Area Forecast Discussion
Issued by NWS Greer, SC

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546
FXUS62 KGSP 241900
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
300 PM EDT Wed May 24 2017

.SYNOPSIS...
An area of low pressure to our west will push a warm front northward
through the area tonight. Unstable conditions south of the front
will lead to showers and thunderstorms producing heavy rain. A few
more showers may occur Thursday before drier and warmer conditions
return to end the work week.

&&

.NEAR TERM /THROUGH THURSDAY/...
As of 240 PM EDT Wednesday: Atmosphere freely convecting across most
of the western Carolinas and northeast Georgia. Storms not quite
severe yet, but the SPC Mesoscale Analysis shows sfc based CAPE in
the 1000-2000 J/kg range and sufficient effective bulk shear and
storm relative helicity to support supercellular structures. Only
problem right now might be the lack of a cap, which is allowing too
many cells to develop instead of concentrating more of the available
energy into a fewer number of stronger storms. Still expect storms
to get better and better organized over the next hour or two. The
1630 UTC update to the Day1 Outlook brought the Enhanced Risk across
most of the area east of the Blue Ridge and that seems wise. The
best tornado risk appears to be the I-85 corridor. Entire TOR Watch
still looks good.

Situation appears very dynamic over the next 12 hours as a deep
upper low drops down across the mid-MS Valley region to the TN
Valley region through the overnight hours. All manner of mid/upper
forcing will be brought to bear with excellent upper divergence as a
140+ kt jet streak lifts out of the upper trof and a strong short
wave also rotates around and lifts NE across the fcst area. Once the
initial round of supercellular convection moves on out, we could
still get some redevelopment closer to the actual cold front as seen
on upstream radars over middle TN and nrn AL. Severe weather will be
less likely with this activity as it will be moving in as the
instability will be on the wane. Cannot rule out flooding in any
areas that have repeat storms, given the wet soil conditions, so the
Flood Watch will be left in place for the time being. Convection
should pare back to the mtns late tonight once the main wave lifts
out and the upper divergence also moves NE.

On Thursday, the upper low will rotate past our area and then off to
the Mid-Atlantic region. We retain enough instability with cold air
aloft and enough forcing with the cyclonic flow that some shower
activity can be expected. Thunderstorms will probably be confined to
the Carolina Piedmont. Temps should be below normal.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
As of 230 PM EDT Wednesday: The sharp trough, and associated upper
level low pressure system, we have been dealing with will shift to
the east of our forecast area (FA) Thursday evening. The last of the
convection should be ending Thursday evening in the NC mountains.

As the upper low lifts northeast into New England we will remain in
a broad northwest flow aloft. Meanwhile high pressure from the Gulf
of Mexico will build north into the Tennessee Valley. Moisture will
be on a downward swing, and with the high pressure system close
enough, we should have a dry Friday.

The upper flow will deamplify Friday night and Saturday, with
ripples of energy riding east/southeast from the Mississippi Valley
into parts of the southeast. At the same time, a weak frontal
boundary will try to drop south (from Kentucky and Virginia), but
will encounter the southern end of the westerlies and halt.

Nevertheless there appears to be a boundary of sorts (depending on
model preference) which allows some moisture to pool and
precipitation to develop. The GFS is the fastest, with the NAM and
European Models slower. Since the Gulf of Mexico is cutoff from this
scenario, we have sided with the slower precipitation guide Friday
night, limited late in the NC mountains. We will have chance, or
small chance, pops Saturday, perhaps reaching down to near the SC/NC
state line and NE Georgia.

Interestingly the SB Capes from both the GFS and NAM are hefty for
Saturday. However, a stout cap is in place. Therefore without a
strong trigger it might be difficult to realize the instability.

In the temperature department our thermal profile signals a warming
trend into Saturday, which we will follow.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
As of 215 pm EDT Wednesday: the extended fcst picks up at 00z on
Sunday with flattening upper ridging over the southeast and another
broad upper trof digging down over the Western Great Lakes. The long
range models still have some lobes of mid to upper lvl shortwave
energy passing just to our north on Sunday, however the main trof
will be slow to progress eastward and isn`t expected to move over
the fcst area until late Tues into early Wed. At the sfc, a cold
front will approach the CWFA early Sunday but is not expected to
move thru the CWFA until Monday. It appears to become stationary
just to our SE by early Tues and lingers over the area until another,
more robust cold front pushes thru the CWFA on Wed into early Thurs.
As for the sensible fcst, no significant changes were made. We still
have solid chances for convection on Sunday and Monday, and slight
to solid chances on Tuesday and Wednesday with the highest POPs
generally over the higher terrain. Temps will remain above climatology
thru the period with a slight cooling trend towards the end of the
period.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
At KCLT and elsewhere: Ceiling continues to lift from MVFR to VFR as
boundary layer warms early this afternoon. Meanwhile, convection was
firing across most of the region though not at any TAF sites. Think
most locations will end up VFR prevailing with MVFR in convective
elements, and that is how we will play it. Winds could be strong and
gusty inside storms as well. The timing of the best chance is
generally through 00Z, but a second round could happen this evening
from the west closer to the front. Several of the storms could be
severe, and a Tornado Watch remains in effect for the entire fcst
area through 22Z. In the wake of the convection, ceiling should come
up to VFR for several hours, before dropping back down in the early
morning hours. Winds should be southerly through the evening hours,
and then with a switch to SW or WSW in the early morning hours (NW
in the Mtns).

Outlook: Scattered showers and thunderstorms will continue on
Thursday afternoon as the upper trough passes. Drying is expected to
finally occur Thursday night through Saturday.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       Med   66%     High  87%     Med   75%     High  91%
KGSP       High  88%     High 100%     High 100%     High  91%
KAVL       High  87%     High  96%     Med   73%     High 100%
KHKY       Low   58%     Med   76%     Med   70%     High  93%
KGMU       Med   73%     High 100%     High  95%     High 100%
KAND       High  88%     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...Flash Flood Watch until 10 PM EDT this evening for GAZ010-017-
     018-026-028-029.
NC...Flash Flood Watch until 10 PM EDT this evening for NCZ033-
     035>037-048>053-056>059-062>065-068>072-082-501>510.
SC...Flash Flood Watch until 10 PM EDT this evening for SCZ001>014-
     019.

&&

$$
SYNOPSIS...JPT
NEAR TERM...PM
SHORT TERM...TS
LONG TERM...JPT/Wimberley
AVIATION...PM



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