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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
143 am EDT Thursday Jul 31 2014

Synopsis...
Canadian high pressure will begin to slide east of the area and
weaken through Friday. As the high weakens...moisture will return
for the weekend increasing the chances of widespread showers and
thunderstorms. Conditions return to those more typical of late
Summer by the middle of next week.

&&

Near term /through today/...
as of 130 am EDT Thursday...weak shower activity across the Midland
continues to slowly decay per latest radar trends. Most recent
NAM/cams guidance indicates redevelopment of showers across portions
of the the upstate later this morning. Confidence is not notably
high regarding this...however did adjust probability of precipitation to include slight
chance mention for the upstate and northeast Georgia. Otherwise...no
changes made to the forecast with this update other than tweaking
temperatures and dewpoints in the extreme near term.

Previous discussion...

915 PM update...cirrus continue to stream across the forecast area...coming out
of the shortwave over Oklahoma/Arkansas. Revised sky grids to better
reflect the presence of these clouds...which are largely opaque.
Some midlevel clouds have developed beneath them as well. Latest
expectation is still for even lower clouds to form overnight...per
the southeasterly flow pattern seen on 00z NAM as well as earlier models. In
light of these facts...and noting slightly warmer temperatures than previous
forecast at this hour...I raised min temperatures perhaps a degree. Some cloud
cover had already been anticipated. As far as the convection over
the midlands...it waned more or less on schedule...taking some time
to exhaust the instability. However some light activity continues to
fire where the best convergence is evidently present just south of
Charlotte. Extended the pop mention in Union NC a little longer.

As of 215 PM...in regard to the big picture....some changes are
anticipated to gradually take place by late Thursday. The upper trough
axis over the eastern U.S. Is forecast by the models to become
re-established to the west of the Appalachians as energy from the
central rockies dives southeast. At the same...a stationary front near the
coast will begin to move slowly westward as a warm front. Hence by
late tonight...boundary layer winds will begin to veer to the east-southeast
which will advect Atlantic moisture into the area and provide week
mechanical lift across the higher terrain. Plus by Thursday afternoon...a
vorticity maximum is forecast to approach from the SW and swing across the
mountains Thursday afternoon. All this combined will give US an
increasing chance for showers on Thursday....with the best coverage over
the NC mountains during the afternoon hours. Will limit probability of precipitation to the
chance range at this time as instability appears limited with only a
couple hundred joules of cape expected. Only widely scattered
showers are expected outside the mountains. With increasing clouds
tonight...min temperatures should be several degrees higher than last
night. Considerable cloudiness is anticipated on Thursday which will
continue to hold maximum temperatures below climatology.

&&

Short term /tonight through Saturday/...
as of 220 PM...on Friday...moisture across the southeast U.S. Should
increase sharply as Atlantic moisture advects west and merges with
moisture associated with a fading Southern Plains low. Forecast
soundings indicate that precipitable water will increase to around 1.75 inches by
Friday morning...with LCLs below 2 kft. Nam12 shows isentropic lift
will strengthen between 305k-315k Friday morning...persisting through
the afternoon. Low level lift will likely be augmented along the
east facing slopes as winds remain from the southeast around 20 kts.
In addition...a broad area of middle level q-vector convergence will
remain across the southeast...east of an amplifying l/west trough centered
across the Mississippi River valley. The combination of the deep
lift...abundant moisture...and weak instability should support
numerous showers and thunderstorms on Friday... I will forecast
generally likely probability of precipitation along and west of I-85...high chance east. The
lack of instability across the mountains and foothills may support only
showers...thunderstorms appear possible along and east of I-85.
Periods of rain...east-northeast winds...and thick cloud cover should limit
daytime heating...keeping highs in the 70s across the mountains to
western Piedmont.

On Saturday...the ridge of surface high pressure should trend weaker
through the day. However...shallow ridging is expected to linger
along and east of the high terrain. Deep forcing is expected to
continue across the forecast area...aided by the approach of the
amplifying middle level trough. I will forecast a range in probability of precipitation...likely
probability of precipitation across the mountains to high chance east. Low level winds are expected to
veer from the southwest...supporting temperatures 4 to 6 degrees
warmer than Friday. Forecast soundings indicate weak instability
across the forecast area...supporting a mention of thunderstorms for
all zones.

&&

Long term /Saturday night through Wednesday/...
as of 200 PM Wednesday...the medium range will start out with a
fairly deep 500 mb trough axis extending from the Great Lakes south to the
lower MS valley at 00z Sunday. The trough will gradually weaken/shear out
early next week...but some degree of trofiness will linger through
Wednesday. A quasi-stationary frontal zone will linger parallel to
the southwesterly upper flow along/just inland of the East Coast...keeping the
County warning forecast area in a moist/unsettled environment into early next week. The
front will wash out by Wednesday...per the consensus of the medium
range guidance.

So blending in the new guidance and wpc guidance...I will forecast above
normal probability of precipitation for Sunday and Monday (high-end chance to likely)...and
tapering somewhat to shotgun low-middle chance for Tuesday and Wednesday.
Temperatures will start out with below normal highs (due to
clouds/precip)...and near normal lows through Monday...then afternoon highs
will trend toward normal by Wednesday with continued near normal
lows.

The expected decent coverage of precipitation over several periods looks to
be mainly supported by persistent upper divergence within right
entrance region of a nearly stationary jet streak along the
Appalachians. However...qg forcing and low level triggering will be only
marginally supportive...with the better middle level forcing to the
north...and low level jet and instability to the east. So confidence is low on
any severe weather or heavy rain threat at this time.

&&

Aviation /06z Thursday through Monday/...
at kclt...a mixture of VFR/MVFR conditions will prevail through this
taf period. Initialized taf with broken low/middle level ceiling which
carries through 14z. At that time...expecting moisture advection
from the east to spread an MVFR stratus layer over the
region...lasting through around noon where low VFR stratus will
prevail. Marginal upglide combined with weak instability could
produce rain showers/thunderstorms and rain at the airfield late in the period...therefore
opted for prob30 for thunderstorms and rain at 00z where ceilings also lower back to MVFR
levels. Expecting ceilings to recover as precipitation subsides near the end
of the taf cycle with low VFR prevailing yet again. Winds through
the period will be a little tricky. Initialized winds at calm
before increasing southeasterly flow dominates through much of the
day. Late in the afternoon to early evening...expecting winds to
back northeasterly somewhat as high pressure wedges in.

Elsewhere...a mixture of VFR/MVFR conditions will prevail at all
sites through the period with kavl actually lowering to IFR levels
late in the period. Low/middle VFR ceilings will lower to MVFR levels
around daybreak as moisture advects in from the east. These ceilings
will likely carry through the middle/late morning hours before lifting
to low VFR. Models have some disagreement regarding precipitation this
morning at the SC sites...thus due to low confidence kept out of
taf. That said...later in the day models do agree on potential for
scattered rain showers/thunderstorms and rain at all sites. Thus...vcsh/vcts mention is carried at
all locations with kavl actually prevailing -shra during the
afternoon where upglide/upslope/instability will be greatest along
the terrain. Expecting ceilings to lower back to MVFR levels as precipitation
commences at each location...and remain there due to substantial
moisture in the boundary layer. Also included IFR ceilings at kavl late
in the period as guidance is in agreement regarding low stratus
development along the high terrain overnight. Winds through the
period will remain light and southerly at kavl...and east
northeasterly elsewhere.

Outlook...moisture will continue to increase through the
remainder of the work week as a warm front approaches from the east
and southeasterly flow continues. Unsettled weather with enhanced
precipitation chances along with morning fog/stratus are expected
through the weekend.

Confidence table...

05-11z 11-17z 17-23z 23-05z
kclt high 100% medium 79% high 92% low 45%
kgsp high 100% medium 72% high 88% low 54%
kavl high 94% medium 62% medium 75% low 52%
khky high 91% medium 78% high 100% low 45%
kgmu high 100% medium 75% high 88% low 48%
kand high 87% medium 73% high 93% low 49%

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: (must be lower case)

Www.Weather.Gov/gsp/aviation_tables

&&

Gsp watches/warnings/advisories...
Georgia...none.
NC...none.
SC...none.

&&

$$
Synopsis...Arkansas
near term...cdg/lg/Wimberley
short term...Ned
long term...Arkansas
aviation...cdg

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