Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 230619

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
219 AM EDT Mon Apr 23 2018

A slowly moving low pressure will bring a strong, moist flow in from
the Gulf and Atlantic today. The abundant moisture will then
gradually decrease into Tuesday. The atmosphere will generally dry
out Wednesday before another moist area of low pressure crosses the
region Thursday through Friday.


As of 150 AM: Water vapor imagery depicts the closed upper low
spinning slowly east over the lower/middle MS River Valley early
this morning. This circulation center will make very little progress
east, reaching only the central TN/KY border area by 12Z Tuesday.
Meanwhile, regional radars are showing the first heavier rain bands
entering the southwest part of the forecast area. The HRRR and other
convection-allowing models are in agreement with this timing and
they indicate the very best bands of moisture and associated heavy
rainfall crossing the region this morning through mid afternoon.
Based on these developing conditions, and little change to event
total QPF values, we see no reason to adjust the Flood Watch at

The east-southeasterly 850 mb jet will maximize this morning at
nearly 50 kt (with the NAM a seeming outlier at up to 60 kt). A high
elevation Wind Advisory is being considered, but it still appears
that advisory or better gusts will affect only elevations from 4500
feet up, plus morning ridge top reports have been a modest 35 to 40
mph max thus far.

As the rainfall becomes steadier across our area, in-situ or hybrid
cold air damming will get better established through the day. This
should help shield us against much of the upstream convection, and
the operational NAM thus appears too aggressive in uncovering warm
sector instability later this afternoon. A secondary surface low
moving through the Midlands/piedmont tonight could help wrap the
wedge front back to the northwest, but only slight chances of
thunderstorms will generally be featured along the southeast fringe
late today through tonight, and sfc to 3 km shear will lessen during
this period. Expect very little diurnal range on temperatures with
the wedge developing and holding relatively firm.


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: Deeper moisture associated with the cyclone
to the west of the Appalachians will invade from the southwest
through the morning hours. Expect a steady uptick in shower coverage
and intensity, along with lowering MVFR cigs. The heart of the
heavier rain bands will cross the region this morning through the
middle of the afternoon and produce visibility restrictions and
persistent IFR to LIFR ceilings. Also, an establishing cold air
damming layer will permit ENE to NE gusts into the 20s at time,
except SE gusts at KAVL. LLWS will flirt with the KAVL region mainly
during the passing of the best SE low level jet circa 10Z to 14Z
this morning. Will generally keep all late day/nighttime
thunderstorm mention well southeast of the terminals with this

Outlook: Some improvement is expected on Tuesday as precipitation
continues to shift northeast of the terminal forecast area. Some
moisture will linger, however, and unsettled weather will continue
on Wednesday and Thursday as a trough of low pressure persists
across the eastern part of the country.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  97%     Med   75%     Med   74%     High  90%
KGSP       High  90%     High  84%     High  90%     High  96%
KAVL       High  84%     Med   66%     Med   78%     High  92%
KHKY       High  80%     High  85%     High  82%     High  91%
KGMU       High  87%     High  86%     High  91%     High  90%
KAND       High  90%     High  84%     High  87%     High  95%

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:


Although precipitation rates do not appear sufficient in any 3 to 6
hour windows to produce flash flooding, duration flooding remains
possible, especially in the most upslope preferred locations and
over the adjacent foothills. Storm totals of 3 to 4 inches are still
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 today, 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 are accessible at


GA...Flood Watch through Tuesday morning for GAZ010-017-018-026-028-
NC...Flood Watch through Tuesday morning for NCZ033-049-050-053-059-
SC...Flood Watch through Tuesday morning for SCZ001>007-010.


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