Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210902

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
502 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 300 AM EDT Sunday: 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 Ridge.

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: Only change to the KCLT TAF for the 09z AMD
is to increase sky cover. Timing for MVFR/IFR cigs at 10z still
looks good. Otherwise, convection and widespread restrictions can be
expected for most of the TAF sites today, with area of TSRA
spreading north from KAND/KGSP/KGMU toward KAVL at TAF time ahead of
an approaching front. Additional convection in GA will work its way
east through the early morning hours. Have prolonged periods of SHRA
VCTS for most sites, though of course the TS may be more
intermittent. For KCLT and KHKY, convection will likely hold off
till closer to noon. Mainly MVFR conditions will prevail, but IFR
also expected this morning for most sites, and occasional IFR to
LIFR conditions can be expected in +SHRA. Winds SSE to start the
period will slowly veer SSW, generally around 5kt, and subsiding
again overnight, but brief vrb gusts in TSRA cannot be ruled out.

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

Confidence Table...

            09-15Z        15-21Z        21-03Z        03-06Z
KCLT       High  88%     High  88%     Med   66%     High  84%
KGSP       Med   61%     High  84%     High  83%     Med   79%
KAVL       High  87%     High  87%     Med   78%     High  89%
KHKY       Med   75%     High  93%     High  83%     High  80%
KGMU       High  94%     Med   77%     High  80%     Med   75%
KAND       High  86%     High  84%     Med   66%     Med   74%

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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