Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 212338
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
738 PM EDT Fri Apr 21 2017

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH SATURDAY/...
730 PM Update...Public fcst is in good shape. Made some minor
lowering tweaks to PoPs across the non/mtns and fthills. Will
monitor radar trends for possible isol -shra moving up the I85
corridor aft 03z or so. Expect general thunder activity to continue
in a weakly forced environ across the nrn zones thru the next
update.

500 PM Update...No sigfnt changes needed to the fcst. Isol tstms
are remaining aligned within h5 flow and low instability has
prevented deep development. Don/t see much strengthening thru the
next update with llvl atmos past peak heating...perhaps a couple
more SPS level storms for small hail and elevated wind gusts.

As of 245 PM EDT Friday:  A cold front continues to approach
the region from the northwest this afternoon, currently moving
into/through the TN valley.  Meanwhile, flattened southwest
flow aloft dominates across the southeast states, essentially
oriented parallel to the sfc front mentioned above.  Because of
this, little additional movement to the southeast of the front is
expected today/tonight, likely stalling along the TN/NC border.
However, a warm/moist and diurnally unstable airmass exists out
ahead across Northeast GA and the Western Carolinas.  Shear is very
weak across the board, however maximized along and ahead of the TN
Valley frontal axis.  Thus, given this and modest CAPE spread about
the region, convection continues to develop along the I40 corridor
region across Western NC.  Said convection is in an environment
capable of producing deep updrafts, however CAPE in the HGZ is
generally poor, therefore although a few severe storms cannot be
ruled out, think hail/winds will remain sub severe on the whole.
Pops have been aligned with current radar imagery which features
categorical pops along I40, stretching west to the TN line, with
high end likely pops featured just south across the Smokies.
Elsewhere, pops taper down to slight chances further southeast
into the I85 corridor region.  Pops will lower outside of the mtns
fairly soon after sunset tonight as heating is lost, thus only
slight/chance pops remain along the TN line where the boundary
will be stalled out.

As 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.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
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.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
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.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and Elsewhere:  VFR conds outside of -shra/tstms all sites
except KAVL and KHKY where early morning BR/FG is likely. For now
looks MVFR range with continued mixing and cloudiness. Pockets of
ulvl energy may induce passing shra/tstms across the non/mtns btw
03z-06z. Main area of precip and thunder activity shud only affect
KAVL/HKKY Sat afternoon...but with warm front continuing to lift
north...addressed this with VCTS aft 18z. Mainly high clouds and
some Cu across the other terminals during the latter TAF period.

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...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Low   22%     High 100%
KHKY       High 100%     High 100%     Low   39%     High 100%
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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...LEV
NEAR TERM...CDG/SBK
SHORT TERM...JDL
LONG TERM...LEV
AVIATION...SBK



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