Area Forecast Discussion
Issued by NWS Greer, SC

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900
FXUS62 KGSP 230001
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
801 PM EDT Thu Sep 22 2016

.SYNOPSIS...
A weak area of low pressure lingering along the coast of the
Carolinas will drift east on Friday. A broad upper ridge and surface
high pressure will build over the region during the weekend as
temperatures rise well above normal again. A slightly cooler, wetter
trend will commence next week as a frontal boundary approaches the
area.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 8 PM: The center of the coastal low this evening is very close
to its position 24 hours ago. The low is forecast to weaken through
Friday. However, the sfc pressure pattern will continue to feature
low pressure across the coastal plain and high pressure across the
mountains and foothills. This pattern should support steady NNE
winds through Friday. A few weak showers should track east of KCLT,
possibly passing as a VCSH through 3z. KAVL has observed dewpoints
in the mid 60s, setting up a crossover of around 4 degrees late
tonight. KAVL may see LIFR conditions by sunrise Friday. I will
increase the mention of mtn valley fog in the forecast. The support
for fog.

As of 230 PM EDT Thursday:  A stacked sfc/upper level low pressure
complex anchored along the Carolina coastline continues to dominate
the wx pattern this afternoon.  With that, moisture continues to be
advected around the rotating low, lofting across the NC/SC piedmont.
Increasing low/mid level lapse rates have allowed for modest mucape
development, upwards of 500-1000j/kg per latest spc meso, which
in turn has yielded increased coverage of shallow shra across
these areas.  Thus, did increase pops through the afternoon to
feature scattered/numerous shra with an isolated rumble of thunder
not to be ruled out.

Pattern evolution through the near term will be highlighted by an
approaching/building upper ridge from the west, while the coastal
low pressure final shifts east and begins to weaken. As a result,
pops are allowed to taper from the west overnight leading to no
mentionable pops by just after daybreak.  However, bl moisture
will be sufficient to support patch fog across the region beneath
llv stratus, with the most dense fog being in the mtn valleys,
and the lowest stratus likely along/east of I77.  Beyond sunrise
on Friday conditions will improve as the aforementioned stratus/fog
erode leading to partly/mostly sunny skies.  Guidance continues to
hint at increasing instability beneath the approaching upper ridge,
maximized across the sw NC mtns by mid/late afternoon on Friday.
Despite low confidence thanks to the rising heights, did allow for
an area of slight chance pops to build atop these zones as a few
shra/tsra cannot be ruled out if weak capping erodes.  Temperatures
through the period will remain 5-10 degrees above normal levels.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
As of 225 PM EDT Thursday...Saturday still looks hot and dry with
ridging in place surface and aloft. With the warmth and ridging,
atmosphere should be capped. However, isolated convection could
develop in the upslope area near the NC/SC/GA border. Although upper
ridge remains in place on Sunday, short waves rotating over the mid
Atlantic will push a weak back door cold front into the area from
the NE. There is little in way of forcing with this front. However,
low level flow will turn more southeasterly possibly leading to
moisture pooling ahead of the front and over and near the mountains.
Some guidance is developing quite a bit of instability with the
moisture pooling, while others keep it weak. Either way, best
potential for convection will be along and near the Blue Ridge where
upslope flow and instability will maximize. Isolated convection will
be possible elsewhere. Although good consensus exists with low temps
nearly 10 degrees above normal, there is more uncertainty regarding
highs. Clouds with the low level moisture and lowering thickness
values from the front will likely lead to lower temps across the
board with a strong gradient from SW to NE. The actual values are
still in question. For now, have gone around 5 degrees above normal.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 230 PM EDT Thursday...medium range guidance currently presents
an unusually high level of uncertainty. There has also been little
in the way of run-to-run consistency in each individual model. No
one model solution has risen to the top as of yet, so the medium
range forecast is constructed of a multi-model blend, with some hand
tweaks to construct a more realistic package as far as
pops/temps/sky cover. Having said all of that...significant changes
were not made to the previous forecast due to the high level of
uncertainty.

As far as sensible weather features...there is decent agreement that
the extended period will begin 00Z Monday with the forecast area
generally positioned in the warm sector of an approaching deep layer
low pressure system. Southeasterly flow will moisten the area
considerably...and the implied upslope flow will trigger chance pops
across most of the forecast area, with higher-end chances in the
higher terrain. This is where any semblance of a model consensus
ends. Dramatic differences in the depth/speed of an approaching
upper trough make for a mess as far as determining a clear surface
frontal passage with the approaching trough...so I`ll just mention
that a front/surface boundary will be in the general proximity of
the area Monday-Wednesday. Pops were therefore raised to high-end
chance each afternoon (following the best instability) across most
of the forecast area through the first half of next week. This does
not seem to be an unreasonable solution considering some form of
upper level forcing and surface moisture will be present in our
through most of the period...though depending on whether or not a
block is able to set up across the CONUS, the forecast could trend a
bit wetter than what is currently reflected in the grids.

Finally...the max/min temperature forecast also has high bust
potential next week, but the overall trend will be lowering heights
over the southeast, so temperatures start just above climo on Monday
and drop to climo by the end of the week.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
At KCLT/KGSP/KGMU/KHKY/KAVL: The center of the coastal low this evening
is very close to its position 24 hours ago. The low is forecast to
weaken through Friday. However, the sfc pressure pattern will
continue to feature low pressure across the coastal plain and high
pressure across the mountains and foothills. This pattern should
support steady NNE winds through Friday. A few weak showers should
track east of KCLT, possibly passing as a VCSH through 3z. The main
forecast challenge will be the development of restrictive ceilings
tonight into Friday morning. Using NAM forecast soundings, I will
forecast MVFR ceilings by late this evening, lowering to IFR at
times around dawn. KAVL has observed dewpoints in the mid 60s,
setting up a crossover of around 4 degrees late tonight. KAVL may
see LIFR conditions by sunrise Friday. Restrictive conditions should
dissipate within an hour or two of sunrise.

KAND: The terminal should remain west of the llvl moisture. I will
forecast VFR conditions.

Outlook: Meandering low pressure along the Carolina coast will
persist thru Friday. This will allow isolated to scattered diurnal
convection to continue especially east of I-77, and periodic
chances for restrictive cigs. Meanwhile, a new cold front will be
approaching from the northeast Sunday, but with sparse precipitation
expected at this point.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High  82%     High  97%     High  90%     High 100%
KGSP       High  80%     Med   74%     High  89%     High 100%
KAVL       Med   61%     Low   53%     High  82%     High 100%
KHKY       High  89%     Low   49%     High  85%     High 100%
KGMU       Med   79%     Med   70%     Med   79%     High 100%
KAND       Med   78%     Med   62%     Med   70%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...LEV
NEAR TERM...NED
SHORT TERM...RWH
LONG TERM...LEV
AVIATION...NED



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