Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 260548

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
148 AM EDT Wed Apr 26 2017

An area of low pressure will gradually weaken and drift northeast
along the Mid-Atlantic coast through tonight. Warm and dry
high pressure will filter in behind the low through Wednesday. A
dissipating cold front crosses our region Thursday, then warm and
humid high pressure builds back in for Friday and Saturday.


As of 145 AM EDT: Low clouds lingering east of the I-26 corridor and
may continue through daybreak. If the clouds linger, fog will be
less likely. To the west skies have cleared, so fog is more likely.
However, it is still uncertain whether any dense fog will develop,
but it is possible.

The southeast ridge axis will be solidly atop the area on Wednesday.
Any boundary layer drying will be brief during the day before
southerly return flow starts up under the ridge in the afternoon.
Mid-level temperatures should be too warm to permit any convective
development despite the good heating - with temperatures rebounding
into the 80s in most areas.


As of 215 PM Tue: A deep shortwave trough will cross the Mississippi
River Wednesday night. Heights fall accordingly over the CWFA thru
Thursday morning. Though the shortwave and associated sfc low are
displaced by the western Atlantic ridge, tracking toward the Great
Lakes, the system`s cold front should reach the west side of the
Appalachians Thu morning. This appears slightly sooner than depicted
on runs from yesterday. Initially we are impacted by the warm
conveyor belt, providing some upglide in conjunction with DPVA early
in the day that warrants small PoPs over the mtns, NE GA, and western
Upstate; low-level cold advection is essentially nil. Overall, the
strong subsidence inversion seen Wednesday will remain over the area,
and convection looks unlikely until perhaps late afternoon when
cooler midlevel air pushes in behind the front and erodes the capping
inversion. Instability of a few hundred joules will likely develop as
that happens, and hodographs indicate storms may organize, perhaps
becoming capable of damaging winds. Current SPC D3 outlook includes
the far western CWFA in an expansive marginal risk along the front,
but I would not be surprised if tonight`s new D2 expands the risk
area further east, seeing the current model trends. Max temps will
run above normal overall, perhaps a couple degrees in the cloudier
west and as much as 6 degrees above in the east.

By early Friday a weak area of high pressure will have built back
into most or all of the area behind the front. Over the course of the
day, heights rise above it as the western Atlantic ridge begins to
strengthen again. This most likely will form a cap over the area and
limit convective coverage to the higher terrain Friday aftn. However,
the NAM depicts a developing low near the Ozarks as lifting the old
cold front back through our FA as a warm front, with a bit of QPF
response to match. Thus a low PoP will be advertised in the morning,
over mainly NE GA and the Upstate, to handle this possibility. With
max temps nearly 10 degrees above normal, and favorable lapse rates
above the cap, CAPE values above 2000 J/kg are progged over most of
the area. Where this instability is realized, vigorous tstms are
expected. Shear is expected to be too strong for pulse storms but too
weak for supercells, so in light of the dry mid to upper levels seen
on profiles, a multicell cluster producing damaging winds is not out
of the question.


As of 230 PM EDT Tuesday: Agree with the previous assessment that
our weekend will start out feeling very summerlike under a Bermuda
high pattern. Diurnal convection will gradually diminish Friday
night and have tried to indicate a lull overnight and early
Saturday, before scattered mainly afternoon/evening convection fires
up again on Saturday. Temps will be well above normal and may push
the upper 80s or near 90 over the Piedmont. Most of the convection
will die off Saturday evening, but a developing upslope E/SE flow
may keep some shower activity going overnight near the Blue Ridge

The forecast gets a bit more tricky on Sunday as the upper pattern
amplifies with a deep upper low coming out over the Srn Plains and
an upper ridge off the East Coast. Upslope flow continues to
strengthen on Sunday and the model guidance responds with more
precip development emanating from the Blue Ridge. Precip probability
ramps up into the likely range in the upslope areas. Temps are
problematic. If the clouds and precip develop as in the model
guidance, the max temp will be something on the order of 5-10
degrees cooler than Saturday, but if not, we will be well above
normal again.

The most interesting day looks like Monday, as the progression of
the upper low lifting to the upper Midwest drives a cold front
eastward. Of note is the possibility of organized severe weather
along the front to our west as it approaches. There is good upper
support with this strong front, although a weakening trend is noted
in the guidance as it moves across our region. Timing is the key.
The operational GFS suggests that some strong storms could move in
from the west late Sunday night, but then redevelopment would happen
over the wrn Piedmont of the Carolinas Monday afternoon. The
environment would be supportive of more strong storms with
sufficient shear but marginal CAPE. Will have to keep an eye on this
one. If the system slows down as suggested by the latest ECMWF, our
threat will be greater.

The front should blast across the region late Monday, clearing us
out for Monday night and Tuesday, with temps cooling back close to


At KCLT and Elsewhere: Low level moisture and weak low level WWA
flow is keeping MVFR to low VFR clouds east of the I-26 corridor.
These clouds may linger through the morning at the NC sites. If so,
fog will be less likely, but MVFR cigs will be more likely. Have
kept conditions MVFR there for now. With clearing skies at the SC
sites, at least MVFR fog is more likely with some cig restrictions
due to the fog. That said, IFR or even dense fog may develop where
clearing can occur. Given the uncertainty have not gone with any IFR
for now, but will amend as needed. Any restrictions dissipate by 13Z
or so with clearing skies and developing S wind. KAVL will see N
wind. Stronger S to SW wind develops this afternoon with mainly
cirrus overhead. Winds diminish this evening with increasing
cirrus coverage.

Outlook: Return flow moisture will develop 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...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     Med   66%     High 100%     High 100%
KGSP       High 100%     High  83%     High 100%     High 100%
KAVL       High 100%     Med   72%     High 100%     High 100%
KHKY       High  93%     High  83%     High 100%     High 100%
KGMU       High 100%     High  83%     High 100%     High 100%
KAND       High 100%     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:




SHORT TERM...Wimberley
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