Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 262306

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
706 PM EDT Wed Apr 26 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 630 PM EDT Wednesday: IR satellite imagery showing cirrus
encroaching upon the cwfa, the onset of the expected top down
moistening accompanying the mean transition to a quicker sw flow
through a deep layer tonight. Well defined cold front was analyzed
near the mid-Mississippi valley at 22z and remains progged to
cross into the TN Valley by around daybreak, with a weak/broken
line of convection to accompany. The front/line will then move
eastward in the high terrain by mid morning, likely weakening
due to the nature of the nocturnally stabilized airmass out
ahead. After that however, 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 I40, 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:  VFR through the evening at all sites.
Shortwave ridging along the southeast coast will promote mostly
sunny/clear skies and dry tafs through daybreak, all ahead of an
approaching cold front.  With that, llv moisture is sufficient
to yield low vfr cu this afternoon, eroding to high based cirrus
overnight.  Winds during this time frame will remain swly in the
4-7kts range at all sites.  Beyond that, moisture associated
with the approaching cold front will lead to lowering cigs,
possibly mvfr/ifr by around daybreak at the westernmost sites,
spreading east into the mid/late morning hours.  Convection is
expected along/ahead of the front, therefore all tafs aside for
KCLT feature mention of precip by way of shra/tsra prob30s.

Outlook: Return flow moisture increases ahead of the next
system Thursday into Friday with the potential for thunderstorms
increasing. Scattered, diurnal thunderstorms will then continue
over the weekend.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High  93%
KAVL       High 100%     High 100%     Low   55%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     Med   68%
KAND       High 100%     High 100%     High 100%     Med   68%

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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