Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 261255

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
855 AM EDT Mon Sep 26 2016

A cold front will approach the area from the west today, and slowly
cross the region tonight through Tuesday. An upper level low will
drop south into the Ohio Valley by Wednesday and linger over the
Central Appalachians through the end of the week.


As of 830 AM...have to throw in the towel and significantly amend
the fcst for metro Charlotte to include a high precip probability
for ongoing convection along the stalled front, now that more
redevelopment has occurred over eastern Gaston and Mecklenburg
County. This activity is still expected to weaken during the
mid-morning lull. The rest of the forecast as of right now looks
like it is in good shape.

Otherwise, as for the rest of the near term, surface high pressure
ridging down the Eastern Seaboard has allowed this back door cold
front to move into the region. In the meantime a strong upper low is
diving out of Canada and over the Great Lakes this morning, and will
sweep an occluded cold front into the Ohio Valley and Appalachians
today. Downstream ridging aloft ahead of this upper low will briefly
increase today, and with low-level WAA picking up as flow shifts
around to southerly, the back door front should retreat to the
northeast (does that make it a front door front?) as the Great Lakes
system approaches. With this comes a surge in instability across the
area today with near-term guidance generally agreeing on a max of
about 1500 J/kg, though the NAM as usual is more aggressive. Expect
an increase in convection across the mountains late morning into
early afternoon, spreading east across the area this evening. CAMs
disagree on exact timing but the general look is the same. Lack of
deep-layer shear and poor mid-level lapse rates will limit severe
potential, but with strong mechanical lift plus increasing surface
buoyancy, an isolated strong or low-end severe storm cannot be ruled
out. Other concern is for isolated heavy rainfall, with PW values
increasing to 1.6-1.7" - well above the 90% threshold and pretty
close to the daily max. The limiting factor here will be increasing
winds aloft which will allow storms to actually start moving.
Nonetheless, locally heavy rainfall will be possible with storm
total basin-average QPF approaching 1.5" for the Day1 period.

Back to the increasing WAA today and the retreating back door cold this lifts northeast, despite the increasing moisture and
cloud cover, expect temperatures to rise again today especially with
the increasing amplitude of the downstream ridging aloft over us and
corresponding increase in thicknesses. MET guidance was warmest of
the pack yesterday and still too cool. It is again on the warmer
side today so leaned that direction and brought in a little bias-
correction. Might see a little cooling of overnight lows tonight in
the northern mountains as the front starts to push through, but the
main pattern change will start in the short term.


As of 300 AM EDT Monday...There is still a lot of uncertainty on the
short term forecast, as a large upper low will drop south across the
Great Lakes to the Ohio Valley. From there, models disagree on
how far south the low will get, and how quickly it will lift back
north. Even a slightly more westward position of the low could
result in lingering moisture and precip chances thru Wednesday. The
00Z ECMWF continues to show this type of solution. Meanwhile, the
GFS has the low slightly east and also keeps it drifting east across
the central Appalachians into the medium range, while the ECMWF
stalls it out around Louisville, KY.

So with all that looks like the front will be slower to
push through on Tuesday into Tuesday night, resulting in solid
chance PoPs across most of the area. There will be some instability
for TSTMS, but shear looks weak, so severe threat should be low.
While an isolated excessive rain threat may exist with the
slow-moving front, generally most areas will recent much needed
rainfall. For Wednesday into Wednesday night, I kept relatively low
PoPs that are more in line with the GFS, as the EC still more of
an outlier.

Max temps will continue to be above normal Tuesday and Wednesday,
with lows near normal in the Piedmont and slightly below normal in
the mountains.


As of 315 AM EDT Monday...An upper low is expected to be near the
area Thursday, then wobble toward the north or northeast Friday thru
the weekend. A weak surface low under the mid and upper low will
spin north of the area, either in the Upper Ohio Valley or the
Central Appalachians. So much lower thicknesses and generally dry
air will work into the area from the west.

For Thursday and Friday...other than some westerly to northwesterly
upslope showers possible along the TN/NC border, the forecast
looks dry. Temps will be below normal for the first time in a while,
with highs in the 50s and 60s in the mountains and lower to mid 70s
across the piedmont. Lows in the 40s in the mountains and 50s
piedmont. The weekend looks nice with mostly clear skies and a
slight rebound in temps to around normal.


At KCLT: TSRA in progress at issuance time should move off by 12Z,
although additional convection developing overhead cannot be
entirely ruled out. Expect mainly MVFR ceiling with some IFR
conditions in and around thunderstorms. Should see slow lifting
trend later this morning as the back door front begins to wash out,
but stronger cold front approaching from the west will approach
later today. Transitioned former PROB30 to a TEMPO TSRA, with
predominant -SHRA after about 00z. Cannot rule out brief MVFR fog
Tuesday morning with low-level moisture remaining in place before
the front scours it out. Winds should remain mainly S/SE
transitioning to S through the period, though early overnight could
see some brief oscillation to ENE.

Elsewhere:  Trends similar to that of KCLT, but with cigs not as low
and lifting earlier. With the approach of the cold front this
afternoon, included VCTS for this afternoon for KAVL/KHKY,
transitioning to -SHRA, but with VCSH for the upstate TAFs. As with
KCLT, some fog/low stratus possible Tuesday morning and introduced
this at KHKY/KAVL.

Outlook: Cold front will slowly cross the region tonight through
Tuesday. This should result in better chances for convection. Then
dry high pressure builds in behind the front on Wednesday.

Confidence Table...

            13-19Z        19-01Z        01-07Z        07-12Z
KCLT       High  80%     High 100%     High  96%     Med   74%
KGSP       High  84%     High 100%     High 100%     Med   75%
KAVL       High  93%     High 100%     High  86%     Med   76%
KHKY       High  89%     High 100%     High  93%     High  93%
KGMU       High  84%     High 100%     High 100%     Med   75%
KAND       High  94%     High 100%     High 100%     High  88%

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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