Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 241821

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
221 PM 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 140 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. 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 looks good.

Otherwise, the Flash Flood threat remains, altho is less than the
Tornado threat, mainly because of the wet soil conditions. 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

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: 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...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       Med   66%     High  83%     Med   77%     High  89%
KGSP       High  80%     High  98%     High 100%     High  88%
KAVL       High  88%     High  95%     Med   76%     High  93%
KHKY       Med   79%     High  83%     Med   74%     High  87%
KGMU       Med   71%     High  98%     High  96%     High  95%
KAND       High  84%     High  99%     High 100%     High  99%

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 until 10 PM EDT this evening for GAZ010-017-
NC...Flash Flood Watch until 10 PM EDT this evening for NCZ033-
SC...Flash Flood Watch until 10 PM EDT this evening for SCZ001>014-


SHORT TERM...Wimberley
LONG TERM...Wimberley
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