Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 260542

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
142 AM EDT Mon Sep 26 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: Back door cold front has stalled across the area with
isolated to scattered SHRA/TSRA developing, mainly NW of KCLT, but
will keep VCSH with TEMPO -SHRA for a couple more hours. Low MVFR
cigs have developed over the Piedmont with IFR cigs expected within
a couple of hours, expanding to the west overnight. Should see a
slow lifting trend later this morning as the backdoor front begins
to wash out, but stronger cold front approaching from the west will
approach toward the end of the period. Continued PROB30 TSRA for
this afternoon, transitioning to prevailing -SHRA after 00z. TSRA
still possible at that point but am already over TAF length. Winds
should remain mainly SE transitioning to S through the period,
though early overnight could see some brief oscillation to ENE.

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, with only some brief MVFR fog possible
at KAND. With the approach of the cold front this afternoon, kept
VCTS through the end of the period at KHKY.

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...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High  81%     Med   75%     High  85%     High 100%
KGSP       High 100%     Med   75%     High 100%     High 100%
KAVL       High 100%     High  85%     High 100%     High 100%
KHKY       Med   69%     Med   75%     High  85%     High 100%
KGMU       High 100%     Med   75%     High 100%     High 100%
KAND       High 100%     Med   61%     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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