Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
1009 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 1010 am EDT Friday...a widespread area of rain is approaching
the forecast area from the south. There are patchy areas of
lighter...high based precipitation ahead of the main band of rainfall...and
this lines up well with the gradually increasing probability of precipitation that we have
in the grids for the rest of the day.

As of 645 am EDT...the forecast remains on track this morning with
rapidly increasing middle and high clouds over the region in advance of
the southern stream trough axis moving into the lower MS River Valley.
The best precipitation coverage remains largely confined to the Florida
Panhandle and southern Georgia...but with a few light returns showing up on
regional radars from time to time 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 /14z Friday through Tuesday/...
at kclt...mostly lowering middle and high clouds are expected this
morning...but some pesky lower VFR ceilings in shallow moist upglide
could flirt with the airfield. The higher level VFR overcast will
steadily lower and thicken through the day...with a rapid transition
to MVFR then IFR ceilings through the evening hours as the rainfall
begins. Expect IFR conditions to get locked in for much of the
overnight hours. 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 pressure gradient.

Elsewhere...VFR ceilings are expected through much of the day at all
locations but kand...where MVFR bases could sneak in on shallow
moist upglide over the developing cad layer. Expect mainly NE winds
throughout today...except toggling between southeast and nearly at kavl. Ceilings
should deteriorate rapidly through the evening hours as the
precipitation onsets from the S...with IFR conditions likely locking
in overnight. 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 tonight.

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...

14-20z 20-02z 02-08z 08-14z
kclt high 100% high 88% medium 61% medium 76%
kgsp high 100% high 81% medium 67% high 84%
kavl high 100% high 80% high 84% high 86%
khky high 100% high 100% medium 67% high 90%
kgmu high 100% medium 67% medium 64% high 86%
kand medium 76% low 57% medium 65% high 80%

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/McAvoy
short term...Arkansas
long term...Arkansas
aviation...hg