Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
345 am EDT Tuesday Sep 16 2014
a cold front will cross the area today...enhancing chances for
showers and thunderstorms through tonight. In its wake...seasonably
cool air will spill into the region Wednesday. The cool conditions
and mostly dry weather will last into the weekend.
Near term /through tonight/...
northwest flow will become established by late this morning...in the wake
of departing middle-level short wave/attendant surface trough. A bit of
middle-level dry air should also filter in...with the upshot being
minimal chances for precipitation for the remainder of the morning...
although a stray shower will be possible across the far western NC
mountains within northwest/upslope flow. The northwest flow should also act to scour
the low clouds in fairly quick fashion within a couple of hours of
sunrise...and most locations should see no worse than partly cloudy
skies by late morning.
A series of subtle short waves is expected to ripple through the
developing eastern trough over the next 24 hours...with a model
consensus suggesting the first one will approach our area by early
this evening...along with an accompanying cold front. These features
will find an air mass with less deep-layer moisture content than
we/ve been seeing in recent days. However...the additional heating
(with maxes expected to peak near climo) will result in moderate
destabilization across the region this afternoon. Thus...expect scattered
convection to develop across the mountains by late afternoon...with
activity trucking eastward fairly quickly in moderately fast west/northwest
flow. Speaking of which...deep layer shear is expected to be rather
respectable today...with forecast soundings depicting bulk 0-6km
shear of around 30 kts. However...instability is not expected to be
very robust (sbcape 1500-2000 j/kg)...but some degree of mesoscale
organization and a brief severe wind gust or two cannot be ruled
Convective chances will likely reach a nadir during early evening...
then gradually diminish toward the overnight hours. With plenty of
low level juice lingering...min temperatures should average a little above
climatology for one more night before the cooler air arrives.
Short term /Wednesday through Thursday night/...
as of 300 am Tuesday...an amplified upper pattern will be seen over the
Continental U.S. During the period...keeping most of the eastern states under a
longwave trough. However the pattern does keep moving as the ridge
moves from the intermountain west into the plains...with a weak surface
high building into the eastern Continental U.S. Downstream of it. Our area will be
under the influence of the weak high Wednesday before a larger
Canadian high slides in for Thursday. A couple of weak shortwaves
buried in the upper flow are also shown on the NAM/GFS/ec...resolved
more distinctly on the NAM on account of its higher resolution.
Moisture profiles appear insufficient for these waves to produce any
precipitation on their own...and they are poorly timed to enhance diurnal
convection within the County warning forecast area. In fact on Thursday the subsidence in the wake
of a wave appears to be its biggest impact. Though lapse rates are
not that bad either day...models support schc probability of precipitation across much of the
area Wednesday. The greater subsidence Thursday lends confidence to a dry forecast.
Temperatures will be a couple categories below climatology both days though
clearer skies Thursday warrant slightly warmer maxes. The surface high and
tightening height gradient within the downstream upper trough line up
Thursday night to allow cad to form which persists into the weekend.
Long term /Friday through Monday/...
as of 200 am Tuesday...as upper trough progresses eastward through the Canadian
Maritimes...earnest cold air damming continues over our region
reinforcing seasonably cool temperatures Fri-Sat. Easterly flow may produce
some cloud cover but precipitation is not expected. Parent high moves
offshore fairly quickly but The Wedge holds on through Saturday.
Falling upper heights take their toll on it by Sunday
morning...bringing return flow ahead of a low pressure system
crossing the Great Lakes. 16/00z GFS and 15/12z ec continue to
depict this system differently but both feature a cold front moving
out of the tenn valley and into our area early in the week. Each
model agrees with the pattern seen on their own previous runs.
Beyond that the general consensus is for an amplifying upper
pattern...with a deep eastern Continental U.S. Trough carving out by next Tuesday.
Temperatures trend upward through the period as The Wedge gives way and the
frontal passage occurs...but at their warmest are around climatology.
Aviation /08z Tuesday through Saturday/...
at kclt...another night...another less-than-confident forecast...
although the next 6 hours appear to be slowly coming into better
focus. IFR ceilings have already been observed early this morning...and
with plenty of low level moisture lingering...see no reason why the
next 6 hours should bring any improvement to the terminal. In
fact...conditions will probably get a little worse before
daybreak...and expect the flight category to at least flirt with
LIFR ceilings periodically. Visby is a bit more uncertain...especially with the
widespread cloud cover already in place...but expect visby to
generally remain in the MVFR range...with periods of VFR and IFR
possible. The good news is that with increasing downslope flow
combining with daytime heating...ceilings are expected to lift/scatter
rather quickly after sunrise...with VFR conditions likely by late
morning. That being said...more heating spells more instability and
scattered convection should develop across the area during late
afternoon/early evening...warranting a prob30 for rain showers after 22z.
Elsewhere...Greenville and Asheville area terminals have already
been bouncing in and out of IFR/LIFR conditions this morning...and
expect these to continue for the balance of the next 4 hours or so.
Some rain showers will also be possible near the upstate SC terminals
through about 08z...as a surface trough moves through the area.
Speaking of which...the winds will shift to the west/northwest behind this
trough...which is the primary reason that forecast confidence tanks
after 09-10z...as it is not out of the question that this developing
downslope (up-valley at kavl) wind could result in lifting ceilings
toward daybreak. We have therefore featured slight improvement at
these terminals by 10z...but at least a tempo for IFR will be
carried through 12z. Otherwise...kand/khky should eventually see
periods of IFR/LIFR conditions this morning as well. The downslope
flow combined with daytime heating should result in ceilings lifting/
scattering rather quickly after sunrise...with VFR conditions likely
by late morning. That being said...more heating spells more
instability and scattered convection should develop across the area
during late afternoon/early evening...warranting a prob30 for thunderstorms and rain
at all terminals beginning around 20z give or take.
Outlook...a return to mainly VFR conditions is expected Tuesday night
through middle week. The best chance of low clouds each morning will be
in the mountain valleys. Another round of cold air damming could
produce some flight restrictions late this week.
07-13z 13-19z 19-01z 01-07z
kclt medium 73% high 85% high 100% high 100%
kgsp medium 67% medium 75% high 100% high 100%
kavl medium 64% medium 73% high 100% low 49%
khky medium 60% medium 79% high 100% high 100%
kgmu medium 68% high 81% high 100% high 100%
kand medium 72% high 84% 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)