Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 261757

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
157 PM EDT Mon Sep 26 2016

A cold front will cross our area tonight and be near the Carolina
coast by Tuesday evening.  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 130 PM...convection has fired over the NC mountains, which the
fcst has well in hand. Will make some minor adjustments for the lack
of deep convection at this time east of the Blue Ridge, otherwise
the fcst is in good shape.

For the rest of the near term, surface high pressure ridging down
the Eastern Seaboard has allowed a back door cold front to move into
the region. In the meantime a strong upper low diving out of Canada
and over the Great Lakes 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 this afternoon with
near-term guidance generally agreeing on a max of about 1500 J/kg,
though the NAM as usual is more aggressive. Expect the convection
across the mountains to spread 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: Somewhat convectively inactive to start with as we are
dealing with a lull brought about by convective debris in the wake
of morning convection. Cannot rule out an MVFR-level broken ceiling
but think that VFR will prevail, so this detail will be left out in
favor of including TSRA this evening. The CAM guidance, particularly
the 4km WRF, shows a band of convection moving across metro CLT this
evening, so a TEMPO from 00Z to 04Z was included. After this
activity moves east, we may have to deal with some visibility
restrictions in the pre-dawn hours thru daybreak if we get rainfall
at the site. Have hinted at this with a 6SM vis and scattered low
cloud. The cold front should hang up to the west thru the period,
which will keep wind S to SW, and allow for destabilization with
some heating on Tuesday morning. TSRA may develop again in the late
morning, so have included a PROB30 for that possibility.

Elsewhere: Trends similar to that of KCLT, with timing based on the
4km WRF. The exception is KAVL, where a thunderstorm is already in
the vicinity, so they begin with a TEMPO for TSRA. The models are
hitting on the possibility of MVFR/IFR ceiling and vis across the
mtns Tuesday morning, so this was included at KAVL. As with KCLT,
some fog/low stratus possible early Tuesday morning and introduced
this at KHKY/KAND.

Outlook: Cold front will slowly cross the region through Tuesday.
This should result in better chances for convection. Dry high
pressure builds in behind the front on Wednesday, which should bring
VFR conditions in most places.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  93%     High 100%     High 100%     High  97%
KGSP       High 100%     High 100%     High  97%     High  99%
KAVL       High 100%     High  96%     High  83%     High  88%
KHKY       Med   66%     High  82%     High  82%     High  85%
KGMU       High 100%     High 100%     High  97%     High  97%
KAND       High 100%     High 100%     High  97%     High  89%

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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