Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
225 am EDT Thursday Jul 28 2016
broad surface high pressure will linger over the southeast, before
sliding offshore as a surface low develops and moves northeast
across the Midwest and Great Lakes regions. A surface front will
remain off to our north and linger within a region between the Ohio
Valley, lower Great Lakes, and mid-Atlantic regions. This front may
finally move into our area over the weekend.
Near term /through today/...
as of 225 am: short range models indicate that a mid level short
wave will slowly ripple east across Kentucky/Tennessee today. Guidance indicates
that a band of shras and tsras will develop along and ahead of the
short wave across eastern KY/TN. Coverage of convection will gradually
increase across the southern Appalachians during the mid to later
afternoon. Forecast soundings east of the mtns indicate weak capping
will remain through the afternoon, limiting coverage to isolated.
Deepening sfc low pressure across Virginia should yield a 2-3 mb pressure
gradient across the County Warning Area this afternoon. The combination of afternoon
mixing and the shallow gradient should result in a period of
marginal gusts from the SW.
0220 UTC update: winds were updated with a blend of the latest NAM and
adjmav. Temperatures and dew points were updated from observations
and model data. Pops were adjusted per radar trends, with a
decrease over SC and Georgia.
Previous discussion: convection is taking it's time developing
across the County warning forecast area...mainly confined across the nrn zones near a sfc
bndry and some periodic pop ups across the ridgetops. The lower
atmos has mixed out a little more than expected across the nrn
upstate and srn NC Piedmont. TD/S are in the m60s leading to a
SBCAPE minima and shallow cu development. The best activity area
this afternoon/evening will be across the nrn zones and perhaps
across the nrn srv where higher end cape is developing.
The overnight will see gradual moist adv from the SW due to a
subtrop disturbance being lifted north toward the Tennessee Valley. Most of
the heavy precip will be aligned south and west of the forecast area...but the
NC mtn spine counties shud see enhanced -shra/tstm activity
lingering through daybreak. Some isol activity is also possible
across the nrn fthills and NC Piedmont with the llvl bndry remaining
The fcst for Thu is a bit tricky compared to the previous few days
as mid and upper level forcing will increase somewhat as a broad h5
trof moves across the lower Midwest and an h85 low associated with
the aforementioned St disturbance continues to advance closer to the
wrn zones. This will increase llvl shear values to arnd 25kts or so
during the afternoon...and along with modest SBCAPE values...some
storms could become loosely organized and produce relatively long
lived cold pools...esp across the wrn zones. This would produce
more of a straight line wind threat as opposed to the localized
microburst activity seen lately. Max temps will be a couple degrees
cooler tomorrow and shud preclude another heat adv issuance based on
similar TD/S across the area.
Short term /tonight through Saturday/...
as of 200 PM EDT wednesday: at the start of the period, subtropical
ridging will remain in place over the southeast and portions of the
western Atlantic, with upper trough extending down the Mississippi
Valley. The initial shortwave will lift northeast on Friday as mean
troughiness continues across the east, with a secondary shortwave
pushes out of the plains for the latter half of the period. With the
general southwest flow aloft over the southern Appalachians, plume
of tropical moisture that moved onshore over the lower Mississippi
yesterday will stretch northeast up the Ohio Valley, over the upper
ridge. Attendant surface front will slowly work its way toward the
Appalachians but with a continued Lee trough across the NC
foothills/Piedmont serving as a focusing mechanism for afternoon
convective development. The secondary wave and slug of moisture
pushes across the Tennessee Valley Saturday as the mean trough
slowly pushes east. Fairly good agreement in synoptic fields on the
operational models, but as usual some differences in details and
sensible weather impacts, with the GFS remaining slightly wetter
than the European model (ecmwf).
Overall, best deep-layer moisture will remain to our west with the
front but with the addition of synoptic lift from the broadening
wave, should see a slightly higher than climo trend to pops through
the short term. Instability looks fairly typical of Summer, with
mixing across the Piedmont contributing to slightly lower cape
values than those across the mountains. Low-level lapse rates will
continue to support an isolated wet microburst or two, with daytime
high temperatures hovering just slightly above normal, and overnight
lows about a category above.
Long term /Saturday night through Wednesday/...
as of 230 PM EDT wednesday: aforementioned slug of moisture
associated with secondary shortwave will push across the southern
Appalachians at the start of the extended period, with the longwave
trough axis continuing to slowly move east and centered over our
area around the start of the work week. Upper ridge strengthening
across the plains will ridge northeast as the the trough becomes
increasingly positively-tilted, remaining stalled over the area
generally through the period. Corresponding surface trough will
enhance (or possibly itself be enhanced by...Chicken-or-egg thing
here) the Lee trough that has been in place the past couple of days.
This will keep a higher-than-climo diurnal trend in convection
across the region, with continued support of isolated to widely
scattered strong to severe storms in the form of wet microbursts.
Temperatures trend more toward climo with expected increases in
cloud cover and convection, but will continue to see some variation
Because the GFS is showing it approaching the NC coast very late in
the period, will note that invest of a disturbance off the coast of
Africa has been given a 40% chance of tropical cyclone formation
through 5 days. Model solutions vary *widely* as this pushes west-northwest
toward the leeward/Windward Islands - this is way too far out to
make any prognostications at this time!
Aviation /07z Thursday through Monday/...
at kclt and elsewhere: clear sky and light winds may favor the
development of mtn valley stratus and fog around dawn, I will
highlight with a tempo from 11z-13z. Short range models indicate
that a mid level short wave will slowly ripple east across Kentucky/Tennessee
today. Guidance indicates that a band of shras and tsras will
develop along and ahead of the short wave across eastern KY/TN. Coverage of
convection will gradually increase across the southern Appalachians
during the mid to later afternoon. Forecast soundings east of the
mtns indicate weak capping will remain through the afternoon,
limiting coverage to isolated. Deepening sfc low pressure across Virginia
should yield a 2-3 mb pressure gradient across the County Warning Area this
afternoon. The combination of afternoon mixing and the shallow
gradient should result in a period of marginal gusts from the SW.
Outlook: falling heights aloft and the arrival of Gulf moisture will
support increasing chances of precipitation and perhaps associated
restrictions going into the weekend.
06-12z 12-18z 18-24z 00-06z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 92% high 100%
kavl high 86% high 93% high 100% high 100%
khky high 97% high 97% high 100% high 100%
kgmu high 100% high 100% high 92% high 100%
kand high 100% Med 77% high 92% 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: