Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 140256

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1056 PM EDT Sun Aug 13 2017

A stationary front will slowly move north over the region on
Monday and Tuesday with abundant moisture remaining in place
over the area. Slightly drier high pressure will arrive during
the middle part of the week. Increased moisture is expected
to return again ahead of the next cold front that approaches
from the northwest on Thursday.


As of 1035 PM: Showers and storms have just continued to expand in
coverage and intensity this evening, mainly along and near the Blue
Ridge. This is aided at least in part by increasing, albeit weak
upper support, lingering sbCAPE in the 1000-2000 J/kg (owing largely
to surface dewpoints in the mid/upper 70s), PWATs near 2", and a
E/SE upslope flow in the lowest 5kft. Strongly veering low-to-mid
level flow is also resulting in slow cell movement. That being the
case, am concerned about the potential for flash flooding overnight,
particularly in the NC foothills. The main limiting factor is that I
just don`t have a good feel for how much longer convection will
linger in these areas. The current activity in the NC foothills is
generally trending downward, and later redevelopment is uncertain at
this time. A mention of the flash flooding potential has been
included in the HWO for the overnight. Convection will be less
concentrated in other areas, but localized heavy rain will be
possible just about anywhere overnight.

Monday looks like a near carbon copy of today (Sunday), with more
channeled vorticity streaming thru west-southwesterly upper flow,
while the sfc front remains across the Midlands. The models have
perhaps a slightly stronger sfc wave develop near Augusta, and all
have widespread QPF response across the area during the AFTN. It`s
difficult to say how unstable we will get with all the cloud cover,
but models show 1000-2000 J/kg of SBCAPE again. So I won`t change
the solid CHC to likely PoPs that I inherited. Heavy rain and
isolated flood threat will continue. Temps will be near to slightly
below normal under mostly cloudy skies.


As of 230 PM EDT Sunday: I would say the forecast continues to
look like a record that is stuck, but then I would wonder how
many people younger than about 30 years old would know what I was
talking about. At least the latest model runs have stepped away
from the flash flood potential for Monday night with less of an
upslope SE low level flow, instead expect a slow decline through
late evening. Tuesday certainly looks like a continuation of the
recent pattern with a broad upper trof axis to the west and yet
another weak wave moving through, acting on plentiful and deep
moisture to produce scattered/numerous showers and thunderstorms. We
continue to be more worried about the flood potential with high PW
and slow moving storms, rather than the severe weather risk. Expect
the diurnal tendency to remain Tuesday night, so precip chances
gradually taper off to a chance only over the mtns. Wednesday is a
bit more uncertain because of subtle differences in the guidance. It
might be not as active if the GFS is correct with showing a short
wave ridge moving through, or it might be just as active with
another short wave moving across as in the NAM. Prefer continuity
at this point, as we will probably not see an air mass change,
so the fcst will keep a chance across the whole fcst area. High
temps will remain near normal and low temps will stay on the warm
side of normal with relatively high dewpoints. Looking ahead,
one can only hope the pattern changes before a week from tomorrow.


As of 215 PM EDT Sunday: the extended fcst picks up at 00z on
Thursday with fairly weak upper ridging moving over the region
from the west. By late Thurs/early Fri, the ridge is expected
to flatten as another upper trof begins to dig down over the
Western Great Lakes. The long-range models move the trof axis
to our north by early Sunday as upper ridging persists to our
west. At the sfc, more wet and unsettled weather is expected
to continue thru most of the period with the Bermuda High keeping
moist SLY low-lvl flow over the fcst area. A low develops over
the Great Lakes on Thursday and moves its associated cold front
towards the fcst area. The current run of the ECMWF is quite
a bit more progressive than the GFS or Canadian and moves the
front into the CWFA by late Thursday, whereas the other models
are about 24 hrs slower. It still appears that the front will
stall out just to our SE by late Friday/early Saturday, however
the models are now showing a greater degree of drying as
reinforcing high pressure slides to our north over the weekend.
As for the sensible fcst, no significant changes were made,
save for lower dewpoints and PoPs over the weekend. Otherwise,
still expecting scattered diurnal convection each day, with
the higher PoPs on Thurs and Fri as the front moves into the
fcst area. Temps will likely start out just above normal and
cool a bit towards the end of the period.


At KCLT and Elsewhere: Showers continue to gradually expand in
coverage and slowly move southeast along the Blue Ridge this
evening. The most likely areas to see convection this evening will
be KAVL (over the first couple of hours of the forecast period), and
KHKY (after 02Z or so). Tempos for SHRA w/ restrictions are carried
at those two sites. The convective potential is much more uncertain
at the other terminals, but opted to continue inclusion of a VCSH at
those sites from 02-03Z into the overnight hours. Otherwise, low
stratus (w/ mtn valley fog) is expected to expand over the area
toward daybreak, with IFR/LIFR conditions anticipated at the western
NC terminals, and mostly MVFR at the Upstate SC sites. Winds will
generally remain light NE overnight, gradually turning toward the SE
on Monday. Additional convection is anticipated Monday afternoon,
with PROB30s warranted at all sites.

Outlook: Chances for restrictions will remain elevated through late
week as a moist pattern persists.  Therefore rounds of
afternoon/evening shra and tsra are favored each day, with morning
visb/cig restrictions possible as well for locales that experience
recent rainfall, as well as in the mtn valleys to affect KAVL.

Confidence Table...

            03-09Z        09-15Z        15-21Z        21-00Z
KCLT       High  95%     Med   77%     High  85%     High 100%
KGSP       High 100%     Low   56%     High  86%     High 100%
KAVL       Low   51%     High  90%     High  81%     High 100%
KHKY       High  86%     Med   78%     High  88%     High  98%
KGMU       High 100%     Med   79%     High  88%     High 100%
KAND       High 100%     Med   71%     High  91%     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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