Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
356 am EST Monday Nov 24 2014
a coastal low will develop off the southeast coast and move up the
Atlantic Seaboard tomorrow night and Wednesday...bringing
precipitation chances back to the area. Drier high pressure will
spread back over the region on Thursday and Friday and linger well
into the weekend.
Near term /through tonight/...
a region of upper height falls will continue to spread into the
eastern states through the short term...with an attendant cold front
gradually pushing its way across our region through this evening.
The NAM continues to develop a healthy axis of SBCAPE across the
Piedmont by this afternoon. However...forecast soundings continue to
depict a substantial layer of warm air in the mid-levels. Under the
influence of increasing upper divergence southeast of intensifying
upper jet...some of the short term and convection-allowing models
respond with some quantitative precipitation forecast within this instability axis this afternoon...
but primarily east of our forecast area. Nevertheless...feel it/S
Worth a slight chance mention across the southeast third of the County Warning Area
this afternoon. Thunder will be omitted in light of the very warm
Small probability of precipitation will also be carried across the western mountains...to account
for the potential of showers developing along the front. However...
even if a few showers do manage to develop along the front...weak
low level convergence will make it difficult for them to be
maintained east of the mountains maximum temperatures will be about 10 degrees
above climatology today in the deep SW flow/pre-frontal thermal ridge
Any lingering cloud cover should clear quickly behind the front
(except perhaps within shallow moist northwest flow regime across the
mtns)...with min temperatures expected to average a little warmer than
climatology...as cold advection takes its time to become established.
Short term /Tuesday through Thursday night/...
as of 330 am Monday...dry high pressure will prevail over the County warning forecast area most
of Tuesday while an expansive upper trough covers the eastern US. Cold air advection
will continue into the southeast with cold front oriented along the
South Atlantic coast and southwestward into the central Gulf. A shortwave
rounding the base of the longer trough will encounter the front and
start to spin up a surface low over Florida by Tuesday evening and it is
this low which will bring increasing precipitation chances Tuesday night into
Wednesday as it rides up the southeast coast. Though there are still
differences of perhaps 50-100 miles in the low track between the
GFS/NAM/CMC/ec...the 00z cycle of guidance has come into fairly good
agreement...though the NAM remains a slight outlier in taking the
low more offshore and not deepening it as quickly. There is
consensus on the timing of peak probability of precipitation...a little before daybreak Wednesday
in the lower SC Piedmont zones...and late morning in the I-77 corridor.
We will go ahead and advertise categorical values in these areas
given the increasing consensus.
Pre-existing cold advection will be enhanced as the low passes
Wednesday. In the western portion of the County warning forecast area partial thicknesses
will already be marginal for wintry ptypes early Wednesday morning...but will
plunge further before the low lifts away. With the entire area
remaining on the cool side of the system...it still appears that
either rain or snow will result with no elevated warm layers seen on
any of the available model guidance. Surface T or tw values only favor
snow reaching the surface in the mountains and higher foothills. There is
fairly good consistency between the various sources of quantitative precipitation forecast guidance
but the blended values ended up close to the GFS ensemble mean.
Liquid values do not look excessive...well under an inch even in our
southeastern/eastern zones nearest the low. Snow totals in the mountains look to be
advisory-level at this time. Sufficiently cold air is only questionable early
in the event leaving liquid quantitative precipitation forecast as the main factor in snowfall.
Provided that the models maintain the coastal track I have pretty
good confidence in these totals.
By Wednesday night an additional push of cold air and moisture will reach
the mountains as a shortwave embedded in the trough exits the Ohio
Valley. Low level winds will veer to northwesterly across the area by Thursday morning and
forecast soundings suggest a minor snowfall event near the tenn
border. Additional accums of less than an inch are expected. Flurries
may continue on the windward slopes through Thursday before moisture
finally diminishes Thursday night. Downsloping winds across the area Thursday
afternoon will help temperatures rebound into the 50s across the Piedmont.
Long term /Friday through Sunday/...
as of 345 am Monday...Canadian high pressure moves back into the region
Friday and this keeps things dry for most of the weekend. The upper
flow flattens out over the weekend...and even as the surface high shifts
offshore Saturday it appears return flow does not bring back much in
the way of moisture into the area. Instead...both the 24/00z GFS and
ec bring the next shot at precipitation with a weak cold front trailing a
northern-stream low Sunday. We will continue to feature below normal
albeit moderating temperatures through the medium range.
Aviation /09z Monday through Friday/...
at kclt and elsewhere...a very complex/low confidence forecast over
the next 6-10 hours...as guidance has done an extremely poor job in
handling the ongoing erosion of cold air damming air mass...and
associated scattering of sub-MVFR ceilings (at least in most areas). A
smattering of IFR does persist near the Blue Ridge early this
morning...but even this area is becoming increasingly small. Warm
front has basically lifted through the entire area...allowing very
Stout S/SW winds to develop...and these shouldn/T be relaxing
anytime soon...as the surface gradient will remain respectable in
advance of approaching cold front. With that in mind...fog should
not be much of a concern overnight...and the visby forecast has been
improved dramatically from the previous forecast. Additionally...the
strong low level flow should act to keep ceilings from crashing to LIFR
(or lower) as was earlier predicted. However...the ceiling forecast is
the most uncertain component of the forecast...and it is difficult
to say if ceilings will remain MVFR or return to IFR later this morning.
Considering the absence of appreciable low level lift...it is
appearing more likely that IFR conditions will primarily be confined
to locations nearer the Blue Ridge...where some upslope lift will
occur. Therefore...tempos for IFR have been included at all
terminals through about 10z...the exception being kclt...which will
not see any effects from the upslope flow.
Gusty S/SW winds will continue through the day ahead of the front...
which should finally push east of the area by the end of the period.
An isolated shower or two may accompany the front this
afternoon...but this probability appears too low to include in the
forecast at this time.
Outlook...brief drying follows a cold front Monday night-Tue...with
rain (piedmont) and/or wintry precipitation (mountains) possibly returning
Wednesday. More settled weather will return to end the week.
09-15z 15-21z 21-03z 03-06z
kclt high 100% high 97% high 100% high 100%
kgsp high 100% high 91% high 100% high 100%
kavl high 100% high 91% high 100% high 100%
khky medium 67% high 88% high 100% high 100%
kgmu high 89% high 91% high 100% high 100%
kand high 94% 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)