Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210530

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
130 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 1030 PM EDT: increasing coverage of mainly showers across the
extreme western zones of the CWFA so far this evening, as expected.
This is only the beginning of a few days of unsettled weather for
the region. As upper ridge axis shifts east of the area, a prolonged
fetch of deep SW flow will set up atop the CWFA, bringing
anomalously high moisture (PWATs 2-3 standard devs above
climatology) and multiple rounds of convection tonight and thru the
day on Sunday. A cooling boundary layer and moistening profiles
should result in somewhat modest buoyancy tonight, while moderate
(at best) shear is expected, so the severe storm potential appears
minimal tonight. The high PWATs should create a favorable situation
for cells that are efficient rain-makers, and would not rule out a
localized flash flood threat by tomorrow morning if cells can train
over a particular area. Min temps tonight will be well above normal.

For Sunday, the situation in terms of the pop/qpf/severe storm
potential is quite muddy, as forcing should remain relatively weak,
and trends will be dictated largely by buoyancy and high moisture
content. The instability forecast tomorrow is highly uncertain due
to the expected morning convection, what areas will be most impacted
by said convection, and whether any thermodynamic recovery will
occur in the afternoon, by which time a cold front should be
approaching the southern Appalachians from the TN Valley. Overall,
likely to categorical pops do appear warranted across most of the
area, but amounts and timing remain highly uncertain. As far as the
severe potential, certainly cannot rule out an isolated threat, but
shear will remain too limited and the instability picture too
muddied for there to be too much of a concern. The larger threat
should continue to be a slowly evolving hydro threat, but this
should be limited to a localized flash flood potential through the
near term. That being the case, feel the best approach is to allow a
little bit of rain to fall before any decisions are made re: hydro


As of 230 PM EDT Saturday: With upper-level troughing remaining to
the west of the forecast area Monday through Tuesday, and above
climo moisture persisting over the southeast, the weather pattern
will remain quite unsettled through Tuesday.

A deep, closed, upper-level low pressure system will wobble east
over the Great lakes Sunday night through Monday. Meanwhile, the
associated trough axis will drive an approaching surface cold front
into the NC mountains by daybreak Monday morning. Deeper moisture
and forcing just ahead of the approaching cold front will impact the
region primarily Sunday night. Rainfall totals for the Sunday night
period should close in on an inch in many locales, but the maximum
will be hard to pin down given the embedded convective rates. The
frontal system will then be very slow to clear the eastern piedmont
through Monday afternoon. Instability just ahead of the
slowly-moving front could reach 2000 to 2500 J/kg of sbCAPE during
the afternoon hours near the I-77 corridor and in locations
southeast of I-85 for an isolated severe threat to accompany a
continued isolated hydro threat where any training develops.

A piece of upper-level energy will then dive southward behind the
Great Lakes system Monday night through Tuesday and close off
another low center over the Midwest. Meanwhile, the recently
departed surface front will stall across the deep south and wave
formation is likely there. Another round of deep forcing and
moisture ahead of the redeveloping trough axis may well return in
the early morning hours Tuesday as the surface wave slides northeast
along the stalled frontal zone. Brief drying will likely wrap back
in from the southwest late Tuesday, but uncertainty remains high on
this timing, and any drying should be fairly short-lived.

Hydro problems could develop just about anywhere through Tuesday,
but we`ll need to see where the earlier period heavy rainfall lines
up before any later period Flood Watches can be considered. Also,
instability and shear could overlap in the southeast half of the
forecast area on Tuesday to provide a severe tstm/isolated tornado
threat, but the timing of this also remains uncertain. Will
advertise the heavy rain threat for both Monday and Tuesday in the


As of 145 pm Saturday:  Within the front side of a deep mid-Conus
trough, broad and moist sw flow will likely be lingering atop the
region to start off the period.  Timing details remain uncertain at
this point, but the going sensible wx fcst of numerous showers and
cooler than normal max temperatures looks to be on track for
Wednesday. Confidence is increasing that the warm conveyer belt pcpn
will have shifted downstream of the cwfa by Thursday. However, with
the passage of the upper trough axis and proximity of the eastward
wobbling of the closed low, decent shower chances remain, although
more scattered in nature and with low QPF.  The medium range model
consensus for Friday and next Saturday is featuring deamplifying and
weakening flow top the the SE conus. Will plan on a dry fcst for the
new day 6 and 7 with max temperatures returning to normal next


At KCLT and elsewhere: 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...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     Med   75%     High  80%     High  81%
KGSP       High 100%     Med   63%     Med   68%     Med   75%
KAVL       High 100%     Med   63%     High  90%     High  87%
KHKY       High 100%     High 100%     High  90%     High  87%
KGMU       High 100%     Med   73%     Med   68%     High  81%
KAND       High 100%     Med   68%     Med   73%     Med   75%

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:




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