Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 270530

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
130 AM EDT Thu Apr 27 2017

Warm and dry high pressure will persist over the region through the
evening. A dissipating cold front will cross our region on Thursday,
with warm and humid high pressure building back in for Friday and
Saturday. Another front approaches from the west on Sunday.


As of 130 AM EDT Wednesday: Fog product IR imagery depicting
developing stratocu within the increasing llvl waa flow. Stratocu
should continue to develop northeast across the cwfa and especially
in and near the high terrain overnight. In addition, thickening
cirrus and convective debris will likely overspread the cwfa toward
daybreak. Pre-frontal deep convective band now east of the
Mississippi River and convection allowing models prog it`s remnants
to work into the ga/nc mtns early tomorrow morning. Later on
Thursday, heating will aid destabilization as mid lvl lapse rates
steepen, leading guidance to suggest upwards of 1-2k j/kg sbcape
atop the region, likely concentrated east of the mtns.  Said
instability will be maximized along the I85 corridor, however
further north toward I-40, abundant cape supportive of deep
convection will also be present. Given the instability progs, and
roughly 40-50kts 0-6km Bulk Shear in place, strong/severe convection
is certainly possible with the greatest threats being from damaging
winds and large hail, although an isolated tor will not be ruled
out. For this reason, SPC has placed nearly the entire region in the
marginal risk for severe wx.  As for pops, trends will rapidly
increase after daybreak, where by midday likely pops prevail atop
the Blue Ridge southward into the Upstate. These pop trends will
lower to the east with time into the mid/late afternoon, however
mainly due to increasing range.  Temperatures on Thursday will be a
few degrees cooler than today thanks to the increasing sky cover
early in the day, yet still slightly above normal.


As of 200 PM Wednesday: Convection is expected to taper off diurnally
Thursday evening. A weak continental high will build over the area by
early Friday, and heights rise steadily through the remainder of the
period as the Bermuda high regains strength over the Southeast. A
warm front will move north out of the Gulf states and into the CWFA
Friday morning as subtropical air returns. Guidance suggests some
light precip may develop with this feature, so a small PoP is
advertised early in the day. This transitions northward to the Blue
Ridge and vicinity by the afternoon as the front lifts further north
and as diurnal destabilization occurs. That said, a rather strong
subsidence inversion is progged, so convection should remain isolated
if not totally suppressed.

Warm southerly flow continues Friday night. This does not look
capable of generating precip, though some low cloud cover will
develop and/or remain trapped beneath the inversion. Accordingly we
will advertise increasing clouds along and south of the Blue Ridge
overnight. Temps/dewpoints will trend warmer by Sat aftn, and lapse
rates continue to be pretty good in the mid to upper levels. While
the GFS/EC suggest the capping inversion will no longer be present,
and respond with QPF as a result, the NAM is capped and totally dry.
It seems suspicious that the cap would erode in the face of a
building ridge, but nonetheless I have kept PoP at slight chance
across the CWFA. In the event storms do fire, weak shear, dry
midlevels, and impressive SBCAPE values indicate the potential for a
few pulse storms producing damaging winds.


As of 230 PM EDT Wednesday...the main concern out in the medium
range will be the passage of a strong cold front on Monday and the
potential for severe weather. Before we get to that, the upper
pattern is expected to continue to amplify on Saturday night and
Sunday with an anticyclone parked off the Southeast coast and an
upper low deepening as it moves out across the Srn Plains Sunday and
then lifts northeast toward the upper Midwest Sunday night. This
pattern should be favorable for pulling in moisture from the wrn
Atlantic early in the period, and then from the Gulf as the cold
front approaches late on Sunday. Altho mainly diurnal convection
will weaken across the region with sunset on Saturday evening,
expect the improving moisture and mechanical lift to maintain a
chance of showers near the Blue Ridge Escarpment overnight, with
precip chances gradually expanding from there on Sunday, eventually
reaching the likely range in the upslope areas and chance elsewhere.
The significant mid/upper forcing should remain well to our west
through the day, but the GFS/GEFS suggests the air mass will support
CAPE on the order of 1500-2000 J/kg...thus scattered thunderstorms
looks like a really good bet for the afternoon/evening. The model
also shows just enough shear/helicity to make one think about the
possibility of organized storms, but the lack of a trigger or
boundary could be the missing ingredient that keeps our storms from
getting really interesting while we probably watch a severe weather
event unfold to our west. Temps were nudged up just a bit.

Which brings us to Monday. Timing will be the key. It is worth
noting the operational GFS continues a trend of slowing down the
arrival of the cold front, which suggests an improving chance of
organized severe storms. At the same time, the model continues to
show weakening of the system as it reaches us on Monday, suggesting
that our potential will not be as great as the potential along the
front to our west on Sunday. That being said, even though the
potential will be less on Monday over the western Carolinas, the
model still suggests strong shear on the order of 40-50 kt and SRH
on the order of 200-300 m2/s2 to go along with CAPE in the 500-1000
J/kg range. This should be sufficient to support the development of
organized storms, maybe even mini-supercells. If the system slows
down even more, the potential for greater instability goes up and we
will have more of a problem. Altho SPC does not indicate a threat
right now on D6, it stands to reason that if the environment
develops as shown in the models, we will have a severe threat on
Monday, at least over our eastern zones. For now, file this one away
until we are done with tomorrows potential event. The front should
move thru quickly and we dry out from the west by mid-evening.

The rest of the fcst is quiet as weak high pressure moves in behind
the front on Tuesday with nearly zonal flow aloft. Temps will cool
off back to near normal. A weak srn stream system may affect us by
late Wednesday, so precip chances climb back into the chance range
by the end of the period.


At KCLT and Elsewhere: Low VFR clouds have developed in the moist
southerly flow across the area. These clouds will continue through
the morning even as mid and high clouds move in from the west. This
should keep fog to a minimum, but some can`t be ruled out over the
more foggy locations or where the clouds are delayed. Guidance now
in good agreement that MVFR clouds develop through the morning in
the continued moist southerly flow into the Blue Ridge ahead of the
approaching cold front. Convection moves in with the front by early
afternoon across the west and late afternoon at KCLT. Have retained
the PROB30 to indicate the best timing for convection. Low VFR cigs
linger behind the convection before scattering out over the west
during the evening. S to SW winds this morning become gusty for the
afternoon ahead of the front. Speeds diminish some behind the front
but remain S to SW, except at KAVL where they turn NNW.

Outlook: More in the way of limited diurnal tstm chances are
expected Friday with the potential for thunderstorms increasing
progressively over the weekend.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High  94%     Med   64%
KAVL       High 100%     High 100%     High  94%     High 100%
KHKY       High 100%     High 100%     High 100%     Med   78%
KGMU       High 100%     High 100%     Med   69%     Med   71%
KAND       High 100%     Med   61%     Med   69%     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:




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