Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 201357
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
957 AM EDT Thu Apr 20 2017

.SYNOPSIS...
More seasonable conditions return today in the form of a warm and
moist southerly flow of air, with showers and storms breaking out in
the afternoon. A cold front will make a run at the region on Friday,
leading to still higher chances of showers and storms. The front
is expected to stall over the area this weekend, with unsettled
weather continuing as a result.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 950 AM EDT Thursday:  Latest GOES-R Vis indicates erosion
trends of river valley fog across the NC/SC Piedmont regions,
which is backed up by latest observations.  Thus, will allow the
DFA to expire on schedule at the top of the hour.  Also increased
pops along the TN line where KMRX radar indicates light shra.
Otherwise, made a few slight tweaks to hourly t/td and sky to
account for recent observations and left the remainder of the
immediate near term as it was for this update.

As of 645 AM Thu: Sfc high pressure will drift further off the
East Coast this morning, with upper ridge axis just east of the
Appalachians, bringing diffluent upper flow. For these reasons,
the shallow CAD wedge remaining over the area should break
down. Some areas of dense fog developed in much of the Piedmont
before dawn. One contiguous area does exist around Charlotte and
suburbs, and that area will not likely see improvement until around
9-10 AM. Dense Fog Advisory in effect; the Upstate zones included
in the advisory are seeing more patchy fog.

Otherwise, today we will see southwesterly return flow restore
springlike conditions around the area. A modest amount of buoyancy
should develop by midday over the mtns, leading to scattered
t-storms forming there and advecting generally eastward. Revised
PoPs reflect this trend, but with hi-res models not depicting
much activity near/south of I-85, so chances have been reduced
there. Shear and dry air are lacking so the main concern with the
t-storms looks to be locally heavy rainfall. Max temps should be
two or three categories above normal. Tonight, skies should be
generally mostly clear in the Piedmont; however, as low pressure
moves through the Great Lakes, the system`s warm conveyor belt is
expected to bring increasing midlevel moisture over East TN and
the NC mountains, so clouds are retained there. PoPs also increase
toward dawn but remain below slight-chance until after 12z.

&&

.SHORT TERM /FRIDAY THROUGH SUNDAY NIGHT/...
As of 250 AM EDT Thursday: A very active short term in store for the
area, and it will begin with the upper ridge axis off the East
Coast, with WSW flow aloft over us. A fairly stacked and weakening
midlevel low over the Great Lakes and Ontario will have its
attendant front draped down the Ohio Valley toward the Southern
Plains, where it will be in the process of merging with a developing
warm front from a storm system pushing off the Rockies (more on that
later). The cold front will sag down the western slopes of the
Appalachians on Friday as temperatures in the low-level WAA regime
across the Southeast increase in to the mid-80s, only kept in check
by increasing clouds from the incoming moisture. sbCAPE values will
increase as well, with even the GFS sporting over 1000 J/kg across
the NC Piedmont. Convection most likely nearer the proximity of the
front across the mountains, but at least possible just about
everywhere. The front itself is progged to weaken, but enough
dynamics still with it that deep-layer shear values of over 30kt
forecast to exist over the mountains during peak heating. Generally
still expect convection to remain pulse-like, but some multicell
clusters may be possible as well with the limited shear. New SPC
Day2 outlook has introduced Marginal across this area and that seems
well-warranted, with isolated strong to marginally severe storms
certainly not out of the question. Lapse rates not impressively
supportive of hail but should see some weak CAA at 500mb, so
although the primary threat would be wind, some small to marginally
severe hail possible as well.

The front stalls over the area Friday night, and as the Plains
system continues to wind up, downstream ridging will increase,
lifting the front back to the north generally more along the NC/VA
line. This continues the northerly trend we have seen in new
guidance the past couple of days, and will help mitigate the heavy
rain threat with the warm front remaining to our north on Saturday.
Nevertheless, upper diffluence (jet level dynamics) plus increasing
DPVA, and a corresponding increase in deep-layer shear all with the
warm sector solidly over our area will lead to another round of
convection developing Saturday afternoon. CAPEs should increase
quite a bit during the afternoon, and disturbingly both the NAM and
GFS have 1500-2000 J/kg across portions of the western NC Piedmont.
Add to this the proximity of the warm front, along which weak
mesolows east of the slopes will be possible resulting in enhanced
low-level vorticity, and result is not a bit surprising that
suddenly we are seeing SigTor values >1 along and just north of I-
40. The gradient in CAPE will be pretty strong with the warm front
somewhere in the area, and certainly some convective bust potential
depending on where the front ends up, but concern is growing for
potential for severe convective activity on Saturday (in addition to
Sunday). New SPC Day3 outlook has introduced 5% probs across the
area and with the amount of sbCAPE, low-level helicity, at this
point any manner of severe cannot be ruled out, though main threats
would be wind and hail.

So speaking of Sunday, the surface low will works its way generally
up the TN Valley Saturday and Saturday night, and should cross the
Appalachians sometime early Sunday. ECMWF a little faster than the
GFS, NAM a little slower still. What is problematic (well, one
portion of the problem) is a surface high ridging east along the
Great lakes that, under confluent flow over New England, should be
enticed to ridge down the Eastern Seaboard, pushing the warm front
back south toward SC as a wedge front (interesting to watch the
evolution of forecast soundings at KHKY with CAPE/shear on Saturday
transitioning to low-level inversion/wedge by Sunday morning). The
approaching surface low has little wiggle room in this case. Deep-
layer shear becomes more unidirectional on Sunday and once again
instability will increase in the warm sector - wherever the warm
sector may be, depending on the location of the warm/wedge front.
With the front shifting south, best convective potential will be
closer to the NC/SC state line, with some margin of error of the
eventual actual location. Instability will be the limiting factor on
Sunday with isentropic lift ahead of the synoptic lift resulting in
widespread cloud cover and maybe even some drizzly conditions across
the northern NC Piedmont. Timing of the actual cold frontal passage
will be tricky as well, with rapid stabilization ahead of it.
Another round of severe potential expected Sunday, with again all
severe modes possible. Temperatures will be correspondingly
difficult.

The front pushes through Sunday night, taking the ruckus with it,
with a cooler/drier airmass beginning to push in by the end of the
period.

&&

.LONG TERM /MONDAY THROUGH WEDNESDAY/...
As of 330 AM EDT Thursday: Lucky for us, the extended is pretty
quiet in the wake of the weekend mess. The upper low dives south
over our area on Monday and off the SC coast Monday night, with high
pressure in its wake. The parent high for this ridge is way up west
of Hudson Bay. The high pressure will remain in control with fairly
seasonal temperatures in place initially, but with a gradual warmup
through the end of the period as the high modifies. The 00z ECMWF
does want to bring a front through Wednesday night (the GFS is about
24h slower), but even on the faster ECWMF the southern edge of the
front dries up over our area. Have depicted a slight increase in
cloud cover but otherwise no sensible weather impacts.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
At KCLT and elsewhere: Low stratus and patchy fog likely will
persist until 14-15z per consensus of short-term guidance, but
confidence is relatively low given the rapid changes (multiple
cycles of improvement and worsening) that have occurred so far
this morning. With abundant low level moisture as well as a very
shallow wedge airmass remaining in place, even if a site is MVFR or
VFR at 12z does not necessarily mean they are out of the woods for
IFR. Hence liberal use of TEMPOs in the first couple hrs. Following
the dissolution of the morning restrictions, however, the rest of
the period looks low-impact. Diurnal destabilization is expected
to be more or less climatological today, with the mtns seeing
isold-sct TSRA break out beginning around midday, thus VCTS at
KAVL. These will advect east into the NC Piedmont and may also
affect KHKY in the aftn, but other sites` chances are too low to
mention. Winds generally SW. Currently it appears sfc conditions
will be dry enough to expect VFR Friday morning, but areas that do
get hit with SHRA/TSRA this aftn might have ground fog to deal with.

Outlook: Diurnally driven TSRA are expected again Friday. Another
backdoor cold front is expected to settle over the fcst area this
weekend, with restrictions and precipitation likely.

Confidence Table...

            14-20Z        20-02Z        02-08Z        08-12Z
KCLT       High  88%     High 100%     High 100%     High 100%
KGSP       High  81%     High 100%     High 100%     High 100%
KAVL       High  87%     High 100%     High 100%     High  95%
KHKY       High  84%     High 100%     High 100%     High 100%
KGMU       High  86%     High 100%     High 100%     High 100%
KAND       High  87%     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...Dense Fog Advisory until 10 AM EDT this morning for NCZ036-037-
     057-068>072-082.
SC...Dense Fog Advisory until 10 AM EDT this morning for SCZ008-009-
     012>014.

&&

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



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