Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 261047

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

Warm and dry high pressure will build in across the region today. A
dissipating cold front crosses our region Thursday, then warm and
humid high pressure builds back in for Friday and Saturday. Another
front approaches from the west on Sunday.


As of 645 AM EDT: Low clouds linger over the I-40 corridor with fog
spreading out across the foothills into the northern Upstate this
morning. Mountain valley fog has developed as well. The fog will be
patchy dense, so issued an SPS for the potential travel problems.
The fog should dissipate by 14Z with the low clouds over the I-40
corridor possibly lingering through noon.

Southerly flow develops between a departing high pressure center and
a cold front moving into the MS river valley. Lots of sunshine and
increasing thickness values as a ridge moves overhead will lead to
highs in the 80s across much of the area.

The ridge axis moves east tonight and a short wave approaches from
the west. This wave pushes the cold front into middle TN and AL
overnight. The continued moist southerly flow will lead to an
increase in clouds overnight. Isolated showers may develop across
the NE GA mountains and the far SW NC mountains by daybreak, but the
rest of the area will remain dry. Lows will be around 10 degrees
above normal.


As of 315 AM EDT Wednesday: A negatively tilted short wave trough
will lift NE across the Ohio Valley on Thursday, brushing the NC
mountains early in the day. An associated cold front will bring a
line of decaying convection, which should move into the western half
of the CWFA in the 12-18z time frame. This may leave plenty of cloud
cover which may limit instability for the afternoon. But models do
show enough forcing to keep the convection going (albeit in probably
weak form) into the eastern half of the CWFA. The new Day 2
Convective Outlook from SPC still has a large Marginal Risk from the
Great Lakes to the FL gulf coast, nudged a little east from the old
Day 3 outlook. Temps will be be a little cooler than today`s
readings under the clouds and precip, but still above normal.

Thursday night thru Friday night, the front will wash out and leave
a warm humid air mass in place. Forecast soundings on Friday show a
"loaded gun" type sounding, with a significant cap above the top of
the mixed layer. Guidance has trended less convective, so PoPs were
reduced to just slight CHC, mainly in the mountains where terrain
effects may be enough to pop a few storms. Those that do develop (if
we do get the 2500 J/kg advertised in both NAM and GFS) may be quite
strong. Shear also remains high enough for quasi-organized cells,
but the limited upper forcing and the strong cap should limit
coverage. Highs rebound back into the lower to mid 80s across the
Piedmont. Lows about 10-15 degrees above normal.


As of 330 AM EDT Wednesday: A high amplitude upper ridge will build
along the East Coast, while a Bermuda High pattern sets up over the
Southeast. This will bring summer-like conditions for Saturday into
Sunday, with temps Saturday approaching 90 in portions of the
Piedmont. Persistent low-level southerly flow will bring more
moisture in, and it looks like we will be less capped than Friday.
So PoPs will feature slightly better coverage, but still isolated to
widely scattered in nature. The GFS advertises another day of
1500-2500 J/kg of sbCAPE, while the ECMWF and Canadian show more
like 1000-1500 J/kg. Also, shear and upper support will be weaker
for convection. So pulse storm mode is most likely and severe threat
should be low.

Sunday and Monday, the broad upper trough over the western CONUS
will sharpen and close off an upper low over the Midwest, then lift
into the Great Lakes. This will bring another negatively tilted
trough across the area on Monday. The medium range guidance is in
decent agreement on this, and also on the timing of the associated
cold front early Monday. This front looks to be stronger and more
strongly forced than the Thursday front, but currently looks to
cross during the diurnal minimum for instability. So while
confidence in PoPs goes up, as far as severe threat, confidence
remains low. Temps will also be tricky, as the GFS in particular,
shows plenty of southerly upslope clouds and precip developing on
Sunday, while the ECMWF is drier. If the GFS is right, temps should
be at least a couple categories cooler than Saturday. However, if
the EC is right, Sunday could be another hot day for late April. I
opted to blend in some of the EC temps for Sunday, going slightly
above the Superblend. Temps should be near or slightly below normal
behind the front Monday and Tuesday.


At KCLT and Elsewhere: CLT looks to stay VFR this morning with low
clouds and fog remaining north and west of the air field. Elsewhere,
low clouds and fog this morning will dissipate by 14Z leaving cirrus
in place. Winds go S to SW all areas through the afternoon and
remain there overnight, although with diminishing speeds. Cirrus
increases through the evening with lower VFR moving in from the
west ahead of a cold front by daybreak. Expect any restrictions and
precip to remain west of the area through daybreak as well.

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...

            10-16Z        16-22Z        22-04Z        04-06Z
KCLT       High  83%     High 100%     High 100%     High 100%
KGSP       High  94%     High 100%     High 100%     High 100%
KAVL       High  89%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High  97%     High 100%     High 100%     High 100%
KAND       High  94%     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:




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