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Area Forecast Discussion

FXUS62 KGSP 250729

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
329 AM EDT Sun Sep 25 2016

High pressure that has dominated the Southeast will begin to break
down today as an upper trough approaches from the Mississippi Valley
and a backdoor front moves into the Carolinas from the north.
Unsettled weather with a slight cooling trend will dominate the
first half of the week, before drier and cooler air enters behind
the passing cold front by Wednesday.


As of 300 AM EDT...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

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


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 and elsewhere...A back door cold front will push
southwestward into the NC Piedmont before daybreak, bringing a light
NE wind and potentially some MVFR stratus as far SW as KHKY and
KCLT. The latest model consensus has MVFR CIGS at KHKY developing
around 12z, but not quite reaching KCLT. Will go with SCT020 at KCLT
for the morning hours after 12z. The front will then stall somewhere
in the area, with the NAM keeping in NC, while the GFS pushes it
into the Upstate by early afternoon. The GFS looks to be a southern
outlier, with most of the guidance more like the NAM`s position. So
most of the convection may remain in the NC mountains this afternoon
and then expand east into the NC piedmont this evening. I will put
PROB30 for TSRA or SHRA at all sites, since confidence is still low.
Winds will initially start out NELY, but will veer to SE this
afternoon or early evening. Moist SELY upslope and upglide flow will
likely help the stratus deck expand south and west across the entire
area tonight after sunset.

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...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High 100%     High  92%     High  99%     High  81%
KGSP       High 100%     High 100%     High 100%     High  92%
KAVL       High  90%     High 100%     High 100%     High  81%
KHKY       High 100%     Med   79%     High 100%     Med   78%
KGMU       High 100%     High 100%     High 100%     High  94%
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:






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