Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210538

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
138 AM EDT Mon May 21 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 1035 PM EDT: Stratiform rain and spotty showers behind the
departing/decaying evening piedmont convection will slowly dissipate
through midnight or shortly thereafter. There remains a very small
chance of upslope shower regeneration along the eastern/southern
escarpments overnight in returning light S to SE flow, but the
mesoscale models remain quiet so only isolated showers will be
featured over the extreme southern Appalachians into the early
morning hours. Some fog and low clouds may develop quickly overnight
if the mid/high debris clouds can clear sufficiently. Low
temperatures will be seasonally mild once again.

Persistence looks like a generally good forecast through Monday, but
with some subtle differences in thermal profiles. We should see a
precipitation lull continue through mid-morning Monday. Still expect
some mountain triggering starting early afternoon, but the model
guidance is not terribly keen on developing convection across the
western Piedmont and Upstate on Monday afternoon. This is further
supported by warmer profiles that will limit sbCAPE despite the high
dewpoints. Will still maintain the chance PoPs throughout, except
likely near the TN border in the high mountains, given the low level
moisture. 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: Only a few light showers remain over the area
and they are located southeast of KGSP. With another night/morning of
near saturated profiles, I expect another round of MVFR to IFR cigs
and visby across the area. We already saw KGSP go down to IFR with low
cigs and visby only to come back up to MVFR over the past hour. KAVL
also saw IFR cigs develop a couple of hours ago and they remain IFR.
The other sites will likely see a combination of MVFR and IFR restrictions
thru the morning hours, but it`s difficult to pin down exactly when
they will lower. Otherwise, Monday looks similar to Sunday with mountain
convection beginning in the early afternoon and possibly spreading into
the foothills and piedmont. Winds should remain light to calm thru the
morning, and pick up from the SW during the afternoon. Early Tuesday,
they will back to a more SLY direction.

Outlook: This persistent unsettled pattern will continue through the
week, with flight restrictions possible each day under isolated to
scattered showers and thunderstorms.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       Med   64%     High  94%     High 100%     High 100%
KGSP       Med   64%     Med   75%     High 100%     High 100%
KAVL       Med   64%     Low   36%     High 100%     High 100%
KHKY       Med   64%     High 100%     High 100%     High 100%
KGMU       Med   64%     Med   75%     High 100%     High 100%
KAND       Med   64%     Med   66%     High 100%     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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