Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
244 PM EDT Tuesday Apr 21 2015
dry high pressure slides east of the area Wednesday. Another cold
front will push in from the northwest on Thursday...bringing low
chances for showers. Dry high pressure prevails briefly in its
wake before a warm front brings better chances for rainfall to the
area over the weekend.
Near term /through Wednesday/...
as of 230 PM EDT...a deep cutoff low pressure system continues to
spin over the northern Great Lakes this afternoon. A belt of strong
westerlies has set up south of this system through the southern
Appalachians. These westerlies will both increase winds through the
period and drive a cold front into the region from the northwest late Wednesday.
For tonight...the first lobe of pre/frontal moisture will arrive in
the western mountains toward 12z Wednesday...but mainly at middle levels and higher.
The moisture and clouds should arrive sufficiently late to permit
chilly temperatures in the 30s mountains and 40s Piedmont.
Slightly better moisture should arrive with the frontal zone Wednesday
afternoon. In addition to weak frontal zone lift...westerly upslope flow
will provide forcing across The Spine of the southern Appalachians. The
best forcing and deepest moisture will require the best chance probability of precipitation
in the western mountains there will be less forcing and moisture farther
southeast...but southeast sections will become the most unstable. A pool of 6.5 to
7 degree c/km lapse rates will cross the region during the afternoon.
Although slight mixing out of dewpoints is expected...the decaying
cap aloft and the steepening lapse rates will warrant at least
isolated thunder chances. Maximum temperatures should recover about 5 degrees
over Tuesday values ahead of the cold front.
Short term /Wednesday night through Friday/...
as of 200 PM Tuesday...weak cold front will continue to push southward through
the County warning forecast area Wednesday night. It still will have only limited moisture...and
elevated instability will be small and waning through the evening. Low
and diminishing probability of precipitation will be advertised overnight over the Piedmont
mainly in case the front is able to kick off any showers...and over
the mountains for any upslope precipitation in the westerly flow in its wake. Looking
further aloft...the front comes in association with a shortwave
rotating around the closed upper low over Ontario. Heights generally
begin to rise over our area in the wake of the front as dry
Continental high pressure spreads back in. Deep drying occurs Thursday
and Thursday night and the height gradient weakens...which should allow
for pleasant and more settled conditions Thursday. Furthermore good
radiative conditions within the already cool airmass will result in
well below normal min temperatures for Friday morning...though records do not
appear to be in jeopardy. On Friday warm/moist advection will occur
into the deep south ahead of a developing low over the plains.
Upglide will get underway by the end of the day over the SW portion
of the area...bringing increasing sky cover and probability of precipitation. Maxes will
remain a category or so below climatology both Thursday and Friday.
Long term /Friday night through Tuesday/...
as of 245 PM Tuesday...extended begins with GFS and European model (ecmwf) in decent
agreement in overall synoptic fields with large upper trough pushing
off the East Coast and midlevel ridging building across the
Mississippi Valley. Cutoff upper low over the Central Plains...
associated surface low...and attendant front will be the main
players for the weekend. Warm front lifts north across the southeast
Friday night into Saturday...with widespread isentropic lift and
moisture advection from the Gulf developing. Upper diffluence under
the right entrance region of the jet adds to the overall lift and
while resulting instability is marginal at best...the big story is
the widespread rain and quantitative precipitation forecast Saturday into Sunday. Details though are
hard to pin down as both GFS and European model (ecmwf) want to split the quantitative precipitation forecast across
the upstate and North Carolina Piedmont late in the day on Saturday
as the closed low weakens and phases with the longwave trough...
keeping the heaviest quantitative precipitation forecast values to the south with another maximum to
the north. Final wave of quantitative precipitation forecast pushes across the southern and central
Appalachians Saturday night. Wpc days 4-5 48hr quantitative precipitation forecast ranges generally
between 0.5-1 inch so overall not a significant concern on the quantitative precipitation forecast
side. Attempted to reflect some of this uncertainty in probability of precipitation but
later forecasts will likely need to bump probability of precipitation up some more with a
high pop/low quantitative precipitation forecast scenario.
Upper ridging develops again up the Mississippi Valley with a very
progressive pattern in general. As the ridge translates east another
trough pushes toward the central part of the country. GFS and European model (ecmwf)
start to diverge a little more significantly toward the end of the
extended but both do bring another system in late Monday into
Tuesday. Too many differences in the details to pin down much more
than that so have deferred to Middle-Range chance probability of precipitation.
High temperatures hover generally a few degrees below seasonal
normals with a reduced diurnal range....with either clouds and
precipitation across the area /for the weekend and again at the end
of the period/ or at least a weak cold air advection pattern /Sunday into Monday in
the wake of the weak front/.
Aviation /19z Tuesday through Sunday/...
at kclt...mixing should continue to provide occasional gusts of 15
to 20 knots through late afternoon...along with west-southwest to SW flow ahead of a
Lee trough. Mainly middle and high clouds will increase through the
latter half of the period. Winds will pick up from the SW Wednesday from
late morning on...with gusts into the lower 20s likely by early
afternoon. The strongest gusts will likely occur just beyond the taf
period with the deepest mixing Wednesday afternoon. VFR conditions will
Elsewhere...middle and high clouds will increase ahead of the
approaching cold front for Wednesday. Low end gusts will likely continue
through late this afternoon before some measure of decoupling occurs.
Winds will pick up through the day on Wednesday with deeper mixing. Will
feature gusts of 20 to 25 knots toward the end of the period...but
substantially higher gusts will be possible after 18z. Will also
keep the tafs dry through 18z...but a few showers may start to
develop near kavl toward the end of the period.
Outlook...a cold front will cross the area late Wednesday and then
gusty west winds will continue into Wednesday night. A dry high pressure
system will build in Thursday through Friday...before another wet
storm system is expected to approach the area from the west over the
18-24z 00-06z 06-12z 12-18z
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)