Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220621

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
221 AM EDT Sat Apr 22 2017

An unsettled pattern will continue as a front stalls over our area,
with showers and thunderstorms possible through the weekend. A low
pressure system will move into the area Sunday bringing widespread
showers and some thunderstorms. The low will slowly shift east of
the area early next week, bringing an end to the rain. A warming and
drying trend will commence through the rest of the work week.


As of 200 AM EDT: Updated PoPs to reflect latest radar trends.
Showers should taper off, as what little CAPE we have dissipates.
Otherwise, only minor tweaks to the grids with this update.

For Saturday, an approaching upper wave and surface cyclone
moving into the MS River Valley region will enhance sly flow
across the Deep South into the mid/late morning hours.
Meanwhile, H5 flow will back slightly further to the sw leading
to warming aloft, while H85 flow backs as well. Guidance tends
to agree that this pattern will support northward movement of
the stalled boundary, likely moving back into SE KY and the
southwestern Virginias by midday. Almost at the same time
further to the north, high pressure behind the front will slide
east allowing for the development of nely flow in the lee of the
Central Appalachians, eventually pushing a back door front
toward the area late in the period. However, profiles will be
rather unstable early on before deep mixing and the
aforementioned warming aloft works to keep convection at bay
across much of the region. The only caveat will be in
association with the intruding back door front into the mid/late
afternoon hours. Said boundary will be moving into a
potentially unstable environment, that lacks any significant
forcing/triggering. This boundary could provide that needed
lift, essentially leading to convective development along/north
of I40 toward periods end. As for pops on Saturday, look for
mid/high chances to the north, while tapering to slight chances
along the I85 corridor. Temperatures will be well above normal
once again, topping out in the lower/mid 80s across the low
terrain, generally 10 degrees cooling in the mtn valleys.


As of 225 PM Friday: As an upper low wobbles from the Ozarks to the
Tennessee River Sat night through Sunday, large scale UVV will
increase across the region. An area of convection will likely
approach the NC mtns after midnight early Sunday, in advance of a
surging cold front. Meanwhile, another part of the frontal zone is
expected to begin back-dooring its way through our area by daybreak
Sunday, as cold air damming becomes established across the Eastern
Seaboard. These factors warrant increasing pops to likely to
categorical across the mountains and northern zones by 12Z Sunday,
with all areas seeing categorical pops during the day. In terms of
how far south the CAD will likely build, the NAM is now in
relatively good agreement with the global models (which have been
quite consistent) in building the CAD to around the SC/NC border,
before the progress of the back door front is slowed by the presence
of developing surface low pressure somewhere in the north GA/
southeast TN/southwest NC vicinity. Where this boundary sets up will
obviously be critical with regard to the severe weather potential on
Sunday. Deep layer bulk shear of 45-50 kts, along with sbCAPE in the
1000-1500 J/kg range is likely in the warm sector Sunday afternoon,
thus the SPS depiction of a Day 3 Marginal Risk across much of the
southern half of our area seems justified.

Meanwhile, there is a growing consensus in the deterministic
guidance that there will be at least a marginal heavy rainfall
threat in the Sunday through Sunday night period. This sort of
stands to reason, owing to a relatively slow moving upper low, an
attendant surface boundary that could provide a focus for training
cells, strong deep layer forcing, and unseasonably high PWATs.

Uncertainty abounds regarding the timing of the end of precip on
Monday due to lack of confidence in the speed of the upper low.
Likely pops will be continued across the I-77 corridor into Monday
morning. Temps are expected to be near to above normal south of the
CAD boundary Sunday and Sun night, and below normal, perhaps well
below normal along the I-40 corridor. The entire forecast area
should be in the cool sector on Monday, and all areas should be at
least a few degrees below climo Monday afternoon.


As of 200 PM EDT Friday: After a very interesting intro to the
forecast, the end becomes relatively quiet. The medium range picks
up at 00Z Tuesday with an upper low attempting to exit the
Carolinas. Some wraparound moisture from the cyclonic flow off the
Atlantic leads me to maintain some pops, mainly in the eastern
zones, through the day on Tuesday. Temps on Tuesday were kept near
average due to lingering cloud cover.

By Wednesday, an upper ridge begins to amplify over the eastern
CONUS while a surface high begins to build over the Atlantic off the
east coast. A front approaches the Appalachians Thursday and Friday,
but the low track is sufficiently northward and moisture is so
minimal that the front...and precip...never really make it into our
area. The pattern continues to amplify significantly through the end
of the work week, with temperatures rising to above average through
the end of the extended period.


At KCLT and Elsewhere: a few showers continue to track thru western
NC, with KHKY and KCLT seeing possibly brief SHRA over the next
couple hours. Earlier rain across the NC mountains should support
fog at KAVL, once the lingering mid clouds shift east. The same can
be said for KHKY. At GSP, despite a good shower earlier, not as
confident on fog, but cannot rule out at least some ground fog
toward daybreak. Later today, another round of afternoon and evening
showers and tstms is expected to develop across mainly the NC
mountains, as an area of low pressure approaches from the west. The
piedmont looks relatively stable thru the day, so no mention of TSRA
will be carried, except at KAVL and KHKY. A backdoor cold front will
push into the area late tonight, bringing winds from SW to NE. Low
clouds will likely develop behind the front, but mainly after 06z.

Outlook: A back-door cold front is expected to push into the fcst
area early Sunday, with restrictions and precipitation likely. A
strong area of low pressure will cross the region Sunday afternoon
into Monday, with widespread low clouds and precipitation expected.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High 100%     High 100%     High 100%     High  92%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       Low   45%     High  82%     High  97%     High  87%
KHKY       High  81%     High  93%     High  97%     Med   78%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     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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