Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 271130

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
730 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.


As of 645 AM: Surface front remains in the vicinity of the
mountains per MSLP/wind analysis, but with the best temperature
still on the TN upwind side. Convection along and ahead of the front
that developed in E TN and crossed the mountains is still going but
has significantly reduced in intensity as it races ahead of the
front toward the Piedmont. Except for these small areas, have
reduced pops through the morning hours, ramping back up to high
chance and likely this afternoon.

Otherwise, as for the bigger picture, 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 HWO.

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.


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.


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.


At KCLT: MVFR cigs should prevail this morning with patchy IFR
around the region, so have continued TEMPO for IFR conditions
through 13z with improvement to VFR shortly thereafter. Expect
another round of TSRA this afternoon as the cold front approaches,
with best timing generally between 21-00z. However, new guidance
continues to slow the system down so that may need to be delayed
depending on convective trends through the day. Winds remain on the
S side, oscillating SSE to SSW, veering more WSW toward the end of
the period.

Elsewhere: Overall similar trend to KCLT but with better morning
conditions across the Upstate and worse farther north toward KHKY.
Timing of TSRA also earlier the farther west you go. After the
convection moves through this afternoon and evening, all guidance is
pointing to another round of mainly vsby restrictions, especially at
KAVL and have trended that direction.

Outlook: Cold front will slowly cross the region today, and will
stall briefly overnight into early Wednesday before pushing out of
the area. Expect at least some convection toward the I-77 corridor
Wednesday afternoon. Dry high pressure will build in slowly behind
the front Wednesday into Thursday, which should bring VFR conditions
in most places through the end of the week.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       Low   55%     High 100%     High 100%     High 100%
KGSP       Low   30%     High 100%     High 100%     High 100%
KAVL       Med   66%     High 100%     High  95%     High  94%
KHKY       Med   61%     High 100%     High 100%     High 100%
KGMU       Low   55%     High 100%     High 100%     High 100%
KAND       Low   55%     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:




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