferred
locations. This will warrant posting a Flood Watch for the easterly
upslope mountain areas and all of the foothills - which will start
tonight and last at least until Tuesday morning.

As the phasing trough lifts northeast, the arrival of the upper
trough axis will steepen lapse rates for Tuesday afternoon. In
addition, while the main surface low moves north across eastern KY,
a secondary surface low will likely transition through our lower
piedmont - possibly helping to move the wedge front northwest. These
features may conspire to increase thunder chances across our
southeast sections Tuesday afternoon. Fortunately, shear values will
decrease by then. Shower chances will continue through Tuesday
night, mainly for the NC mountains in NW flow moisture. Temperatures
should recover a few categories Tuesday over the chilly wedge
temperatures on Monday.


As of 230 PM Sunday: Relatively cool and unsettled wx will continue
across the region for the latter half of the week, as a deep
longwave trof persists across the eastern CONUS. A series of potent
shortwaves will dive SE into the mean trough and cross the Southern
Appalachians. The first dives way south into the Gulf states then
pivots as a neutral or slightly negatively tilted trof as it crosses
the Carolinas on Thursday. This feature will have some moisture to
work with, but the low-level flow remains fairly weak. Scattered
showers are expected as the wave passes; although the 12z GFS hints
at a more widespread rain shield Thursday. The second wave will quickly
follow and will be associated with the northern stream. This will
push a cold front thru the area, with mainly scattered showers
across the high terrain, and more isolated in the piedmont. Heading
into next weekend, the longwave trough axis should start to pivot
east and heights rise across the Southeast, resulting in drier
conditions. Temps Thursday thru Saturday will be slightly above
normal for lows, and below normal for highs. Highs on Sunday may be
slightly above normal.


At KCLT and elsewhere: VFR still barely hanging on at all TAF sites,
though with precip and more mid/low cloud beginning to enter the
area from the southeast, our restriction-free period is rapidly
coming to an end. -RA/SHRA/MVFR cigs will spread slowly across the
area by this evening, and with the pressure gradient across the area
expected to tighten as cold air damming develops, winds will remain
elevated overnight and increase early tomorrow morning, with some
gusts possible this evening and again for most of tomorrow. Winds at
KGSP/KGMU/KAND will be NEly through the period due to the CAD
pattern, and MVFR/IFR cigs with -RA and SHRA will persist through
the valid TAF period, with vsbys also dropping to MVFR/IFR
intermittently tonight and tomorrow as occasional heavy rain spreads
through the area.

Outlook: Conditions will deteriorate tonight with widespread rain
overspreading the area. IFR/MVFR conditions will be likely thru the
day on Monday, with some improvement on Tuesday as precip largely
shifts NE of the area. Some moisture will linger, however, and
unsettled weather will continue on Wednesday and Thursday as a
trough of low pressure persists across the region.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High  88%     High  83%
KGSP       High  94%     Low   58%     Med   71%     High  85%
KAVL       High  80%     Med   76%     Med   77%     High  81%
KHKY       High 100%     High  98%     High  83%     Med   79%
KGMU       High  94%     Med   61%     Med   75%     High  88%
KAND       High  83%     Med   77%     High  82%     High  89%

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:


Storm total QPF values have been increased slightly for the upcoming
event. Although rates do not appear sufficient in any 3 to 6 hour
windows to produce flash flooding, duration flooding is looking more
likely - especially in the most upslope preferred locations and over
the adjacent foothills. Storm totals of 3 to 4 inches are now
expected, with localized 4 to 6 inch amounts in the most upslope
preferred areas.

Flooding of the most common flood-prone and low-lying areas may
steadily develop from the southwest on Monday, with main stem
flooding of the Upper French Broad River and associated tributaries
quite possible. Flooding of main stems and streams in the Upstate
may also develop, especially along the Broad and Saluda rivers, with
crests not occurring until Tuesday and perhaps early Wednesday.
Additional significant rises may develop along the Catawba River
chain in western NC during and immediately after the heavy rainfall.
Water level forecasts for these rivers will incorporate the total
forecasted rainfall later this morning and will be accessible at


GA...Flood Watch from midnight EDT tonight through Tuesday morning
     for GAZ010-017-018-026-028.
NC...Flood Watch from midnight EDT tonight through Tuesday morning
     for NCZ033-049-050-053-059-062>065-501>510.
SC...Flood Watch from midnight EDT tonight through Tuesday morning
     for SCZ001>007-010.


NEAR TERM...Carroll
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