Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 252103

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
503 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 450 PM, pops have been scaled back in most areas, as the
nature of the cu field outside the mtns does not hold much promise
for development of deep convection anytime within the next couple of
hours. Even in the vicinity of a backdoor front draped across
western NC, the cu field looks meager. Otherwise, a pretty
impressive temperature gradient exists across the boundary, with mid
90s noted across the Upper Savannah River Valley, and mid/upper 70s
across the northwest NC Piedmont. Temps have been tweaked a bit to
account for the latest trends.

Moving along, pops will remain elevated across the western NC
piedmont overnight as the boundary lingers, with precip eventually
becoming more isentropically forced atop the wedge boundary.  As for
Monday, parent high pressure responsible for the wedge will erode 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:  A mixture of vfr/mvfr and ifr possible through this taf
cycle as a back door front meanders about the region providing
focus for convergence/lifting of moisture.  As such, low stratus
this morning has eroded and will continue to do so to the northeast
while convective cu builds across the Piedmont.  Guidance favors
convection along/near the front through the late evening thus taf
features vcsh with a 3hr tsra tempo, all ahead of a shra with a tsra
prob30 into early/mid morning.  By that time cigs will be falling
into the mvfr/ifr range, lingering through mid/late morning with
occasional mvfr visb.  Winds through the evening/overnight/morning
will remain in the vicinity of 080-100 in the 5-7kts range before
veering southerly late in the period as the front lifts back north
given wedge erosion.

Elsewhere:  Trends similar to that of KCLT above as a back door
front/wedge boundary provides focus for convection/upglide induced
precipitation at all sites this afternoon, with the lowest
chances/confidence at the SC sites thus only vcts was included. Cigs
will lower from the northeast through the night with ifr favored at
all sites aside for KAND where only a sct ifr deck was included in
the taf.  Conditions will improve into late morning as winds veer
southerly given wedge erosion.

Outlook: Another cold front will approach the area from the west
on Monday, then 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 Wednesday.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     Med   66%     Med   66%     Med   75%
KGSP       High 100%     High 100%     Med   62%     Med   68%
KAVL       High 100%     Med   62%     Med   62%     High 100%
KHKY       High 100%     Med   72%     Med   78%     Med   69%
KGMU       High 100%     High 100%     Low   59%     Med   68%
KAND       High 100%     High 100%     High  89%     Med   67%

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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