Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 240539

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
139 AM EDT Sat Sep 24 2016

A broad upper ridge and surface high pressure will remain
in place over the southeast Saturday as temperatures rise well above
normal again. A slightly cooler, wetter trend will begin Sunday as a
frontal boundary settles into the area from the north. Unsettled
weather is likely to linger through the first half of next week
before drier air enters behind the passing cold front by the end of
next week.


As of 1030 PM: Weather conditions continue to feature clear,
calm/light winds, and mild. I will issue a quick update to align
primary weather elements with observations.

As of 750 PM: Latest water vapor loop shows a mesmerizing vort max
over southern GA within an envelop of dry air. A spoke of dry air
was nearly stationary over the Piedmont, with another band of dry
air sweeping south across NC. The sky cover across the CWA remains
clear for many locations, I will update the forecast to decrease sky
cover and PoPs. I will also make small tweaks in temps/dewpoints
based on latest observations.

As of 230 PM EDT Friday:  An upper ridge continues to build
over the east/central CONUS this afternoon as a weak/persistent
low pressure system located along the Carolina coast begins to
open/weaken.  As the pattern evolves, guidance suggests further
dissipation of said coastal low as well as rising heights aloft
across the southern Appalachians as the aforementioned H5 ridge
builds further east.  Currently, low stratus continues to scatter
across the NC/SC piedmont while steepened llv lapse rates due to
differential heating have yielded a modest cu field across the
western Upstate, northeast GA, and the western NC high terrain.
Given modest llv moisture, am expecting said cu to persist until
heating subsides while guidance favors a low end pop across the
mtns by way of ridgetop convection, which is featured in the fcst
this afternoon.  As for pops across the Piedmont, will hold onto
low end isolated pops south of I85 for a few more hours given weak
returns and measurable observed precip, however a dry fcst will
prevail into late afternoon.

Overnight and Saturday, expecting a rather quiet night as all pops
are removed leading to only patchy fog to highlight the fcst by
mid/late morning.  Winds will remain light/northeasterly outside of
the mtns where backed north/northwest flow is expected.  Given the
departure of the low pressure system and its associated moisture,
as well as building height aloft reinforcing surface ridging,
expecting a rather sharp increase in max temps for Saturday as
sunny skies lead to highs nearly 10-12 degrees above normal.


As of 215 PM Fri: A back door cold front drops south into the area
Saturday night and stalls over the area Sunday. Model guidance is in
some disagreement on the amount of low level moisture and the
strength and timing of the developing upslope flow. Weak instability
develops, and this with the moisture and forcing, will be enough for
scattered convection to develop along and near the Blue Ridge, with
isolated convection elsewhere. There is better agreement on moisture
and upglide developing Sunday night with the stalled front in place
over the area and a stronger cold front moving toward the area from
the west. This will result in scattered showers and isolated storms
developing overnight. The back door cold front washes out Monday but
the western cold front reaches the spine of the Appalachians by the
end of the day. This should keep a moist south to southwest low
level flow across the area through the day. Instability returns
leading to continued chances of scattered convection. Best location
for precip remains along and near the Blue Ridge. Little in the way
of shear is shown on either day keeping the chance of any severe
storms minimal at best. Lows remain nearly 10 degrees above normal
both mornings. Highs will take a dip across the I-40 and I-77
corridors with the front moving in. However, they remain around 5
degrees above normal. Highs elsewhere will be lower than Saturday,
but still 5 to 10 degrees above normal. Highs drop again on Monday,
ranging from a couple of degrees to 5 degrees above normal.


As of 230 PM EDT Friday...medium range guidance has continued to
follow an encouraging trend of better agreement. There are still
discrepancies in the speed/depth of an approaching upper low and
surface cold front, but more significant updates to the extended
package were made due to the increased clarity in the past few
cycles of extended period guidance.

As far as sensible weather features are concerned, the medium range
begins 00Z Tuesday with the area in the warm sector ahead of an
approaching upper low and surface cold front and under the influence
of low-level southeasterly flow. Profiles look to be quite moist,
and with the forcing associated with the approaching upper longwave
trough and surface front, pops were increased to high-end chance
across the entire forecast area. Higher terrain will come under the
influence of some upslope southeasterly flow, so pops were raised to
likely in these areas through the day on Tuesday and into early
Wednesday morning. The 12Z GFS is on the slow end of the guidance
envelope while the 00Z ECMWF shows a more progressive solution with
the surface cold front passage, so timing was split between the two
(which, incidentally, is closer to the mean of the ensemble
solutions), and the front clears the area by 12Z Wednesday. At this
time, QPF associated with the front is not overly impressive, though
this appears to be a good chance for most of the area to receive
some badly needed rainfall.

After the frontal passage Wednesday morning, our area dries out
considerably as an upper high builds in over the southeast.
Temperatures will not drop much behind the front due to the rising
heights, but will come closer to average for this time of year. Dry
conditions will persist through the end of next week, and a trend of
lowering heights beginning at the end of the week will encourage
temps that ***may*** end up below average. this is reflected in the
forecast at this time, and future packages will tell if this trend
will continue.


At KCLT and elsewhere...High pressure settling into the area should
result in good conditions for radiation fog, especially in the
mountain valleys during the wee morning hours. There is also signs
of fog trying to develop across portions of the NC piedmont. The
majority of the guidance does not have any visibility restrictions
at KCLT, but trends will be monitored to add a tempo, if needed. A
tempo for IFR fog will be carried at KAVL. After daybreak, dry air
in place should result in mainly SKC with light winds into tonight.

Outlook: A back door cold front will push into the region late
Sunday, bringing increased moisture for early next week. Another
cold front will push in from the west on Tuesday, bringing the best
chance for showers and possibly some TSTMS. Then dry high pressure
returns midweek.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  83%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
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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