Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 231449

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1049 AM EDT Tue May 23 2017

A train of Gulf low pressure systems will track through our
area along a stalled front through tonight. As the front moves
southeast, a broad area of showers and storms crosses our area
Wednesday ahead of a closed upper low pressure. Expect temperatures
to be below normal until the deep upper trough moves out before the
start of the weekend. A drying trend will begin by the end of the
work week and continue until the end of the weekend.


As of 1030 AM EDT: KGSP base reflectivity shows a boundary drifting
southeast across the foothills of the western Carolinas this morning
but with widespread moderate precipitation continuing to the
southeast. Meanwhile, weak surface waves continue to slide northeast
along the boundary to aid shower coverage while a more potent
wave was moving northeast into southwest GA late this morning.
Very high coverage of at least light rainfall should continue
for most of the area, but the focus of the heavy rainfall, and
the better thunderstorm threat, will be over the piedmont as
the wave approaches through the afternoon hours. No changes are
planned to the Flash Flood Watch area at present.

The guidance remains in fairly good agreement that the main wave
will mostly move out tonight (generally after 00z), with a bit of a
lull in the activity for the end of the near-term period. Precip
associated with the approaching upper trough will move in as we
begin the short term. As for continued convective potential, the
widespread rain/clouds will keep temperatures down (highs today
should be about 10 degrees below seasonal norms) and limit
instability, but some areas across southern zones could see up to
between 500-1000J/kg during peak heating. The current Marginal Risk
in extreme southern zones seems reasonable, especially with some
deep-layer shear coming across with the upper jet. Profiles remain
too tropical for much in the way of a hail threat, so if anything it
would most likely be some gusty winds.


As of 300 AM Tue: A mature low pressure system will remain
generally over the middle Mississippi and lower Ohio valleys Wed
and early Thu. Its warm conveyor belt will bring abundant moisture
flux and isentropic upglide atop our area Wed, reactivating the
stalled boundary as a warm front once again, which continues
to push north Wed night. A portion of the area thus will see
warm-sector conditions, featuring subtropical moisture and an
uptick in instability. Strong upper level winds will generate
appreciable deep-layer shear, and t-storms may organize into linear
segments. The cold front will approach the area through the morning,
pushing in by late afternoon. With a shortwave moving northward as
it rounds the upper low, both NAM and GFS generate a mesoscale sfc
wave along the front. This could significantly increase the severe
wind threat by strengthening or backing the low level winds. Note
SPC has now expanded the marginal risk to most of the CWFA for
Day 2, and the slight risk to a portion of the Upstate southeast
of I-85.

Furthermore, both NAM/GFS develop an area of heavy rain in response
to the enhanced convergence with their frontal waves. Unfortunately
they do not agree on when this occurs, though they both suggest the
mountain/foothill zones are the most likely locations to experience
the enhanced precip. Confidence is high on a wave developing,
but confidence is low on the placement. We have opted to give the
next shift a chance to evaluate the need for a flash flood watch
for Wednesday, as well as being able to better judge the areal
extent thereof.

The thunderstorm threat will diminish Wed night following the
front, but moisture remains plentiful in its wake. The upper
low itself will drift overhead at that time, which warrants
a continuation of PoP overnight; westerly upslope flow makes
this particularly necessary near the Tenn border. Excellent
lapse rates and diurnal warming on Thursday bring back enough
instability to expect increasing shower coverage. With over 500
J/kg of CAPE progged over portions of WNC and the eastern Upstate,
thunderstorms producing small or perhaps even marginally severe
hail will be possible. Deep mixing and cold advective flow will
produce somewhat gusty winds. Precip chances finally taper off
Thu night, though continued upsloping may allow some lingering
showers in the mountains.


As of 150 AM Tuesday:  Starting at 12Z Friday weak high pressure
will be in control of our weather to start the weekend as the upper
low crosses New England and the next systems gather over the Rockies
and Plains States. The next closed upper low affecting our weather
will rotate over the North Dakota and Canadian border Saturday and
drop down to Wisconsin Sunday night. This feature will be the
driving force for shortwaves moving from the Plains States to the
Appalachians to start next week. The intensity of the convection
affecting our weather will have much to do with the timing of the
associated cold front. The GFS has the front crossing our region
late Monday which may be near peak daily heating for much of the
area. Therefore, instability would be greater with this model
solution. However, the ECMWF currently has the front crossing Sunday
night which would result in lower chances of storms due to less
instability. There would be less cooling with this upper system
since models are consistent in taking the upper system NE to eastern
Canada once it reaches the Great Lakes. At this point, Day 7 of the
current forecast has much uncertainty due to lack of model agreement
on how quickly the cold front crosses our region. There is some
agreement that by 12Z Tuesday of next week that drying will take
place as high pressure moves in behind the front.

Temperatures will be near or just below normal Friday as the upper
low moves away then temperatures a little above normal through the
rest of the medium range.


At KCLT and elsewhere: TAFs remain complicated late this morning
with abundant moisture and rain supporting widespread IFR, but
lingering pockets of VFR slipping into obs at times. As additional
SHRA overspreads the region, IFR cigs should prevail, with some
improvement to lower-end MVFR possible this afternoon. Could also
see some isolated TSRA for Upstate TAFs and KCLT, but will only
include afternoon TEMPOs for KAND and KCLT at this time. Timing
difficult to narrow down as embedded TS within the larger precip
shield could continue off and on through the day. SHRA moves out
late this afternoon/early evening, but restrictions remain and with
plentiful low-level moisture ahead of the next system, should see
widespread IFR to LIFR again Wednesday morning. Winds still forecast
to be all over the place and truly VRB as this will depend on track
of surface low across the area. For KCLT, will generally keep
conditions ENE before turning more NW later this afternoon, and then
back to NE by 00z.

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...

            14-20Z        20-02Z        02-08Z        08-12Z
KCLT       Med   73%     High  90%     Med   74%     Med   69%
KGSP       Med   78%     Med   69%     Med   72%     Low   55%
KAVL       High  83%     Med   64%     Med   65%     High  85%
KHKY       High  80%     High  82%     Med   76%     Med   76%
KGMU       Med   78%     Med   69%     Med   75%     Med   79%
KAND       High  91%     Med   75%     High  82%     Med   79%

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 GAZ018-026-028-029.
NC...Flash Flood Watch through late tonight for NCZ035>037-056-057-
SC...Flash Flood Watch through late tonight for SCZ001>014-019.


SHORT TERM...Wimberley
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