Area Forecast Discussion
Issued by NWS Greer, SC

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783
FXUS62 KGSP 240849
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
449 AM EDT Wed May 24 2017

.SYNOPSIS...
An area of low pressure to our west will pull a warm front northward
through the area today. 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 TONIGHT/...
As of 400 AM EDT Wednesday: Update quickly after last-minute
consultation with surrounding offices that opted to issue a Flash
Flood Watch, decided to go ahead and issue one today. Confidence is
low still, as indicated below.

Otherwise, the rainfall roller coaster continues through the near-
term, but in this case we are looking at a slightly more clear-cut
situation due to the large-scale ascent/synoptic forcing ahead of
the digging upper low over the Mid-Mississippi Valley. Strong SW
flow aloft ahead of this system with really lovely upper diffluence
will allow the surface low to lift from the Tennessee Valley
northeast up the western side of the Appalachians today, with strong
cold front stretching down the mountains into the Southeast. All
near-term guidance is also inducing mesolow formation along the lee
of the Appalachians across western NC, which will serve to help back
the surface flow today (more on that later). 120-130kt upper jet and
strong DPVA will push across the area today, so in addition to the
upper diffluence, jet-level divergence will also be a factor, as
well as southerly 850mb LLJ (40kt). And of course plenty of moisture
to work with as well.

So what does this mean for convective trends today? Well, after this
morning`s lull, low-level isentropic upglide as well as frontal
forcing will begin to increase convection across especially the
southwest mountains. Despite temperatures a good 5-10 degrees below
seasonal normals, should still be enough surface moisture and
heating to result in sbCAPE increasing today to between 1000-
1500J/kg especially across the Piedmont. Add to this the deep-layer
bulk shear of 50-60kt (70 if you believe the NAM) and low-level
helicity >100m2/s2 and you have an interesting setup. Because of the
depth of the trough aloft, we see some veering with height to about
500mb, but then backing again slightly up toward jet-level. Low-
level hodographs are impressively curved, but then straighten out
aloft. STP values on both the NAM and GFS are over 3 (NAM is over
5). So confidence is increasing in severe convection today, but the
limiting factor in threat advertising is lack of confidence in
specific locations, since the overall pattern lacks a definitive
theoretical arrow, so to speak, pointing to the greatest threat
location. So despite the parameters looking impressive, SPC has
limited the Day1 Outlook to Slight Risk everywhere (though that is
an upgrade for most of the area from the previous Day2 outlook), but
would not be a bit surprised to see an additional upgrade to
Enhanced with later updates. Indeed, the Day1 outlook specifically
mentioned that the tornado threat is highest in the western
Carolinas where low-level flow will be backed as mentioned earlier.
With increasing lapse rates, hail will be a threat today as well,
but locally think that strong winds are the primary threat, but with
the increasing concern for isolated tornadoes.

Heavy rain threat today will be limited to convective elements.
Given that the past two Flash Flood Watches are now 0-for-2, opted
to not extend/expand/reissue for today. While soils remain primed
from recent rain, Flash Flood Guidance remains generally >3" in 6hr,
and 1-2" for 1hr. Again, isolated locations could certainly see this
and expect that small stream/rivers will respond much more rapidly
today with more runoff, but even the Day1 Excessive Rainfall Outlook
just has Marginal across our area, so just do not think it worthy of
another watch at this time. Will leave the option open for the day
shift to issue one, however, if convective trends end up adjusting
more toward widespread heavy rain vs. severe.

If it wasn`t for the thunderstorms, today would be an incredibly
pleasant day with temperatures in the lower 70s (5-10 degrees below
seasonal normals) across the Piedmont and 60s in the mountains. More
of a cool-down expected tonight behind the front, with lows dropping
below seasonal normals with loss of deep layer moisture, as we`ll be
in the dry slot between the front and the approaching upper low
(which we`ll have to deal with for the short term).

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As of 245 AM Wed: The upper low will wobble northeastward into the
Mid-Atlantic states on Thursday. Lapse rates will remain strong
over our area as a result, and a lobe of vorticity will also spin
through. Thus scattered showers and potentially some thunderstorms
will occur over mainly western NC, with chances limited further
south by warmer midlevel temps. Instability is greatest in the
late morning to midday and that is when peak thunder chances will
be advertised; convection should wane in the aftn with rising
heights aloft. With broken cloud cover over much of the area, and
breezy winds, max temps will be similar to today`s, 7 to 10 degrees
below climo. With deep mixing, a few brisk gusts could occur simply
within the gradient, if not due to outflow from the showers/storms.

Heights continue to rise Thu night into Friday. Subsidence will
inhibit convection and skies should remain mostly clear. Temps are
expected to bounce back to near normal. A warm front will develop
over the mid-Mississippi Valley as the next low gets cranking in
the Plains. Guidance generally indicates clouds will begin to return
by early Saturday, with a small chance of showers developing along
the TN-NC border, as a result of a weak wave moving into the Tenn
Valley at that time.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 315 AM Wed: The Bermuda high will remain somewhat suppressed
over the South Atlantic region this weekend, with another broad upper
trough moving east out of the northern Plains. In response the
developing Plains low will activate a baroclinic zone over the
mid-Mississippi Valley on Saturday. The cold front associated with
the trough most likely will approach the area late Sunday or Monday.
Some differences in timing and speed of this fropa do exist between
the 00z GFS and EC. They do generally agree that the front will hang
up over the Coastal Plain Tuesday, so a small PoP is retained then
for the possibility it is slower to do so, or a wave develops along
it in our vicinity. Max temps thru the period will hover near normal,
with mins initially held warmer by clouds/precip, but those return to
near normal by Tuesday night.

Impact-wise, some diurnal thunderstorms are possible each day until
the front passes. Deep shear parameters are on the high side given
the gradient around the trough, but a capping inversion is progged by
the GFS over much of the Piedmont until the front arrives. Thus the
large CAPE values it is generating may not be realized.

&&

.AVIATION /09Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: Update for the 09z KCLT AMD is mainly to
increase winds and allow the TEMPO LIFR to go through 13z. No other
changes at this time. Another tough forecast this morning, where
cigs should be mainly IFR to LIFR, but pockets of MVFR and even VFR
continue to pass through the area, making timing of the up/down
trends very difficult. Area of SHRA moving through at TAF time will
continue to fall apart. Widespread restrictions will gradually
improve through the morning hours to MVFR and even VFR ahead of the
next system. Convection will initiate across the area this
afternoon, spreading east through the afternoon/evening hours. For
now, have left PROB30s with VCTS, but as we get closer, will need to
ramp up that wording as there is some concern of severe convection
today. Winds will generally range from SE to SW and will pick up to
between 5-10kt this afternoon, but will likely see some gusty winds
(possibly severe gusts) in TSRA.

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

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High  81%     High  85%     Med   77%     High  96%
KGSP       High  84%     High  87%     High  87%     High  96%
KAVL       High  86%     High  80%     High  91%     High  93%
KHKY       Med   77%     High  84%     Med   67%     High  88%
KGMU       High  86%     High  82%     High  88%     High 100%
KAND       High  85%     High  82%     High  95%     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 from 10 AM EDT this morning through this
     evening for GAZ010-017-018-026-028-029.
NC...Flash Flood Watch from 10 AM EDT this morning through this
     evening for NCZ033-035>037-048>053-056>059-062>065-
     068>072-082-501>510.
SC...Flash Flood Watch from 10 AM EDT this morning through this
     evening for SCZ001>014-019.

&&

$$
SYNOPSIS...Wimberley
NEAR TERM...TDP
SHORT TERM...Wimberley
LONG TERM...Wimberley
AVIATION...TDP



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