Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 222345

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
745 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 745 PM EDT Sunday: Light rainfall continues to spread west to
east across the area this evening. However, it has been dissipating
in the drier air over the NC Foothills and Piedmont and the Northern
NC Mountains. Expect the atmosphere to moisten overnight with at
least light rain over most of the area by daybreak. Rain will likely
be moderate over the western sections by then. Gusty winds will
continue across the higher elevations of the NC Mountains, but
should remain below advisory levels for all but locations above
5000 feet. Next shift can reevaluate and see if an advisory is
needed for winds dropping into the valleys. Have adjusted forecast
for current precip and temp/dew point trends, but the overall
forecast remains on track.

Otherwise, 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 develops.

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 235 PM Sunday: Model guidance shows that precipitation
will remain widespread Monday evening, but that potential for
significant rainfall will be slowly ramping down Monday night
with the departure of the better upper divergence, isentropic
lift, and deeper moisture. No need to adjust the timing on the
back end of the Flood Watch, though, because we will still be
dealing with runoff issues and water being routed into the main
stem rivers. Precip prob will be slowly lowered from S to N after
midnight, with the pace picking up on Tuesday as most of the dynamic
forcing weakens or moves out entirely. Tuesday looks like a really
tough, low-confidence fcst as we deal with the destruction of the
cold air damming wedge. Its demise will probably have a lot to do
with the eventual track of a triple-point low across the Carolinas
during the day. The NAM still seems overly-optimistic with bringing
the sfc low over the NC mtns/foothills and thus exposing a great
deal of the Upstate/wrn Piedmont to a warm sector. The GFS/ECMWF
solution is more likely, which is to say, taking the sfc low mainly
over the SC Midlands/ern Piedmont of NC in the afternoon. The
difference in track is the difference between seeing sfc-based
CAPE on the order of 500 J/kg (GFS) vs. 1500 J/kg (NAM). We favor
the GFS scenario. Altho shear will have long since diminished,
enough residual low level shear makes this a situation that bears
watching. We could also have a slightly higher FLASH flood threat
over the southeast zones because the convection would have higher
rainfall rates and the shear might favor training storms. The
upper low opens up Tuesday night and gets kicked northeast up the
OH Valley region, bringing the upper trof across the fcst area
overnight. Precip chances will quickly pare back to the NW flow
upslope region near the TN border. A reinforcing cold front will
drop down across the region on Wednesday, accounting for a small
precip chance mainly over the nrn tier.


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 will linger a little while longer, but
MVFR cigs will move in from the west this evening. VSBY will take a
little longer to drop but expect MVFR to develop overnight as well.
end. Heavier rain moves in after daybreak and VSBY should fall to
IFR from west to east. IFR cigs likely to develop by daybreak with
LIFR cigs for much of the day. Moderate to heavy rain will continue
through the day as well. Not much if any improvement Monday evening
at KCLT. SE to ESE wind becomes ENE this evening and remains there
through the period. Expect gusts to develop overnight and winds
speeds and gusts to increase during the morning before tapering off
a little during the afternoon and evening. KAVL will see SE to SSE
winds through the period with low end gusts possible. LLWS likely at
KAVL from near daybreak through the day Monday. LLWS possible
elsewhere given the strength of the low level jet moving in, but
chance too low to include for now.

Outlook: Some improvement expected 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...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     Med   78%     High  93%
KGSP       High 100%     High  83%     Low   41%     High  85%
KAVL       Med   62%     High 100%     High 100%     Med   78%
KHKY       High 100%     High  89%     High  83%     Med   78%
KGMU       High 100%     High  83%     Med   78%     High  85%
KAND       Med   75%     High  89%     Med   75%     High  85%

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 GAZ010-017-018-026-028-029.
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.


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