Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211507

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1107 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 1035 am Sunday: Headache-inducing forecast for the remainder
of the day, as pop trends as well as hydro/potential severe
convective trends are largely driven by mesoscale and thermodynamic
processes, about which forecast confidence plummets beyond the 3-6
hour forecast window. The most interesting feature is an MCV that
can be seen in radar imagery across north GA, and this should be the
main impetus for precip organization potential through the day.
Meanwhile, widespread rainfall has become more focused due east
through north of this feature (i.e., across the SC/NC mtns), but has
become more showery in nature south of there. Meanwhile, eastward
progression of the overall rainshield has been painstakingly slow,
but this trend should gradually break as the upper ridge continues
to flatten and progress downstream of the forecast area. Perhaps the
most uncertain aspect of today`s forecast is how the instability
picture will evolve. Temps this morning are a veritable mess, with
rain-cooled air across the western zones and penetration of the back
door/CAD front into the northeast part of the forecast area
resulting in temps in the mid/upper 60s in those areas, while the
southern NC piedmont/eastern SC Piedmont were already near 80 late
this morning. In fact, the rain-cooled areas have seen a bit of
in-situ cold air damming set up, indicated by a shift to NE winds
throughout much of the Upstate. Meanwhile, moistening profiles/poor
lapse rates do not leave a lot of wiggle room in terms of surface
heating/realization of significant instability, as even the good
surface heating across our SE zones is only yielding 500-1000 J/kg
of sbCAPE (per the latest SPC Mesoanalysis).

The main area to keep an eye on this afternoon will be across the
southwest quadrant of the CWA, where diminishing precip coverage
could yield better potential for marginal destabilization this
afternoon. The hydro threat should continue to be quite localized
through the end of the day. However, the northern SC Piedmont and
foothills, where 1.5 to 3 inches of rain has fallen since around
midnight, will need to be watched closely as rain rates associated
with any robust convection this afternoon could quite easily result
in flash flooding.

All that being said, it appears that the best potential for
significant convection across our area will be across the southeast
quadrant of the CWA, where at least a couple more hours of heating
will occur before denser cloud cover arrives. We could see the
differential heating boundary delineating this area from the
rain-cooled areas to the west serve as a focus for initiation of
more robust convective activity early/mid afternoon. However, even
if we realize some halfway decent instability, shear will be far too
modest to support anything more than a marginal severe threat.

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...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  80%     High  84%     High  81%     High  94%
KGSP       Med   66%     Med   71%     High  80%     Low   45%
KAVL       High  86%     Med   72%     Med   75%     Med   68%
KHKY       High  93%     Med   76%     Med   78%     Med   72%
KGMU       High  81%     Med   75%     Med   78%     Low   56%
KAND       High  98%     High  86%     High  83%     High  83%

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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