Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
256 PM EDT Sat Apr 19 2014

Synopsis...
a moist low pressure system will continue moving off the South
Carolina coast through Sunday. High pressure will briefly build in
Monday...before a cold front pushes across the region on Tuesday. A
broad and dry area of high pressure will then build in from the
north Wednesday and linger through the end of the week.

&&

Near term /through Sunday/...
as of 240 PM EDT Saturday...the 12 UTC runs of the NAM and GFS both
have a potent upper tropospheric jet streak rounding the east side
of the upper low at this time. Strong upper divergence has developed in the
exit region of the jet streak...pretty much over The Heart of the
forecast area. I just saw a pilot weather report issued by the ztl cwsu that
several aircraft have reported moderate turbulence in the vicinity of
kclt with this area of upper ascent. The GFS 18 UTC ageostrophic
vertical circulation over western NC is about as strong as it gets. All
of this explains the increase in rainfall over western NC...especially the
foothills and Piedmont...and the northern upstate. While the precipitable water/S are not
that impressive...the zonal component of the 800 mb flow on the GFS
continues to run 3 to 4 Standard deviations above normal. Based on
the cooling cloud tops on the regional infrared imagery...I suspect that
we will see moderate rain fall across these zones for at least
another 4 to 6 horus. This should yield additional rainfall amounts
of around a half inch...possibly up to three quarters of an inch.
This should not be enough to cause any significant flooding
concerns...though a few larger streams and smaller river may
approach bankfull.

The rain and drizzle will gradually taper off later tonight. Clouds
are expected to linger through middle-morning Sunday over most of the
Piedmont...though there is some hope of clearing over the mountains
and the western most foothills. Still...sunrise services will
probably be cloudy for the most part east of the mountains.
Otherwise afternoon temperatures will moderate a good 10 to 15
degrees from today/S.

&&

Short term /Sunday night through Tuesday night/...
as of midday Saturday...short upper ridge remains over the forecast area Sunday
night as deep low moves off the southeast coast...maintaining
northerly low level flow into Monday. Wraparound moisture from the low
lingering over the eastern half of the forecast area late Sunday thus gets shunted
off to the south overnight...while some low level moisture does advect
into the western forecast area as the flow backs. This moist layer is shallow on
forecast soundings. Convection across the forecast area will likely remain capped by
subsidence within the ridge Monday afternoon. However...late Monday the ridge
collapses as trough swings into the Ohio Valley. Some of the 09z sref
members indicated the cap will break as early as Monday evening...and
some surface based instability could be realized if that scenario
happened early enough. More likely though...the inversion dissolves
closer to Tuesday morning as the trough digs in ahead of impending frontal passage.
Probability of precipitation will be reintroduced into the mountains early Tuesday morning as frontal
moisture spreads in with weak upsloping.

Decent surface based cape does develop over the forecast area ahead of the front on
Tuesday. None of the shear parameters look threatening for severe
weather...and with the stronger forcing being well to our north the quantitative precipitation forecast is
light on all the consulted guidance. Best instability is shown over
the NC Piedmont...with model consensus showing cape peaking around
1000 j. Dcape is also fairly high on GFS which has fairly high cloud
bases...so there may be a low-end pulse storm wind threat. Probability of precipitation
diminish late Tuesday following the diurnal peak and as the front
departs. Very shallow moisture on northwest flow Tuesday night might produce
some additional showers in the mountains winds may also be moderately
gusty in the higher terrain Tuesday and Tuesday night.

Cool wedgy temperatures continue across the area Sunday night...but temperatures
rebound to slightly above climatology for Monday and Tuesday.

&&

Long term /Wednesday through Saturday/...
as of 145 PM Saturday...strong latitudinal upper ridging will
develop to the west early Wednesday and push east over the forecast area
through Thursday. This will bring down a strong Canadian surface high and
allow for deep layered drying while the high settles into an atl
ridge config Thursday. There will be some measure of an airmass mix Wednesday
mainly across the eastern zones where maximum temperatures will probably be held a
couple degrees below normal across the northwest Piedmont. Downslope flow
will aide in warmer than normal temperatures across the rest of the
non/mtns...while the mountains valleys reach right around normal. The
low level flow veers S/ly Thursday and allows a better modification of surface
based Theta/east and temperatures/dew points will respond a couple degrees warmer.
All in all...both days should mostly sunny and pleasant with dew point/S
remaining below 50 f most locales.

The upper ridge axis shifts east Thursday night as an 500 mb low deepens
across the upper Midwest. The op models disagree on the timing and
nature of this low with the 12z GFS closing it off and producing a
slower synoptic pattern by about 12 hours than the 00z European model (ecmwf). The gefs
mean heights/mslp are a pretty good compromise as far as timing and
this was given the most weight in the forecast. There are also notable
differences with the amount of the gom ridging and available moist
flux ahead of the surface front Friday. The European model (ecmwf) sets up a stronger low level
ridge and thus disrupts the moist gom flow moreso than the
GFS...leaving a dry frontal passage by Friday night. In any case there should be
enough isen lift moisture to produce isolate prefrontal showers
beginning early Friday with an increase to scattered coverage across the NC
mountains by the afternoon. Model soundings show the presence of elcape
on the order of a couple hundred joules Friday afternoon...so thunder
mention will be maintained through the evening period. Dont see
enough in the dynamics to suspect a severe potential at this time as upper
forcing remains weak and shear levels are moderate at best. Maximum
temperatures will continue to increase a couple degrees each day.

&&

Aviation /19z Saturday through Thursday/...
at kclt and elsewhere...rainfall intensity and coverage is
increasing over western NC...though the heavier rain is located a
little ways to the north and west of the kclt airfield. As long as
the upstream airmass continues to be saturated by precipitation...IFR ceilings
should continue to affect the airfields...especially at kclt and the
upstate sites. IFR ceilings were kept at kclt through 21 UTC with MVFR
developing thereafter. However...I suspect that IFR could continue
longer than this at kclt. Gusty northeast winds will continue
through the night from kclt down to kand. Wind gusts of 25 kts
should be common. Ceilings should begin to lift by late evening once the
precipitation starts to diminish.

Outlook...dry high pressure will then develop through Monday...with a
fairly wet frontal system expected on Tuesday.

Confidence table...

19-01z 01-07z 07-13z 13-19z
kclt medium 70% high 82% high 96% high 100%
kgsp low 59% high 82% high 96% high 100%
kavl medium 69% high 85% high 100% high 100%
khky high 86% medium 74% high 100% high 100%
kgmu low 56% high 84% high 94% high 100%
kand medium 70% high 83% high 97% 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: (must be lower case)

Www.Weather.Gov/gsp/aviation_tables

&&

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

&&

$$
Synopsis...sbk
near term...McAvoy
short term...Wimberley
long term...sbk
aviation...McAvoy