Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211142

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
742 AM EDT Sun May 21 2017

Widespread showers and a few thunderstorms are expected today
through at least Monday as a large frontal system passes
through the area.  This frontal passage will bring cooler
temperatures as well with Tuesday through Thursday highs 5 to 10
degrees below normal. A deep upper trough approaching in mid week
will result in another area of heavier showers and storms Wednesday
and Wednesday night. Expect dry high pressure and milder
temperatures next weekend.


As of 700 AM EDT Sunday: Widespread rain continues across the
western half of the area (roughly I-26 and westward). WPC MPD that
came out just over an hour ago mentioned the MCV over west GA, and
once this initial convection moves through, HRRR and other hires
models are developing second round sometime around 15z, give or take
a couple of hours, with a final round this afternoon. Pops are a
mess and it`s really hard to describe all of this in the grids; made
some minor adjustments (up) this morning but otherwise will let most
trends ride as they are. Temps/dewpoints look fine for now.

Otherwise, we remain in the warm sector this morning, with backdoor
cold front draped from VA into northern NC, and cold front
approaching from the west, attached to a surface low over the Upper
Mississippi Valley. High-amplitude ridging in place up the Eastern
Seaboard will be squished and pushed east from strong upper low over
the Northern Plains, pushing the cold front toward the Appalachians
this morning. Add to this the complication of an MCS over the Deep
South which may leave a remnant MCV to move over the area today. PW
will also be on the increase today, with values of 1.8" or so this
morning, which are pretty close to the daily max. Southerly winds
ahead of the front will help to pool this moisture against the Blue
Ridge, with plenty of orographic lift in play. As the front
approaches, low-level flow will begin to veer, losing the orthogonal
component and more mountain-parallel by midday today. Still, plenty
of moisture and synoptic lift combined with surface flow that will
remain backed through the end of the day will still allow for some
decent rainfall totals to start adding up. WPC QPF from 18-00z
tonight is over 1.25" (basin-average) for parts of the northern
mountains. 1hr flash flood guidance is about 2" for those same
areas, and excessive rainfall outlook has kept slight risk over the
area. WPC seems to have slightly higher QPF than some of the global
models, but adding in the convection-allowing schemes and could see
2-3", possibly up to 4" through the near term along the Blue Ridge.
With that and after collaboration from neighbors (specifically FFC
and RNK), have opted to hoist a Flash Flood Watch along the Blue

As for convective trends today, initial round of rain came through
early this morning, with another round moving into the western
portions of the forecast area at AFD time. Hires guidance has this
weakening but marching across the area through the morning hours,
and then as the remnant MCV moves across, depicting a flare-up in
intensity this afternoon in almost a QLCS-looking feature. Shear is
minimal, but even with rainfall and clouds in place, should see
instability increase to around 1000J/kg at least across the Upstate
this afternoon. While heavy rainfall remains the primary threat,
cannot rule out some gusty winds. We remain only in general thunder
on the Day1 Convective Outlook, but SPC did mention the possibility
of adding a risk area with later updates.

Temperatures will be highly dependent on the few, if any, breaks
that develop in the clouds, especially over the Catawba River
Valley, where it will take a little longer for rain to develop.
Across the rest of the area, should see temperatures at least 10
degrees cooler than what we saw yesterday, and some areas in the
mountains will be over 15 degrees cooler. Plentiful low-level
moisture will result in reduced diurnal range so not significantly
cooler overnight tonight.


As of 330 AM Sunday: A cold front will cross the southern
Appalachians Monday morning, weakly pushing through the
CWFA. Abundant moisture and low level convergence will continue
to warrant elevated PoPs, tapering as the frontal zone sets up
east and south of the area Monday night. Synoptic models depict
embedded shortwaves moving across the Deep South within the broader
upper trough, subsequently causing cyclogenesis along this stalled
boundary early Tuesday. As usual there is disagreement in the
exact location thereof, thus causing wide spread in the timing
and expanse of the next wave of precip for the area.

Some drying could occur in the wake of the Tuesday low, particularly
if the GFS verifies. However there is enough spread between
the various solutions that it is prudent to maintain PoPs until
the cyclogenesis process more or less repeats Tuesday night and
Wednesday. That occurs as the next embedded shortwave encounters
the front in a similar position relative to our CWFA. Furthermore,
Wed morning the closed upper low in the core of the trough will be
just to our west, and a secondary shot of dynamic forcing and/or a
cold front will impinge on the area. PoPs finally begin to decline
late Wednesday.

Obviously, hydrologic impacts will continue as rounds of rainfall
continue across the area. NAEFS ensemble mean PWAT values peak
around 2 SD above climo (above 1.75"), and NAM and GFS each depict
a PWAT plume of 1.8" to 2.0" preceding each low. Currently the most
likely track of these lows will keep the best moisture confined to
the Piedmont, a positive note given the expected heavy rainfall
today in the mountains and foothills. I can`t rule out that an
extension/expansion of the Flash Flood Watch won`t be needed,

Thunderstorms, primarily but not exclusively diurnal in nature, are
forecast each day during the period over most of the area. Sharp
differences in depicted instability limit confidence Tue and Wed.
Relatively small shear on Monday indicates little severe threat,
but hodographs look much more interesting Tue-Wed as the flow
strengthens aloft and with the possibility of the lows enhancing
the low level winds.


As of 145 am Sunday:  The axis of the deep upper trough at 12Z
Thursday should roughly be near a line from Cincinnati to Atlanta.
All deep moisture should be off the Carolina coast and moving away.
Moisture will linger under the trough axis mainly over western
North Carolina and in particular the mountains. At 12Z Thursday, the
new GFS has a closed upper low over KY and the old ECMWF has a
closed low over the Great Lakes. Some instability will be in place
Thursday and if storms develop with the low freezing levels, expect
hail. Both models maintain the closed upper low as they move NE out
of our region with rising heights coming from the Mississippi
Valley. Active weather is shown to ride up the west side of the
upper ridge from TX to the Great Lakes next weekend while our region
stays mostly dry. Passing shortwaves over the Upper Miss Valley
brush a little moisture over the NC Mtns Friday night into Saturday
but otherwise no rain expected across our area. Temperatures on
Thursday about ten degrees below normal rising to around 5 degrees
below Friday and at or slightly above on Saturday.


At KCLT and elsewhere: Messy at the TAF sites today with
widespread SHRA and embedded TSRA combined with MVFR to IFR cigs.
First round of convection moving across the area will be joined by a
second round sometime close to 15z or so (give or take a couple of
hours), with a final round (for today) coming through this
afternoon/evening. Narrowing down exact timing of TSRA will be
difficult so  have carried prolonged periods of VCTS for most TAFs,
though did provide rout times for KCLT. Guidance is indicating some
improvement after the second round moves through, with VFR
conditions at KCLT/KAND this afternoon. Restrictions build back in
tonight with IFR expected everywhere toward the end of the period.
Generally S winds (SSE to SSW) through the period, 5-10kt this
afternoon, with perhaps some low-end gusts in TSRA.

Outlook: Periods of showers and thunderstorms with associated
restrictions will continue off and on through the middle of the week.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       Med   78%     High  80%     High  95%     High  87%
KGSP       Low   41%     Med   68%     High  85%     Med   75%
KAVL       High  83%     High  90%     High  85%     Med   71%
KHKY       Med   75%     High  90%     Med   75%     High  94%
KGMU       Med   69%     Med   73%     Med   65%     Med   75%
KAND       Med   69%     High  95%     High  95%     Med   71%

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:


GA...Flash Flood Watch through late tonight for GAZ010-017.
NC...Flash Flood Watch through late tonight for NCZ033-048>050-052-
SC...Flash Flood Watch through late tonight for SCZ001>003.


SHORT TERM...Wimberley
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