Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 192109

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
509 PM EDT Sat Oct 19 2019

The remnants of Tropical Storm Nestor will continue to track across
southern Georgia and South Carolina through the overnight. Brief
drying returns by mid-day Sunday, before an active cold front
crosses the area Monday night into Tuesday. Dry high pressure builds
in on Wednesday, with another moist frontal system affecting the
region late in the week.


As of 500 PM: Radar and satellite data place the center of
circulation of Post-Tropical Cyclone Nestor moving into extreme
southwest Georgia and accelerating to the northeast toward the
central and eastern Carolinas. Moderate to occasionally heavy rain
showers have blossomed atop our region north of the system and this
widespread coverage will continue and strengthen through the late
evening and early overnight hours. Surface instability remains
shunted well to the southeast coastline, and this is fortunate given
the robust shear profiles that are likely to return to Piedmont
locations through the early overnight hours. Still anticipate that
the peak rainfall rates of the event will occur between 00z and 06z
for most of the area, when the best frontogenesis occurs along with
the best southeasterly winds producing the strongest mechanical
lift. Still following WPC QPF guidance closely with the official
fcst, which places a 2 to 2.5 inch maximum in some southeast
sections through the night. No significant hydro problems are
expected given dry antecedent conditions.

With lapse rates now looking more uniformly favorable aloft,
the potential for a few rumbles of thunder tonight is now more
plausible atop the inversion than once thought, but still only
warranting an isolated t-storm mention in the fcst. Convection
would be most likely between 03z and 12z on the forward flank
of the low. The strength of the sfc inversion makes our severe
threat very small despite very strong mid-level flow producing bulk
shear values around 40 to 45 kt by 09Z. Frequent wind gusts are
still are expected overnight, perhaps up to 25 kt, mainly in the far
southeast zones.

Precip will likely end by 12z over most of the area. Winds will be
upslope against the Appalachian spine at that time, supporting a
small PoP there for a few additional hrs. It appears winds will take
some time to go uniformly downslope across the area, and it may take
until nearly noon in the eastern zones for the low clouds to break
up completely. Many aspects of this event bear similarity to CAD
wedge development and erosion. Temp trends thru the overnight hours
are no exception; values likely will remain nearly steady overnight
if not rising slightly when precip diminishes. Aided by downsloping
and the clearing skies, temps tomorrow will be a remarkable change
from today, generally rising into the 70s, with values closer to 80
over parts of the Savannah River Valley and western Upstate.


As of 143 PM EDT Saturday: With what`s left of Nestor long gone by
Sunday night, our attention will turn to the next system, that being
a strongly-forced cold front approaching from the west on Monday
and crossing the region early on Tuesday. Probably. Before we get
there, Sunday night will be quiet as high pressure ridges down from
the north in the wake of the Nestor remnants, but low temps will
remain on the order of 5-10 deg above normal. On Monday, as the
front approaches, a southeasterly upslope return flow will develop
through the day, so shallow rain showers are still a possibility
close to the Blue Ridge Escarpment. The fcst favors a small chance
in that vicinity per the model blends and statistical guidance, in
spite of the operational model output being mainly dry through 00Z
Tuesday. The cold front will be making its way east at a steady clip
on Tuesday evening, so precip chances will quickly escalate from
west to east during the Tuesday night period. Precip looks like a
safe bet across the mtns and foothills early Tuesday morning...but
at that point, model differences start to come into play, casting
some uncertainty. The GFS and ECMWF generally have a fropa early
in the day on Tuesday, essentially east of the fcst area by 18Z,
while the NAM is six hours slower. The problem with the NAM is
that it would raise the stakes for some severe weather threat,
at least over the eastern zones in the afternoon as the Charlotte
metro area would be uncovered and might realize sbCAPE upwards of
500 J/kg while shear on the order of 35-40kt was still around. The
faster GFS does not develop this high-shear/ low CAPE environment,
at least not until the front is well to our east. The fcst favors
the faster solution for the time being, as the GFS ensemble also
has the front to the east by 18Z. Thus, precip chances end by the
end of the day Tuesday.


As of 130 PM EDT Saturday: No major changes were made to the going
fcst. The upper pattern becomes zonally flat early Wed which will
support a broad 1023 mb sfc high across the SE CONUS Wed thru most
of Thu. Cross sections show deep-layered dry air in place and PWATs
dropping below 0.35 inches both days. Thus...good insol and mixing
of td/s will be had...while temps reach near normal levels. Winds
will remain two rather pleasant fall days are in store.

Moisture begins to increase within the upper levels Thu afternoon
ahead of a deepening upper trof across the Midwest. This system will
be the next precip maker for the area early Fri into Sat as it
interacts with a weak sfc wedge leftover from the exiting cP
airmass. The models continue to show differing ideas with the
evolution of the system...with the ECMWF much quicker and phased
better than the GFS and the CMC solns. The GFS is more aggressive and
moist with an elevated warm front Fri...which coincides with the
pre/frontal moist seen on the PoPs were kept in the high
chance range thru most of Fri. On Sat...confidence continues to
decrease as the ECMWF crosses the frontal zone east of the FA while
the GFS lingers good lift and moisture over the region. So...will
mention low-end chance PoPs for now. Thunder chances will be small
due to warm mid-levels...however the GFS is showing some elCAPE
across the ern zones as the wedge retreats which could lead to a few
general tstms in the later afternoon into the evening. Temps will be
held a cat or so below normal Fri/Sat due to high amts of moisture
and decent coverage of precip.


At KCLT and elsewhere: Tropical Storm Nestor will transition to a
frontal low tonight, passing SE of KGSP/KCLT, then moving through
the Carolina coastal plain Sunday morning. Heavy rain showers are
ongoing at the southwest TAF sites, and will spread into the KHKY to
KCLT area through the evening hours. Some bouncing between VFR and
MVFR is expected early, but with solid MVFR likely as the rain
showers spread in, with IFR quickly following. The best forcing from
the low will shift across the terminals this evening and very early
overnight, and frequent gusts of 15-25 kt are likely at times. Even
as the rain rates taper off from west to east overnight following
the remnant of Nestor, the low clouds and some areas of fog will
remain until after daybreak; downsloping flow will develop and
promote scouring of the low clouds, but improvement likely will be
slow. A few low clouds will linger into the aftn at KCLT, with
northerly winds.

Outlook: Generally VFR late Sunday, but some nocturnal restrictions
may develop Sunday night. The next system approaches late
Monday night with precipitation and periodic flight restrictions
likely. VFR will return Tuesday and last for most of the work week.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High  87%     Med   75%     High  88%     High  82%
KGSP       Med   64%     High  83%     Med   64%     Med   63%
KAVL       Med   78%     High  83%     Med   72%     Med   69%
KHKY       High  86%     Med   67%     High  83%     Med   64%
KGMU       Low   53%     Med   79%     Low   57%     Med   65%
KAND       High  90%     High  88%     Low   43%     Med   68%

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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