Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
201 am EDT sun Apr 20 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 today/...
as of 155 am EDT...regional radars show one last training band
moving through the southeast side of Charlotte early this morning.
All mesoscale models depict coverage slowly diminishing through the
early morning hours as the deformation zone dissipates and slips
east. Meanwhile...water vapor imagery shows the swirling upper low
moving over the offshore waters east of Savannah early this morning.
As this upper vortex moves farther offshore today...ridging will
build in from the west. Gradual scattering will allow temperatures to
rebound despite the lingering surface high pressure zone remaining in place.
The NAM has more easterly flow moisture at 850 mb tonight...but the
preferred GFS/ecm Camp keep the moisture confined more to the
coastal plain. Expect maxes in the 60s this afternoon and mins in the 40s
tonight.

&&

Short term /tonight 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 /06z Sunday through Thursday/...
at kclt...mainly light rain in the lingering deformation zone near
the airfield will continue through 09z before dissipating. Low level
dry air advecting in on NE flow is keeping all ceilings and visibility VFR
even in the better precipitation...so will ride on VFR through the
morning hours. Occasional gusts are likely in the NE flow...but with
better gusts developing through the day with mixing. Will allow VFR
ceilings to scatter out by 16z and then lean toward less clouds as
indicated on the GFS/ecm through the evening hours.

Elsewhere...all lingering deformation zone precipitation should
remain east of the foothill and mountain taf sites through the early
morning hours. In addition...slightly drier air advecting in on NE
flow is keeping visibility and ceilings at VFR levels and this should persist.
Kavl to kand are the sites of most concern. Kavl will be sheltered
to the drier NE flow...and kand will see slightly less boundary
layer drying. These locations could experience brief MVFR conditions
but confidence is too low to feature at the moment. Expect
occasional gusts on NE flow at the upstate taf sites
today...especially this afternoon with mixing.

Outlook...dry high pressure will develop through Monday. A fairly wet
frontal system is expected on Tuesday...with drying returning middle week.

Confidence table...

06-12z 12-18z 18-24z 00-06z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 100% high 100% high 100%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 100% 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: (must be lower case)

Www.Weather.Gov/gsp/aviation_tables

&&

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

&&

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