Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 241121

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

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.


As of 700 AM EDT Wednesday: Have made a round of adjustments to all
grids but especially aviation-related parameters. Reduced pops early
this morning and tried to time highest pops moving across this
afternoon, but there is definitely some uncertainty as the HRRR with
each run speeds it up, slows it down, depicts a broken line of
intense convection, has only a couple of get the
picture. Still looks generally like most significant threat will
move from W-E starting around noon or shortly thereafter and march
across the area through 00zish. No other major changes to thinking
at this time.

Otherwise, 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.

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).


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.


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.


At KCLT and elsewhere: IFR to occasionally LIFR cigs continue at TAF
time. Expect slow but steady improvement through the morning hours
to MVFR. VFR late this afternoon into the evening, and with
lingering low-level moisture, should see MVFR again toward the end
of the period for NC TAF sites. Convection will initiate across the
area early this afternoon, spreading east through the aftn/evening
hours. Have carried VCTS at most TAFs for a period of time,
narrowing the greatest threat down in TEMPOs from 17-21z for western
TAFs and 19-23z for KCLT/KHKY, but that could easily change. Winds
will generally range from SSE 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...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       Med   78%     High  90%     High  90%     High  94%
KGSP       Med   78%     High  95%     High  95%     High 100%
KAVL       High  89%     High  95%     High  95%     High 100%
KHKY       High 100%     High  95%     High  90%     Low   58%
KGMU       High  83%     High  95%     High  95%     High 100%
KAND       High  94%     High  90%     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:


GA...Flash Flood Watch through this evening for GAZ010-017-018-026-
NC...Flash Flood Watch through this evening for NCZ033-035>037-
SC...Flash Flood Watch through this evening for SCZ001>014-019.


SHORT TERM...Wimberley
LONG TERM...Wimberley
AVIATION...TDP is the U.S. government's official web portal to all federal, state and local government web resources and services.