Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180235

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1035 PM EDT Wed May 17 2017

Summerlike Bermuda high pressure will persist over the region
through the end of the work week, maintaining above normal
temperatures and increasing chances for mainly afternoon and evening
showers and thunderstorms Thursday and Friday.  On Saturday, a
cold front may drop down from the north to bring slightly cooler
temperatures for the weekend. Another cold front approaches the
region from the west Sunday night, and then should move across
the region on Monday.


As of 1030 PM EDT: bumped up sky grids a bit to match up with
satellite imagery, as some thick cirrus streams by. Otherwise, no
changes needed with this evening update.

As of 820 PM EDT: The forecast continues to remain on track. Expect
quiet weather conditions tonight, with an upper ridge axis in place
atop the region.

The persistent, low-level south to southwest winds will permit
low-level moisture to gradually increase overnight into Thursday.
This may generate some low clouds invading from the southwest,
especially in areas of the extreme southern Appalachians where light
upslope will continue. A few showers will be possible around
daybreak in southwest mountain areas. Expect some measure of lower
stratus to cover at least the southwest half of the forecast area
around daybreak, and then be fairly slow to lift and scatter through
late morning. Meanwhile, the upper ridge will remain relatively
stubborn over the southeast. Model profiles exhibit a bit less
capping over the region and the continued light upslope triggering
may produce scattered mountain convection and isolated coverage
across the foothills Thursday afternoon. Expect mins and maxes to
run 5 to 10 degrees above climo through the period.


As of 205 pm, deep upper troughing will persist across the
Rockies/Great Plains through the period, supporting an area of upper
level high pressure across the Southeast. After starting out the
period suppressed near the Southeast Coast, the ridge associated with
this high pressure will build into our area from the southeast over
the weekend, as large scale heights rise in the wake of a short wave
trough traversing the northeast Conus. This pattern will continue to
support an unseasonably strong Bermuda high ridging into the
Southeast through much of the period.

Low theta-e air within this ridge axis will generally limit moisture
return to areas along and west of the Appalachians through at least
early Saturday, thus sbCAPE is primarily limited to our mountain
zones Friday afternoon. In fact, a model consensus would suggest
CAPE maximizing at a robust 2500-3000 J/kg across much of the high
terrain. Ridgetop convergence/differential heating should therefore
support at least scattered convection Fri afternoon, with improving
deep layer bulk shear supporting an isolated severe weather threat
as well. Convective chances will linger into the evening before
coverage diminishes into the overnight.

Surface high pressure building into the northeast quadrant of the
country in the wake of aforementioned short wave trough is expected
to send a frontal boundary S/SW down the Eastern Seaboard Fri night
through Sat, with a model consensus indicating that the front will
be along the VA/NC border by 12Z Saturday, and to the NC/SC border
by 00Z Sunday. However, with the upper ridge holding tough across
the Southeast, uncertainty is higher than usual (and it`s almost
always high) regarding how far south the front will push before
slowing/stalling. From a pop forecast standpoint, the situation is
further complicated by the fact that short term guidance suggests
much of the warm sector will be capped or even stable, owing to the
continued impacts of deep-layer ridging. In fact, the NAM depicts
only a very narrow ribbon (i.e., 20 miles or so) of positive sbCAPE
within the moisture pool immediately along the boundary. We settled
on 30-40 pops across western NC and slight chances across SC by Sat
afternoon. Otherwise, temps will remain above normal through the
period, although temps falling through the 70s are possible across
portions of the NC Piedmont and foothills Saturday


The advertised backdoor cold front may briefly pass through most,
if not all, of the forecast area by Sunday morning before quickly
retreating north as a warm front during the day on Sunday in
advance of the long-advertised cold front, which is currently
forecasted to pass through the area Monday afternoon.  If you have
been following along, model guidance (ECMWF, GFS, Canadian) has had
a terrible time trying to come to an agreement on timing all week.
There is still a decent spread of 12-18 hours between the fastest
Canadian (front through by 12Z Monday) and the consistently slower
ECMWF (front through by 06Z Tuesday).  The current forecast takes
a blend of the GFS and ECMWF timing with fropa generally complete
by 00Z Tuesday.  Forecasted instability with the front is not be
terribly impressive at this time. Total QPF for this system will
favor the SE-facing upslope areas along the Blue Ridge Escarpment
with 1-2 inches there as amounts taper quickly into the Piedmont.
Accumulations will generally occur from Sunday afternoon through
Monday afternoon.

The progressive upper-level trough associated with the above system
will act to suppress the subtropical ridge as a deeper longwave
trough and associated Great Lakes closed low digs in quickly behind
it heading towards mid-week next week. A brief drying period on
Tuesday gives way to chance pops area wide heading into Wednesday
in advance of the next cold front approaching from the west.
Once again, there are significant timing disagreements amongst
the trio of primary models, with the GFS slower to propagate the
closed-low into the upper Midwest and slower with an associated
frontal passage by about 12-18 hours or just beyond Day 7.
The current forecast is a therefore blend of the guidance and
is therefore faster with fropa occurring overnight Wednesday.
I`m certain there will be many tweaks to the forecast in the
days ahead.

Regardless, the overall pattern (the respite from the strong
subtropical high) will result in lower thicknesses and cooler
temperatures through the end of the period, with much of the
area staying at or below normal (by as much as 5 degrees) through


At KCLT and elsewhere: It should be a quiet evening, with periods of
high clouds and light south to southwest winds. The main concern
will be increasing low level moisture in the persistent southerly
flow. Models agree fairly well on bringing in at least MVFR cigs
across the Upstate SC TAF sites, while perhaps scattered low clouds
survive to KHKY and KCLT. While guidance agrees on the moisture,
there is disagreement on how low the cigs will be. KAND has the best
chance of IFR cigs around daybreak Thursday, but any of the Piedmont
sites may see IFR conditions, if the NAM/HRRR are correct. Any vsby
restrictions should be confined to the southwest mountain valleys.
There should be a little less capping Thursday, with isolated to
scattered SHRA/TSRA possible in the mountains. Only KAVL will carry
a PROB30 for now. Expect continued S to SW surface winds throughout.

Outlook: Chances for diurnal convection and restrictions will
increase into Friday ahead of a cold front that approaches the
forecast area over the weekend.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High  86%     High  87%     High 100%
KGSP       High 100%     High  87%     High  86%     High 100%
KAVL       High 100%     High  93%     High  95%     High 100%
KHKY       High 100%     High 100%     High  91%     High 100%
KGMU       High 100%     High  87%     High  86%     High 100%
KAND       High 100%     High  87%     High  85%     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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