Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 230218

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1018 PM EDT Mon May 22 2017

A train of Gulf low pressure systems will track through our
area along a stalled front through Tuesday night, bringing
thunderstorms and the potential for heavy rainfall. Another system
will move through Wednesday, again bringing widespread heavy rain
and below average temperatures. A drying trend will begin by the end
of the work week and continue through early next weekend.


Weak cold front is analyzed extending roughly from the ATL metro
area northeast into the NC Blue Ridge. Widespread convection has
moved east of the area. A few strong storms will linger across the
NC Piedmont. Scattered showers will continue moving east across the
area early this evening.

More widespread convection is anticipated overnight as the first in
a series of mid-level speed maxes and associated moisture surges is
expected to bring another round of moderate to heavy rain with
embedded convection. This should bring another solid inch of rain
(with locally higher amounts) to the area by the end of Tuesday
morning. The Flash Flood Watch looks well placed to indicate the
threat of additional heavy rainfall. Shear profiles will be
improving throughout this period, and with the surface boundary
expected to establish itself somewhere near the southern border of
our area, cannot rule out a low-end high shear/low CAPE severe
threat southeast of I-85. However, with at least elevated convective
elements expected, convection producing excessive rainfall rates
will again be the larger concern.

While guidance is not in the greatest of agreement regarding the
details (not surprising considering the pattern), it`s plausible
that another wave of precip will waft (or begin wafting) over the
area at some point later Tuesday. Thus, categorical pops continue
through the afternoon. Widespread 1-2 inch amounts appear likely btw
00Z Tue and 00Z Wed, with locally higher amounts possible likely in
areas that see training convection. This may be enough for later
shifts to expand the Flash Flood Watch farther north.


As of 230 PM EDT Tuesday: The short term begins 00Z Wednesday with
the Gulf surface low exiting the area to the northeast. Pretty much
all major models now agree on a bit of a lull in rainfall in the
SC/NC Piedmont Tuesday night into early Wednesday morning, so pops
overnight were trimmed down to chance in those areas. The main upper
low/surface low will rotate through the Midwest as the main cold
front slides through the southern Appalachians, bringing with it yet
another round of heavy rainfall during the day on Wednesday. Many
locations could pick up an additional inch or so of rainfall
Wednesday, which could create some flooding issues in locations that
present with especially wet antecedent conditions. Models continue
to show some destabilization Wednesday afternoon, though the current
feeling is that this is overdone, considering the widespread heavy
rainfall moving the area during the day Wednesday. Thunder was
included in all zones, but with relatively poor shear profiles, the
threat of severe weather is not impressive at this time.

By late Wednesday night, the surface front will pass through the
forecast area, bringing with it the best chances of heavy rainfall.
Some moisture will linger over the mountains as the upper low dives
towards the SC/NC border and rides up the spine of the Appalachians
as a wedge of drier air works into the SC/NC Piedmont on Thursday,
so the best pops were maintained in the higher terrain through
Thursday evening. Though not all that impressive, enough instability
will linger Thursday that thunder was included in areas that feature
pops. Convection on Thursday will be more scattered in nature than
that associated with the better synoptic forcing earlier in the
week. Temperatures will hover about 5-10 degrees below average.


As of 200 PM EDT Monday: The medium range picks up 00Z Friday with a
shift towards a drier pattern. Slight chance pops linger for the NC
mountains into early Friday morning as some moisture wraps around
the back side of the exiting surface low, but drying during the day
on Friday should be quick as the upper longwave trough axis/upper
low exit off the east coast of the CONUS. The influence of surface
high pressure and a weak upper ridge will nudge in briefly for
another dry day on Saturday before a series of shortwaves begin to
cross the Appalachians as upper heights fall on Sunday. The NC
mountains will get in on precip during the day on Sunday as a
surface low and weak cold front march eastward ahead of an
approaching upper trough, with more widespread showers and possibly
storms spreading through the entire forecast area Sunday evening.
Thinking for the previous forecast was that moisture would not
arrive in the SC/NC Piedmont zones (where better instability will be
Sunday afternoon) until after peak diurnal heating, which would
greatly limit any severe threat. With both global models also coming
in a bit drier overall, I see no reason to make many adjustments to
the current forecast.

Substantial global model differences exist from this point onwards
in the medium range. The ECMWF brings the main surface low and cold
front through the area Sunday night, leaving Monday mostly dry
through the whole area. Frontal passage in the new GFS is almost a
full 24 hours later, which presents the interesting question of
Monday afternoon. If the GFS is correct, the front would be
approaching the area on Monday near peak diurnal heating. Dynamics
with the front aren`t all that impressive at the moment, but decent
instability in the SC/NC Piedmont zones coupled with the forcing
associated with the front could present a very marginal isolated
severe threat. Considering the greater than usual uncertainty at the
end of the medium range and the currently dry ECMWF, forecast
details will continue to be re-evaluated in later packages.

Temperatures begin the medium range near average and finish about 5
degrees above average early next week.


At KCLT and elsewhere: Convection will continue to avoid TAF sites
this evening even as scattered showers move east across the area.
Cannot rule out a brief restriction with the showers, but chance is
low. Better chance for restrictions overnight as the first in a
series of waves of low pressure developing along a stalled frontal
boundary is expected to bring a round of widespread showers and
embedded thunderstorms to the area after midnight. That said, timing
any TSRA is problematic so left precip as SHRA for now. Cigs should
also deteriorate to IFR or lower tonight around the time that the
showers move in. Vsby may fall to IFR as well. There may be another
lull in the precip after sunrise Tue, only to be followed by
afternoon convection once again. Despite any lull in precip,
restrictions will likely only slowly rise of MVFR by afternoon.
Winds are very tricky with the waves of low pressure expected. KAVL
should see NNW wind through the period. Elsewhere, expect winds to
generally vary between SE and SW.

Outlook: Periods of showers and thunderstorms with associated
restrictions will continue off and on through Thursday. Drying is
expected to finally occur through the end of the week.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       Med   77%     High  90%     High  82%     Med   66%
KGSP       High  84%     High  80%     High  84%     High  85%
KAVL       High  95%     High  91%     High  84%     High  91%
KHKY       High  86%     Med   77%     High  82%     Med   69%
KGMU       Med   77%     Med   73%     High  83%     Med   77%
KAND       High  88%     High  88%     Med   79%     High  84%

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 Wednesday night for GAZ018-026-
NC...Flash Flood Watch through late Wednesday night for NCZ064-065-
SC...Flash Flood Watch through late Wednesday night for SCZ001>014-


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