Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 251752

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
152 PM EDT Sun Sep 25 2016

An upper level trough will influence our weather through at least
mid week. Expect scattered to numerous showers and a few storms,
especially late Monday, ahead of the approaching cold front.  Expect
more seasonal temperatures behind the cold front for the second half
of the week.


As of 150 PM EDT Sunday:  No sig changes needed/made to the
immediate near term fcst with this update.  Did tweak t/td/sky
to better align with recent obs/sat which favored stratus erosion
across the northern NC Piedmont as well as convective cu development
regionwide.  Otherwise, full fcst discussion to follow within
the hour.

Previous Discussion:  A vigorous trough will advance across the
Upper Midwest, helping to break down the upper ridge over the
eastern states today. At the surface, a back door cold front was
analyzed just entering Davie County, NC, and should push thru most
of our NC zones by midday today. From there, the front is expected
to stall, with most of the guidance indicating it stopping before
reaching I-26, then becoming rather diffuse on the wind field. A
large stratus deck behind the front should at least push into the
NW NC Piedmont, but whether it gets any further southwest is still
in question. The edge of the cloud deck may help sharpen the temp
gradient this afternoon, and be a focus for convection. However,
forecast soundings do not show a lot of CAPE. The best instability
and trigger still looks to be in the high terrain, where scattered
showers and perhaps an isolated TSTM or two can be expected. Across
the Piedmont, mainly isolated showers expected, mainly in NC
and the eastern Upstate. Highs were bumped up per latest MOS,
ranging from the upper 70s north of I-40 to lower 90s in the Upper
Savannah Valley.

Tonight, the stalled front may continue to be a trigger for
convection, with southerly upglide flow developing over the
boundary. Also, some easterly upslope component is seen in the
lowest 2000 ft or so of the forecast soundings. Both the NAM and
GFS show expanding low cloud cover with deepening moist layer
overnight.  So PoPs will actually expand across the northern
2/3RDS of the forecast area. Lows will remain well above normal
under the cloud cover.


As of 300 AM EDT Sunday: As we begin the short term period, the
back door/wedge front will remain stalled across the area - though
not for long - with surface high pressure ridging down from the
northeast. Strong upper trough/closed low pushing through southern
Canada toward the Great Lakes will sweep a cold front into the Ohio
Valley at the beginning of the period, with a fairly significant
pattern change in its wake for the extended.

For Monday, as the cold front approaches, surface winds will veer
around to the SE and S during the day, with low-level WAA competing
with the increased cloud cover from the incoming moisture.
Instability will be on the increase as well, with the GFS between
1000-1500 J/kg in pockets across the area, especially just to the
east of the Blue Ridge, and the NAM going gangbusters with over 3000
J/kg. Luckily we have no deep-layer shear to speak of. Some warming
at 500mb contributes to poor mid-level lapse rates even with decent
low-level ones. SPC Day2 outlook just has us in general thunder and
overall think that works well, though naturally cannot rule out an
isolated strong to possibly severe storm or two. QPF continues to
inch upwards, with now almost 3/4" basin-average for Day2 across the
mountains and foothills, and well over 1" combined Days 1-3. Not
much to write home about overall but we`ll take what we can get at
this point. As for pops, with those QPFs, see no reason to not paint
a large swath of likely across the mountains Monday afternoon and
evening, spreading into the northern Piedmont by early Tuesday. A
shortwave rounding the base of the trough early Tuesday should serve
to pull the front back to the west ever so slightly Tuesday
afternoon and evening, with a corresponding brief increase in pops
across the Upstate and toward the Charlotte area. The longwave
trough rapidly kicks this out of the way however, with all pops out
of the forecast area by the end of the period.

Should see a slow but steady cooling trend through the period, with
highs back to near normal levels for Tuesday. Big changes ushered in
Tuesday night with cooler and drier post-frontal airmass, with
overnight lows progged a good 4-8 degrees colder Tuesday night than
Monday night.


As of 315 AM EDT Sunday: Cooler and drier is the rule for the
extended with temperatures dropping below normal after the front
pushes through and remaining on the cool side through the end of the
period, with highs actually progged a good 5-6 degrees below normal
Thursday and Friday. Continued discrepancies however could result in
a variety of impacts to the sensible weather. The ECMWF continues to
want to cut off the upper low over the Southern Appalachians and
spin it around and around, dancing across the mountains and back up
toward the Ohio Valley. With the steepening lapse rates convective
showers would likely develop over the mountains. The GFS, while
kicking the upper low out, does bring it far enough south to warrant
a slight mention of showers across the northern tier Thursday.
However, as WPC noted in the extended discussion, only about 10% of
ensemble members show an ECMWF-like solution, so for now will
continue to discount that.


At KCLT:  A mixture of vfr/mvfr and ifr possible through this taf
cycle as a back door front meanders about the region providing
focus for convergence/lifting of moisture.  As such, low stratus
this morning has eroded and will continue to do so to the northeast
while convective cu builds across the Piedmont.  Guidance favors
convection along/near the front through the late evening thus taf
features vcsh with a 3hr tsra tempo, all ahead of a shra with a tsra
prob30 into early/mid morning.  By that time cigs will be falling
into the mvfr/ifr range, lingering through mid/late morning with
occasional mvfr visb.  Winds through the evening/overnight/morning
will remain in the vicinity of 080-100 in the 5-7kts range before
veering southerly late in the period as the front lifts back north
given wedge erosion.

Elsewhere:  Trends similar to that of KCLT above as a back door
front/wedge boundary provides focus for convection/upglide induced
precipitation at all sites this afternoon, with the lowest
chances/confidence at the SC sites thus only vcts was included. Cigs
will lower from the northeast through the night with ifr favored at
all sites aside for KAND where only a sct ifr deck was included in
the taf.  Conditions will improve into late morning as winds veer
southerly given wedge erosion.

Outlook: Another cold front will approach the area from the west
on Monday, then slowly cross the region Monday night through
Tuesday. This should result in better chances for convection. Then
dry high pressure builds in behind the front on Wednesday.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     Med   79%     Med   73%     High  85%
KGSP       High 100%     High 100%     High  80%     Med   71%
KAVL       High 100%     High  83%     Med   69%     High  84%
KHKY       High  91%     High  85%     High  80%     Med   79%
KGMU       High 100%     High 100%     High  83%     Med   73%
KAND       High 100%     High 100%     High 100%     Med   69%

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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