Area Forecast Discussion
Issued by NWS Greer, SC
FXUS62 KGSP 270853
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
453 AM EDT Tue Sep 27 2016
A cold front will cross our area today, then stall across Midlands
and central North Carolina tonight through Wednesday. An upper level
low will drop south into the Ohio Valley on Wednesday and linger
there through the end of the week.
.NEAR TERM /THROUGH TONIGHT/...
As of 300 AM EDT Tuesday: Mesoanalysis has placed the cold front
pretty close to the TN/NC state line, with a band of showers
developing along it. Most hi-res guidance/CAMs seem to want to break
all of this apart over the mountains and while that may be the case
the area of convection in general does seem to be expanding. Made
some updates to the near-near term to account for this. As for the
bigger picture however, we`ve been advertising this approaching cold
front and parent upper low for several days now, so the synoptic
overview should be no surprise. The initial movement of the front
into the area will stall as a weak shortwave on the southeast edge
of the upper low lifts rapidly northeast, but then the main trough
axis will dip toward the Deep South later today as the closed low
pushes toward the Ohio Valley overnight.
With the brief slowdown/stalling of the front, this will allow the
low-level southerly flow to bring one more surge of warm/moist air
to the area, with another day of above-normal temperatures for the
area. CAPE values should increase to upwards of 1500 J/KG again
today across the foothills and piedmont and as the upper low
approaches, deep-layer shear is no longer negligible, albeit still
very low. An isolated strong storm cannot be ruled out. Luckily with
the additional synoptic forcing the storms will be moving fairly
quickly, but if storms manage to develop over areas that received
heavy rainfall yesterday (read: Charlotte) localized flash flooding
could still be an issue. PW values at AFD time remain anomalously
high, pretty close to the daily max values and at least at the 90th
percentile, so will keep that heavy rain/flash flooding wording in
The front will slowly push off the mountains tonight but in the wake
of the smaller shortwave impulse and before the main kicking
mechanism comes into play, it will have a hard time really getting
through the area. The surface front and associated wind shift itself
are progged to push through by the end of the period, but the
subsequent CAA will not be quite as strong just yet. With this, have
ended up bumping up tonight`s low temperatures a few degrees (same
with dewpoints) to address this.
.SHORT TERM /WEDNESDAY THROUGH THURSDAY/...
As of 300 AM EDT Tuesday...A large upper low will drop south into
the southern Great Lakes on Wednesday, then continue to wobble
around the Ohio Valley thru at least Thursday night. The 00z suite
of operational models shows good agreement on the center`s track,
and even the major vort lobes rotating around the vortex.
Significant height falls ahead of the low should keep backed
deep-layer SWLY flow atop the CWFA. The models continue to trend
slower with bringing the dry slot in from the west. So the eastern
third of the CWFA now looks to support at least isolated to
scattered convection Wednesday afternoon. The NAM soundings, in
particular, show over 2000 J/kg with around 40 kts of 0-6km bulk
shear. So convection may become organized, but mainly push east of
the area by late afternoon. The new Day 2 SPC Outlook has a marginal
risk along and east of I-77, with a decent chance that some of that
area may be upgraded to a Slight later. In any case, PoPs were
bumped up in the east to chance. Temps were also bumped up within
the warm advection SWLY flow.
Wednesday night and Thursday...the first major spoke of vorticity
will pivot thru the southern Appalachians, and provide westerly
upslope showers along the TN border overnight, then possibly across
much of western NC on Thursday with daytime heating. The steep mid
level lapse rates under the low could result in enough instability
for thunder. Will continue the chance PoPs north to less than slight
chance in most of the Upstate and northeast GA. Temps are expected
to cool off to below normal Thursday, but models have trended
warmer, as flow remains out of the SW ahead of the low.
.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 315 AM EDT Tuesday...The models have converged on the handling
of the upper low for late in the week thru this weekend. The low is
expected to slowly drift north from KY into OH Friday and Saturday,
then turn northeast into the eastern Great Lakes Sunday and Monday.
This track should bring a dry slot into the CWFA and basically keep
it there. So other than some initial westerly flow upslope showers
along the TN line, the medium range forecast is dry. Temps are
expected to be coolest Thursday night into Friday, about a category
or two below normal. Then temps rebound to slightly above normal for
the weekend through Monday.
.AVIATION /09Z TUESDAY THROUGH SATURDAY/...
At KCLT: No significant changes for the 09z AMD, with MVFR to IFR
cigs all around the airfield, so will keep current trend but may
need to lower cigs if the rent of LIFR cigs continues to expand. The
first PROB30 will drop off with this AMD so will adjust timing but
leave the second PROB30 as-is. Otherwise, should see slow
improvement to cigs after sunrise, but with the approach of the
front expect to see another round of TSRA this afternoon. Best
chances look to be between 22-00z tonight and ended up with short
prevailing TSRA but will continue to reevaluate that with new data.
Winds remain generally S but with the front approaching may see
brief backing ahead of it, and then just beyond the end of the
period winds will begin shifting W.
Elsewhere: Convection has moved out of the areas, though another
line is developing along the mountains with the front that may
impact KAVL overnight. Bigger concern will be widespread IFR
conditions, with confidence not quite as high across the Upstate
TAFs as for KAVL/KHKY. Improvement after sunrise and with the front
moving into the area should see another round of SHRA/TSRA, and
handled this with a combination of PROB30/VCTS/prevailing TSRA.
Mainly S winds slowly veering SW, and for KAVL/KHKY should see winds
shift NW toward the end of the period.
Outlook: Cold front will slowly cross the region today. Dry high
pressure builds in slowly behind the front on Wednesday, which
should bring VFR conditions in most places through the end of the
09-15Z 15-21Z 21-03Z 03-06Z
KCLT Low 44% High 100% High 100% High 100%
KGSP Low 33% High 100% High 100% High 94%
KAVL Low 55% High 89% High 98% Med 64%
KHKY Med 61% High 87% High 100% High 100%
KGMU Low 55% High 100% High 100% High 100%
KAND High 85% High 100% 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: