Scientific Forecaster Discussion
fxus62 kgsp 200657
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
257 am EDT Tue Aug 20 2019
an area of low pressure and elevated moisture will persist across
the western Carolinas and northeast Georgia today. A moist cold
front will approach the region Thursday and become stationary over
the area on Friday. The front will gradually settle south of the
region through the weekend as high pressure builds in from the north
and maintains cooler than normal temperatures. Moisture could linger
behind the front.
Near term /through tonight/...
as of 245 am tuesday: still plenty of convection lingering across
central Georgia to the central Savannah River area early this morning,
with some of that convective debris cloudiness drifting north into
the forecast area. The western edge of these clouds have cleared east of the
central and western NC mountains, revealing dense valley fog in the
little Tennessee Valley. As the clouds start to thin out over the next few
hours, a light southeasterly low-level flow will help produce some stratus
across portions of the Piedmont. Confidence on how extensive the
clouds will be is still low, but I do expect at least a fair amount
of low clouds around daybreak or shortly thereafter. The clouds will
delay diurnal heating early, but guidance still agrees on another
round of scattered convection this aftn into early evening. Forecast
soundings show a lot more mid lvl moisture than yesterday, and with
Max temps a category or two cooler, Max cape should be more in the
1000-2000 j/kg range instead of the 2000-3000 j/kg yesterday. So I
expect less of a severe threat today. A few storms could still be
strong, especially in the mountains, where they will be a little
removed from the deeper mid-lvl moisture and should have more
sunshine this morning.
Tonight, a weak low will slowly drift NE over the forecast area and bring the
low-level flow around of the SW overnight. This flow, while losing
most of the upslope component, will have a little upglide with it,
helping produce some low stratus across portions of the Piedmont. A
few isolated showers may linger in the upglide flow as well. Temps
will be slightly above normal under the cloud cover.
Short term /Wednesday through Thursday night/...
as of 230 am EDT tuesday: relatively zonal upper-level flow is
expected atop the region on Wednesday, but weak lobes of vorticity
embedded in this flow pattern could provide triggering over the
mountains during peak afternoon heating. Mid-level lapse rates
should spike back up to the 6.5 to 7 deg c/km range, and the
convection allowing models feature activity developing along the
Blue Ridge mountains and running quickly eastward across the
foothills/Piedmont through the evening hours. Warm and moist
profiles look to be a limiting factor for much severe weather, but
very gusty winds will certainly be possible with any downbursts.
Heights will then fall sharply from the upper Midwest through the
Ohio Valley Wednesday night through Thursday night. This will permit
a cold front to lay over north of the region and promote steadily
increasing deep moisture over our area south of the frontal zone.
Profiles exhibit precipitable water values over two inches in many
areas, but positive energy in the profiles looks a bit higher than
on Wed., With an associated uptick in storm strength likely on
Thursday afternoon. Min temps remain two categories above climo
through the period, with maxes about one category above.
Long term /Friday through Monday/...
as of 245 am EDT tuesday: the best dynamic forcing associated with a
deep closed low over eastern Quebec should pass north of the
forecast area Friday through Saturday, but with a trough axis
crossing the southern Appalachians early in the period. An
associated cold front will lay over into the region, along with
continued abundant moisture. Numerous to widespread showers and
thunderstorms will likely develop in this moist airmass as the
frontal zone and upper level forcing arrive on Friday.
The frontal zone draped across the area Friday night will gradually
settle south of the region through the weekend. The GFS remains more
aggressive than the European model (ecmwf) in pushing the boundary southward, but all
solutions keep some measure of moisture lingering north of the
frontal zone to promote clouds and precipitation and associated cold
air damming conditions. Although upper heights will generally build
again from the west through the weekend, some bagginess is indicated
in the height field west of the mountains. This could reinforce
intermittent, weak upglide and help to establish/maintain damming
the rest of the weekend. Indeed, easterly flow and continued
moisture could permit cad-like conditions to linger into Monday.
Temperatures have been trimmed slightly for the Sat-Mon period, but
probably not enough if true damming develops. Will keep above climo
pops as well for mainly light rain showers. Any thunder should be
confined to the far Southwest Mountains.
Aviation /07z Tuesday through Saturday/...
at kclt and elsewhere: plenty of mid to high clouds will be slow to
dissipate, thanks to a weakness in the upper pattern. The western
edge of the clouds have cleared out of portions of the NC mountains,
revealing Mountain Valley fog. Guidance is mixed on how much fog
and/or stratus may develop elsewhere, as the clouds thin out aloft.
But with wet soils acrs much of the region from yesterday's storms
along with a light easterly to southeasterly low-level flow, some
stratus is expected to develop. The highest chances will be in the
west (which would impact the upstate taf sites), but it could expand
east to affect kclt and khky around daybreak. Once the clouds form,
it will take a little while to lift to VFR (per latest forecast
soundings). The very moist air mass in place should support another
round of scattered convection (albeit not quite as widespread or as
strong as yesterday). Will go with prob30 at all sites again this
aftn. The light low-level flow will turn more out of the SW tonight,
losing most of the upslope component, but with an increase in
upglide. So areas of stratus will likely redevelop tonight.
Outlook: the potential for afternoon/evening showers and
thunderstorms is expected to increase later this week, as a cold
front approaches from the northwest into Thursday. Morning fog and low
stratus will be possible each day across the mountain valleys.
Otherwise, expect VFR.
07-13z 13-19z 19-01z 01-06z
kclt high 100% high 100% high 100% Med 71%
kgsp high 100% Med 73% high 92% Med 78%
kavl low 51% high 96% high 100% high 89%
khky high 86% high 96% high 100% high 84%
kgmu high 96% Med 69% high 92% high 80%
kand Med 75% high 82% high 89% 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: