Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 260240

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1040 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 1030 PM, for the past 10 hours or so, the mesoscale models
have been far too aggressive in initiating convection near weak back
door/quasi-stationary frontal boundary, which currently extends
along the SC/NC border in the Piedmont, through roughly the I-26
corridor in the mtns. Yes, there are a few very isolated cells, but
these are so isolated, small, and brief that they hardly even
warrant a forecast mention. Nevertheless, the low level flow is
expected to increase overnight in advance of cold front approaching
the eastern Conus from the west. This is expected to support
establishment of weak upglide after midnight, esp across western NC.
Therefore, expect some degree of shower coverage to continue
overnight, and would not be at all surprised to see this coverage
increase toward daybreak. Therefore, will allow pops to continue to
increase into the chance range, mainly across the NC foothills and
Piedmont. Min temps will remain well above normal.

Otherwise, surface ridging over the East will erode on Monday as an
upper trof and associated surface cold front race across the OH
valley.  Meanwhile, remnant cyclonic flow at H5 atop the FL
panhandle will retrograde west/southwest allowing flow through the
column to veer further thereby yielding southerly return flow. Pops
across the region will increase as the front approaches late in the
day, likely moving into the NC high terrain by periods end. Profiles
suggest any subsidence will be weakest over the mtns, therefore high
chances to low end likely levels are featured for those locales,
with low/mid chances to highlight the Piedmont regions.  Fortunately
at this point it looks as if the best shear associated with this
trof will slide by to the north, thus SPC has highlighted the
central/northern Apps with a mrgl risk, with only general thunder
featured across the cwfa.  Temperatures on Monday will be around 5-7
degrees cooler than today as weak moist upglide along the wedge
periphery yields a slow start to diurnal heating.


As of 230 PM EDT Sunday: Short waves rotating around a strong upper
low over the Great Lakes will slowly push a cold front into the area
from the west Monday night. The front will slowly cross the area on
Tuesday. A moist southerly low level flow will remain across the
area ahead of the front. This will keep copious moisture across the
area leading to areas of moderate instability. This combined with
the forcing will lead to numerous convective coverage, especially
across NC, Monday night. As the front translates eastward on
Tuesday, better precip chances translate east as well. Instability
will approach moderate levels again along and south of I-85. There
is little in the way of shear either period, so severe chances
remain minimal. Excessive rainfall is not expected either. However,
there should be up to three-quarters of an inch of beneficial
rainfall during this period.

The upper low dives south into the Ohio valley Tuesday night and
Wednesday. This helps to spin up a low pressure center along the
slowly moving cold front. The bulk of the guidance suggests that the
front and developing low will be far enough east to keep the
Piedmont precip free. However, any westward adjustment in this
feature would mean lingering showers. That said, there is good
agreement on moisture moving into the NC mountains on Wednesday with
yet another strong short wave rotating around the upper low. Expect
clouds to increase over the mountains during the day with isolated
showers developing during the afternoon. Will keep the dry forecast
outside of the mountains for now.

A cooling trend develops with highs a little above normal on Tuesday
falling to near normal on Wednesday. Lows up to 10 degrees above
normal Monday night fall to varying degrees on Tuesday night. Expect
readings a few degrees below normal over the mountains then near
normal for most locations. The Charlotte metro area being the
exception where lows will be a few degrees above normal.


As of 210 PM EDT Sunday: Cooler and drier weather for the second
half of the week and through the weekend. The forecast leans heavily
toward the GFS as the ECMWF has low confidence. Both of these models
have a similar solution at 00Z Thursday with the orientation of the
upper trough aligned across the Appalachian mountains.  Both models
have a deep closed low at 00Z Friday centered over the TriAd of NC
on the GFS and Murphy NC on the ECMWF. Therefore on Thursday
increase the cloud cover and increase the area covered by slight
POPs over western NC. The models slowly lift the low north with the
GFS taking it to the VA and W VA  border at 00Z Sat and ECMWF taking
it toward Louisville KY. Although both these models open up the low
over the weekend, the GFS is most progressive as the low goes off
the New England coast at 00Z Monday. ECMWF has it over Toledo OH at
that time.  As the low departs the coast on the GFS, surface high
pressure moves down over the Great Lakes and creates a CAD scenario
at the very end of the current forecast.

Temperatures are forecast to be around 5 degrees below normal
Thursday and Friday then near normal over the weekend.


At KCLT:  Slow-moving back door cold front has sagged to the
vicinity of the airfield this evening, and has been responsible for
just a few isolated showers across the Piedmont. The potential
exists for more showers through the overnight, although chances are
looking perhaps a bit less than previously thought. Will allow a
VCSH to persist through about 08Z, with a tempo for -SHRA from
05-08Z. However, that tempo is more geared toward introducing the
potential for development of low cigs, which should begin expanding
north of the front over the next few hours. Expect cigs to
eventually reach IFR or lower during the pre-dawn hours. Winds are
generally expected to remain just south of due east, but they may
oscillate to E/NE from time to time.

Back door front will dissipate through tomorrow, but should still
support chances for SHRA during late AM/early PM on Monday. Another
cold front will then approach from the west, supporting better
chances for convection by the end of the period. A PROB30 for TSRA
is included during the afternoon.

Elsewhere:  Trends similar to that of KCLT, cigs will develop
earlier and be lower at KHKY, and later and not as low (generally
MVFR) at the other terminals. In fact, restrictions are not expected
to develop at all at KAND during this period.

Outlook: Cold front will 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

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       Med   76%     Med   73%     High  81%     High 100%
KGSP       High 100%     Med   66%     Med   69%     High 100%
KAVL       High 100%     Med   73%     High  94%     High 100%
KHKY       Med   76%     High  84%     Med   73%     High 100%
KGMU       High 100%     Med   62%     Med   69%     High 100%
KAND       High 100%     High  95%     Med   73%     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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