Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
issued by National Weather Service Columbia SC
158 PM 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/...
as of 150 PM EDT Tuesday...despite increasingly unstable
environment and encroaching back door frontal boundary...northeast
Georgia and the western Carolinas remain convection free. Satellite
imagery does indicate expanding cumulus field across the lower terrain
where downsloping flow has allowed for abundant
heating/destabilization. Still expecting convection to initiate
within the next few hours along the Blue Ridge southward into
northeast Georgia and the Piedmont regions of the Carolinas.
Thus...opted for no changes to the forecast with this update. Full
forecast discussion to follow within the hour.
As of 845 am...very spotty showers continue across the region...with
slightly better coverage now developing closer to the front over
extreme northwest NC. Will maintain mainly isolated late morning probability of precipitation and
ramp up to more solid scattered coverage with the front this
afternoon. Have also trimmed maxes a degree or so in most areas in
line with the 06z bias corrected MOS and in light of previous days.
Otherwise...the fog and low stratus should improve rapidly given
the light downslope flow.
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 /18z Tuesday through Sunday/...
at kclt...VFR conditions will prevail through the majority of the
forecast cycle with the exception of possible afternoon thunderstorms and rain and
early morning visb/ceiling restrictions. Initialized taf with light
northwest winds and broken low VFR cumulus with a 4hr tempo from 22-02z to
account for afternoon convection in the warm sector ahead of an
approaching back door frontal boundary. Beyond that...kept prob30
in the next from group as latest cams indicated potential for
redevelopment as the front itself slides through later this
evening. Latest NAM runs slow the frontal progression through the
region overnight leading to persistent MVFR ceilings/visb restrictions
through morning with visb improving by late morning. Kept
northerly winds and MVFR ceilings through the remainder of the period
as soundings indicate continued saturation in the moist low
Elsewhere...much the same as kclt above with VFR conditions
prevailing through the majority of the forecast period aside for
afternoon thunderstorms and rain and early morning visb/ceiling restrictions.
Initialized all taf sites with thunderstorms in the vicinity mention with embedded tempo
groups for thunderstorms and rain and 4sm visb with slight timing differences
depending on site location. Continued with thunderstorms in the vicinity mention beyond
tempos at all sites except for kavl as the frontal boundary slides
southeast. Further ceiling and visb restrictions are expected
overnight at all sites...with guidance hitting the mountain
valleys the hardest despite the frontal passage. Thus...carried MVFR
ceilings/visb at kavl but opted for only a tempo for IFR conditions
due to low confidence. Conditions will improve after daybreak as
high pressure continues to build leading to the return of VFR
at all locations aside for khky where lingering moisture will
allow for persistent MVFR ceilings through the period.
Outlook...although the chances for diurnal convection will greatly
diminish after Wednesday...the potential for late night/morning
fog/stratus will persist into late week...especially in the mountain valleys.
Conditions may finally dry out for the weekend.
12-18z 18-24z 00-06z 06-12z
kclt medium 67% high 100% high 100% high 100%
kgsp medium 67% high 100% high 100% high 100%
kavl high 86% high 100% medium 78% high 96%
khky low 59% high 100% high 100% high 91%
kgmu medium 79% high 100% high 100% high 95%
kand high 82% 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)