Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 140607

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
207 AM EDT Mon Aug 14 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 155 AM EDT: The latest runs of the HRRR show decent eastward
progression of the NC foothills/piedmont convection through the
early morning hours. This should keep additional rainfall from
becoming excessive in these areas despite the very heavy rates of 1
to 2 inches per hour in the best downpours. No Flash Flood Watches
are expected.

Upstream to the west, water vapor satellite imagery shows a
shortwave embedded in the central conus trough lifting northeast
from the lower MS River Valley. This, and subsequent, waves will
develop and then dampen as they move east through the near-term
period, but with associated mid level forcing likely crossing the
region at times today through tonight. Meanwhile, at the surface,
weakening 1015 mb high pressure to the north will keep the surface
frontal boundary draped mainly along the lower piedmont through much
of the near term. The boundary could undulate northward at times to
impact more of the region, and this would provide an additional
trigger for afternoon and evening convection. Also at low levels,
the southerly 850 mb flow will start to veer westerly after 10Z this
morning, continue westerly through the morning hours, and then back
toward southerly late in the day ahead of the next wave. The
combination of these features should allow the peak convective
coverage among the improved instability and increased forcing to
center along the Blue Ridge 21Z to 00Z. This activity will likely
sweep eastward across the piedmont, with the best coverage in
western NC through the evening hours, and then lingering isolated
mountain showers overnight tonight. Meanwhile, Tropical system Gert
will pass well east of the eastern seaboard and not impact the area.


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: A cluster of showers and thunderstorms on the move eastward
from the foothills early this morning could well impact the KCLT
vicinity from 07Z to 10Z. The main question is whether or not
thunder will arrive, but the HRRR has a downward trend in intensity,
so will hold onto mainly SHRA at this point. Otherwise, expect IFR
cigs to develop given the abundant moisture and restrictions will be
very slow to lift through MVFR through late morning and the early
afternoon. Will feature mainly PROB30 for late Monday afternoon
convection, but TEMPO could well be needed if robust convection
develops off the Blue Ridge as expected late today. Winds will be
mainly N of E and fairly light through the current period.

Elsewhere: KHKY will see the best chance of an isolated thunderstorm
early this morning, but mainly SHRA are expected as the foothills
convection continues to weaken. Elsewhere, morning precipitation
should avoid the TAF sites, but with IFR cigs likely developing
fairly quickly through 09Z, and then MVFR cigs slow to lift through
late morning. Anticipate mainly lower VFR cigs with showers and
thunderstorms redeveloping from the west this afternoon ahead of an
approaching shortwave. KAVL should see TSRA the soonest, by 19Z,
with activity spreading to the foothills from 20Z through 00Z. Winds
will be NE early and become southeast to southerly later in the day.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       Med   74%     Med   75%     High 100%     High 100%
KGSP       Med   61%     Med   76%     High 100%     High 100%
KAVL       Med   77%     High  81%     High  98%     Med   65%
KHKY       Med   69%     High  88%     High  93%     High  84%
KGMU       High  87%     Med   76%     High 100%     High 100%
KAND       High  82%     Med   79%     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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