Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 221055

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
655 AM EDT Sat Apr 22 2017

An unsettled pattern continues Saturday as a front stalls over our
area, with showers and thunderstorms possible. A low pressure system
will move into the area Sunday bringing widespread showers and some
thunderstorms. The low will slowly shift east of the area early next
week, bringing an end to the rain. A warming and drying trend will
commence through the rest of the work week.


As of 645 AM EDT: Dense fog has developed across the foothills and
western NC piedmont, extending into northern Greenville and
Spartanburg counties. The mountain valleys are also socked in in
fog, per the satellite fog product. I have issued a Dense Fog
Advisory for the areas where it`s widespread until 9 AM.

As of 330 AM EDT: A positively tilted upper low will slowly drift
across the Midwest today, keeping the flow out of the west-southwest
atop the CWFA. A surface front roughly parallel to the upper flow
will move little today, as it remain draped across the Tennessee
Valley to the Mid-Atlantic. A surface low along the front will track
along the front, reaching the vicinity of Chattanooga by early
evening. Guidance is in decent agreement that the NC mountains and
I-40 corridor toward the Triad will become moderately unstable,
while areas to the south will be relatively suppressed today. So
PoPs today will feature a similar gradient to yesterday, with likely
along the TN border to less than 20% across the southern Piedmont.
Temps will also be similar to yesterday, as thicknesses remain high
and there should be plenty of sunshine across the Piedmont (Highs
about 10-15 degrees above normal). As for severe potential, it still
looks fairly low. The SPC Day 1 Outlooks has a marginal risk for the
entire CWFA. An isolated large hail or damaging wind gust cannot be
ruled out.

Tonight, the upper low will continue to drift east into the
Tennessee Valley. An associated 100-120 kt 300 MB jet will set up
just to our north, placing the NC mountains within a region of
strong entrance region upper divergence. This upper support will be
couple with 100-300 J/kg of MUCAPE and strengthening deep-layer
isentropic lift. A backdoor cold front should push into the area
overnight and set up a hybrid cold air damming wedge. The NAM and
GFS disagree on how fast Pops ramp up overnight, with the GFS quite
bullish on 1-3" of QPF possible across the I-40 corridor. The NAM
shows only light amounts thru 12z Sunday. In any case, I ramp up
PoPs to categorical across pretty much the entire area along and
north of I-85, and likely to the south. If SWLY flow can advect
enough moisture into this strongly forced environment, excessive
rain and isolated flooding may be possible early Sunday.


As of 315 AM Saturday: A wet and potentially stormy Sunday and
Sunday night is on tap as an upper low moves from West TN to central
GA. This keeps strong forcing and a deep southerly flow over the
area. This flow extends to the upper levels as the right entrance
region of the upper jet is over the area keeping strong divergence
in place. Low level forcing will also be strong as cold air damming
strengthens over the area and a surface low moves from NW GA to
along the I-20 corridor. The H925 flow easterly flow is up to 3
standard deviations above normal while the H85 southerly flow is up
to 2 SD above normal. This will produce quite a bit of upglide over
the cold dome. All this comes together producing the potential for
heavy rainfall despite PW values mainly just 1 SD above normal.

The threat of severe storms is less certain given the now good
agreement in the guidance showing the TMB associated with the CAD
likely sets up just south of the CWFA. That said, it will be close
enough that some SBCAPE may be realized from Elbert County into the
Lakelands Sunday afternoon. Deep layer bulk shear and helicity will
be strong which would favor organized severe storms. There will be
some elevated CAPE, but even the GFS now shows the depth of the cold
dome up to 3000 ft AGL. While there will be enough forcing to
combine with the elevated instability for rumbles of thunder across
the area, it seems that it would be hard to realize any strong winds
at the surface across most of the area. Of course, all this depends
on exactly where the TMB sets up.

The upper low and associated surface low only slowly move to the
coast Monday then up the coast Tuesday. Therefore, showers will
linger across the area through then but will taper off from SW to NE
through the period.

Highs will be around 10 degrees below normal Sunday with the CAD in
place, rise a few degrees Monday, then rise to near normal Tuesday.
Lows remain nearly steady a little above normal.


As of 345 AM EDT Saturday: Short wave ridging moves over the area
Wednesday in the wake of the short term upper low. A trough passes
by to our NW Thursday with a ridge axis building near the Atlantic
coast Friday. At the surface, dry high pressure briefly returns on
Wednesday. A cold front approaches the area Thursday but never quite
gets here. That said, isolated diurnal convection could develop
across the mountains as moisture moves in ahead of the front. Yet
another cold front approaches the area from the west on Friday.
There is disagreement on how far east it moves, but isolated diurnal
convection could develop across the mountains again as moisture
moves in ahead of the front. Temps rise to well above normal through
the period.


At KCLT and Elsewhere: Areas of fog have developed across the
valleys and foothills, but should stay just north of KCLT early this
morning. This fog should lift out and scatter out by around 14z.
From there, the afternoon thru evening should be VFR with high-based
cu and marginally gusty SW winds. Scattered SHRA and TSRA will
develop across the mountains and out across the I-40 corridor this
afternoon, then additional showers will develop and move in from the
SW across the entire area tonight, as a back-door cold front pushes
in from the north. Cold air damming will set up by daybreak Sunday,
with CIGS lowering to MVFR and IFR. Winds will turn out of the NE
across the piedmont sites and SE at KAVL.

Outlook: A strong area of low pressure will cross the region Sunday
afternoon into Monday, with widespread low clouds and precipitation
expected to persist. Conditions should improve by Tuesday with dry
air working in behind the departing low.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High 100%     High 100%     High 100%     Med   62%
KGSP       High 100%     High 100%     High 100%     Med   75%
KAVL       Med   62%     High 100%     High  83%     Med   69%
KHKY       Med   69%     High 100%     High 100%     Med   69%
KGMU       High 100%     High 100%     High 100%     Med   69%
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:


NC...Dense Fog Advisory until 9 AM EDT this morning for NCZ035-056-


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