Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
446 am EDT Friday Apr 18 2014

Synopsis...
a low pressure system will develop in the northeastern part of the
Gulf today...and track east across northern Florida tonight. This
will spread precipitation into the region tonight into Saturday.
Cool high pressure lingers behind the departing low Sunday and
Monday. A cold front will cross the area on Tuesday...followed by
dry high pressure for the rest of the workweek.

&&

Near term /through tonight/...
as of 440 am EDT...cloud cover has been increased slightly on the
most recent update...as middle and high level moisture continue to
thicken and lower over the region ahead of the southern stream trough
axis moving into the lower MS River Valley this morning. The best
convection and precipitation remain largely confined to the Florida
Panhandle and southern Georgia this morning...but with lighter upglide returns
starting to show up on regional radars over eastern Georgia in improving
upglide. The pop onset may need to be hastened in subsequent updates
if this light rain can overcome the surface based dry layer.

Otherwise...weak surface high pressure lingering along the New Jersey coast will
keep a dry cad pattern over the area early today as the clouds
continue to thicken and lower. Will lean cooler than MOS on maximum
temperatures for today. Upper level divergence will increase rapidly across
the forecast area today into tonight as a 500 mb low center in the
upstream trough closes off over Alabama by evening and then migrates
slowly toward the southeast coastline overnight. The deepest forcing and
moisture ahead of this system will cross the region tonight.
Although the greatest quantitative precipitation forecast swath may be focused just southeast of the
area....deformation zone precipitation and upslope could still focus
some heavier precipitation northward into our area. Two quantitative precipitation forecast maxima may
thus set up...one in the lower Piedmont and a secondary area along
the southern/southeast Escarpment areas...but with nearly one inch rainfall
totals in between as frontogenesis develops from the lower Piedmont
to the Blue Ridge. Precipitable water values are not especially
impressive at a little over one inch through tonight...and given the
lack of a clear terrain focus and the relatively short duration of
the heavier rainfall...a Flood Watch is not expected.

&&

Short term /Saturday through Sunday night/...
as of 300 am Friday...models are in good agreement on the 500 mb low
slowly working its way east rougly along the Florida/Georgia border on
Saturday...keeping the County warning forecast area under strong q-vector convergence through
the day. A surface low will be nearly right under the upper low...with a
stiff NE low level flow and deformation zone precipitation shield likely still
across much of the area at least through the morning. I have bumped up
probability of precipitation through the day...with a slower decreasing trend in the afternoon. Surface
high pressure building into the northeast states will supply cool air...so
taking a blend of the consraw guidance...temperatures were ratcheted
down...staying primarily in the 50s to lower 60s in the west.
Additional quantitative precipitation forecast of up to a quarter inch in the southeast zones is
possible.

Based on latest model trends...it looks like the wrap-around
moisture and low level NE flow lingers Saturday night and even into
Sunday. I have pop taper off to less than 15 percent by Sunday
morning...but clouds slow to clear until about midday in the east.
Given the cool NE flow...I did blend in consraw temperatures to bump them
down...especially east of the mountains

A narrow surface ridge axis settles over the area Sunday night under
rising heights aloft. Temperatures will be near normal...so no frost/freeze
concerns are expected.

&&

Long term /Monday through Thursday/...
as of 215 am Friday...the op models are in good agreement on the
phasing of remnant southern stream energy from a closed upper low in vicinity of
calif and a northern stream shortwave trough digging into the Great Lakes
Monday into Tuesday. The resultant propagating latitudinal trough will
push a cold front through the County warning forecast area on Tuesday. The GFS is most bullish
on instability and convective coverage...with 600-800 j/kg of SBCAPE.
Overall forcing with the front looks modest at best...and the forecast
soundings show a deep-layer unidirectional westerly flow. So not expecting
much more than scattered showers with perhaps a few thunderstorms. The best
coverage will be near the Tennessee line...as downslope may limit coverage
east of the mountains

An upper ridge builds back in behind the shortwave Wednesday through
Friday...with dry weather on tap for the rest of the week.

Temperatures start out near normal Monday...then warming a couple
categories above normal ahead of the front Tuesday...and staying
above normal Wednesday through Friday.

&&

Aviation /09z Friday through Tuesday/...
at kclt...the cloud base forecast remains uncertain this
morning...with models/MOS split on generating MVFR ceilings by daybreak
today versus keeping any ceilings at mainly cirrus levels. The more
optimistic solution is better matching observation thus far...but will
continue to hint at MVFR possible with a sct020 forecast after 09z.
The higher level VFR overcast will steadily lower and thicken
through the day...with a rapid transition to MVFR then IFR ceilings
expected during the last 6 hours of the taf period as rain develops.
NE winds should be less than 10 knots through the first half of the
period...but with increasing flow and gusts late as surface low pressure
over southeast Georgia tightens the gradient.

Elsewhere...mainly VFR middle and high clouds are expected
early...except for a better chance of MVFR this morning at kand in
weak upglide over the cad layer. Expect mainly NE winds throughout
today...except toggling between southeast and nearly at kavl. Ceilings should
deteriorate rapidly late today through tonight from the S as better
upglide moisture and upper support arrive. Anticipate MVFR ceilings by
early evening...with IFR likely after 03z along with moderate to
possibly heavy rainfall. In addition...a tightening pressure gradient north of
the southeast Georgia coastal low will produce northeasterly gusts above 20 knots at the
upstate taf sites late in the period.

Outlook...ceiling and visibility restrictions will linger well into
Sat as a southeast coastal low pressure system pulls slowly away. Dry high
pressure will return Sunday into Monday...with a fairly wet frontal
system expected on Tuesday.

Confidence table...

08-14z 14-20z 20-02z 02-08z
kclt medium 73% high 100% high 88% medium 70%
kgsp high 80% high 100% medium 63% medium 72%
kavl high 100% high 100% high 88% high 98%
khky high 81% high 100% high 100% medium 68%
kgmu high 80% high 100% medium 63% medium 79%
kand medium 71% medium 64% medium 68% medium 75%

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...hg
short term...Arkansas
long term...Arkansas
aviation...hg