Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 202358

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
758 PM EDT Sun May 20 2018

A moist air mass will remain atop the region through at least
Tuesday. This will result in a continued high chance of showers and
storms each afternoon and evening.


As of 745 PM EDT: Outflow driven convection has pushed to the lower
piedmont early this evening and should continue to slip southeast
and slowly dissipate into a trailing stratiform rain shield through
late evening. Any additional gusty wind or hydro issues should be
confined to generally the Rock Hill to Greenwood area. Any lingering
precipitation should steadily end by midnight, however, there is a
small chance of upslope regeneration along the eastern
southern/escarpments overnight in light S to SE flow, but the
mesoscale models are quiet.

Persistence looks like a good forecast through Monday. We should get
a lull through mid-morning Monday. We could end up with some patchy
dense fog overnight in places that had good rain late today. Low
temps will be seasonally mild once again. On Monday, destabilization
will depend on when any lingering convective debris moves away or
dissipates, but in general it should be fairly early in the day over
the mtns. The model guidance is not terribly keen on developing
convection across the western Piedmont and Upstate on Monday
afternoon, however. High temps will be just above normal.


As of 245 PM EDT Sunday: The forecast area will be between a weak
upper low over GA and a subtropical anticyclone off the Southeast
Coast on Tuesday. This will keep a deep, moist southerly flow over
the area as surface high pressure sits off the Southeast coast as
well. Instability will be weak, but the southerly flow and moist air
mass will create conditions favorable for diurnal convection. Given
the weak instability and moist profiles, expect heavy rainfall would
be the greater threat than severe storms. Highest PoP will be across
the mountains, western Upstate, and NE GA, but the rest of the area
will see good chance values. The upper low washes out Wednesday and
the subtropical anticyclone is suppressed as a short wave trough
passes by to our north. This short wave does push a frontal boundary
into the area on Wednesday. The short wave also creates increasing
lapse rates with cooling mid level temps. Dry mid level air filters
in as well increasing surface delta theta-e values. DCAPE values
remain modest, but expect severe chances to creep up with the
moderate instability and drier mid levels. That said, heavy rainfall
cannot be ruled out given the good coverage and frontal boundary.
Lows remain nearly steady 5 to 10 degrees above normal. Highs near
normal Tuesday rise to around 5 degrees above normal Wednesday.


As of 235 PM EDT Sunday: The medium range forecast period begins
Wednesday night, as an overall unsettled weather pattern remains on
tap through into next week. With the Bermuda High once extended into
the Southeast retreating back to the east and an upper trough slowly
exiting from the NE and Canada off to the east, weakening upper
ridge is progged to move out of the central plains and over the
eastern half of the US on Thursday. As sfc high manages to build
south into the OH Valley before being pushed eastward and out to
sea, a weak sfc trough looks to meander around and just south of the
FA Thursday into early Friday. With a very moist airmass in place as
PWATs remain well above one inch, low-level SW flow returns on
Friday as weak upper ridging returns to the north. Expect diurnal
convection to be a daily occurrence as dewpoints remain in the mid
to upper 60s and severe weather parameters, while not overly
impressive, remain somewhat on the map of interest. Thus, would not
entirely rule out an isolated severe storm any of the days, but the
bigger concern will be the continued potential threat for periods
heavy rainfall and localized flash flooding.

Into the weekend, as it has been mentioned over the past few days,
things could get interesting and even more complicated. While
another system looks to move into the upper midwest and gradually
approach the area just past the medium range forecast period,
guidance continues to hint at the possibility of a developing
tropical system. However, plenty of discrepancy remains between the
models so nothing is set in stone. Whether it develop in the GOM or
on the east coast of FL, or not at all, moisture will continue to
infiltrate into the area, and thus the continuation of an unsettled
weather pattern.

Temperatures each day will climb into the low to mid 80s (cooler
across the mountains), with overnight lows above normal.


At KCLT and elsewhere: Thunderstorms are quickly slipping south of
all the terminal locations around 00Z this evening. Will maintain
mainly showers at the foothill and piedmont sites through mid-
evening before precip ends. Anticipate some MVFR cigs and fog
developing tonight in areas that received the heavy rainfall, but
timing will be very hard to pin down, and IFR stratus and fog will
be possible during the early morning hours. Otherwise, Monday looks
to be nearly a copy of today. Wind should remain light S/SW once all
the cold pool outflows dissipate, light/variable overnight, and
then SW again Monday.

Outlook: The unsettled weather pattern will continue through the
week, with flight restrictions possible each day around
isolated/scattered showers and storms.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High  99%     High 100%     High 100%
KGSP       Med   77%     Low   53%     High  94%     High 100%
KAVL       High 100%     High  91%     High  94%     High 100%
KHKY       Med   73%     Low   55%     High 100%     High 100%
KGMU       Med   76%     Low   56%     High 100%     High 100%
KAND       High  85%     Med   61%     Med   76%     High 100%

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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