Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
237 PM EST Tuesday Dec 1 2015
a cool and wet air mass will persist across the area before a cold
front crosses from the northwest on Wednesday. Dry high pressure
will build in on Thursday and remain into the weekend.
Near term /through Wednesday/...
as of 230 PM EST Tuesday...a wet/rainy/foggy afternoon and evening
in store across the vast majority of the forecast area. A closed
upper low rotating over the Missouri Valley will slowly slide
east over the next 24 hours. At the surface...a cold front
tied to a surface cyclone beneath the upper low continues to
push ever so slowly east across west/middle Tennessee this afternoon.
Out ahead...cad wedge that has been in place over the region has
be rather stubborn pertaining to erosion...and thus has kept the
Piedmont regions locked in under low stratus. As a result...a
very moist/saturated boundary layer continues to promote areas of
dense fog across the upstate and northeast Georgia where substantial
precipitation has yet to fall. Thus...extended the ongoing dense fog
advisory through the remainder of the afternoon before the arrival
of heavier precipitation should allow for scavenging. That said...models
remain very pessimistic regarding fog through the overnight with the
flow veering southerly yielding weak/improving warm air advection...thus wouldnt
be surprised to see continued visibility issues. The other main
concern through the near term continues to be Hydro as model quantitative precipitation forecast
remains consistent with an additional 2-3 inches forecast over the
NC/SC/GA high terrain. Not seeing much of a flash flood threat
with this event as llv flow remains relatively weak compared to
other events which will inhibit modest upsloping. Thus think
the primary threats will be to area rivers/streams as recent days
rainfall will allow for ample runoff...therefore the Flood Watch
remains in effect through Wednesday afternoon.
As for the evolution of the system and the forecast...expecting the
prefrontal precipitation shield to continue sliding into the region
overnight...likely centered atop the mountains/foothills by
middle morning. It is at this time that precipitation rates are expected
to maximize as upper jet support complies. Thus...probability of precipitation will
increase through the afternoon with categorical levels featured
over the mountains...tapering to likely levels east. Probability of precipitation will
remain elevated through tonight where by morning categorical
levels are favored nearly County Warning Area wide...before slowly tapering from
the west with time as the front itself slides across the Tennessee line
into the mountains. Beyond that...continued with likely probability of precipitation
along/south of Interstate 85 on Wednesday afternoon as guidance
does favor additional redevelopment along the front over the NC/SC
Piedmont where some minimal cape will reside which could yield
scattered thunder. Temperatures were populated from bcallblend
using a non diurnal curve through tonight...warming ahead of the
front...before falling behind the front Wednesday afternoon.
Short term /Wednesday night through Friday/...
as of 145 PM Tuesday...precipitation will be moving out of the
area by 00z Thursday as the cutoff upper low swings across the
Ohio Valley and Great Lakes...pushing the cold front toward the
eastern Seaboard and eventually offshore. Could see some lingering
moisture in the northwest flow over the northern mountains...along
the southern edge of the upper low...and as temperatures drop
behind the front cannot rule out some brief snow showers in the
northern mountains between 06-12z Thursday. Have not added any
accumulation but do advertise brief snow showers or at least a
rain/snow mix. Moisture lifts north with the strong system as it
pushes toward New England...leaving US dry with temperatures just
a few degrees below seasonal normals on Thursday. Will probably
see some fairly gusty winds especially in the mountains behind
the front early in the period...but gradient does not look strong
enough to support Wind Advisory levels.
Cold Thursday night with 20s in the mountains and at best middle 30s
across the upstate with mainly clear skies as surface high pressure
builds in under confluent flow aloft. Not much change into Friday
with quiet conditions remaining in place. A weak shortwave does
push through the central Appalachians Thursday night into Friday
which might be enough to bring a slight increase in cloudiness to
the northern mountains but for now have left skies mostly clear.
Long term /Friday night through Monday/...
as of 200 PM Tuesday...as we push into the extended...guidance
becomes increasingly at odds with several synoptic features. First
of note is shortwave at the end of the short term that...on the
GFS...cuts off over the central Appalachians...but on the European model (ecmwf)
remains an open wave and pushes out to sea. However even if the GFS
is correct it looks like the shortwave is fairly moisture-starved
and should not be much of an impact...just perhaps an increase
in cloud cover especially across northern zones. Overall expect a
pleasant weekend across the southern Appalachians with temperatures
pretty close to seasonal normals...maybe a couple of degrees below.
Deep trough digging off The Rockies into the plains over
the weekend will be the next feature of note through the
extended. Some significant differences in how this trough evolves
as it moves east. Both GFS and European model (ecmwf) close the low off late in
the weekend...though the GFS is a bit farther south. Moisture
return with this system increases on Monday and both models
bring decent quantitative precipitation forecast amounts to the area. However the European model (ecmwf) begins to
spread precipitation across the area Monday morning and keeps it
on the north side...while the GFS holds off for 18 or so hours and
brings the heaviest rain across our southern zones. Both eventually
develop a Miller-a low off the coast but the GFS is a little more
significant across our area with the more southerly track of the
upper system. Concern would of course be on the northwest side of
the surface low especially in the upper deformation zone but for
now temperatures remain above freezing. However if nothing else
could see another round of heavy rain at some point early next week.
Aviation /20z Tuesday through Sunday/...
at kclt and elsewhere...IFR/LIFR will be the predominate headline
for this taf set as cad regime remains dominant early on...before
erosion gives way to an encroaching cold frontal axis. Guidance
remains very pessimistic with this taf cycle with all sites
prognosticated to remain in the LIFR range until late tomorrow morning.
The only exception will be early on this afternoon at kavl where
The Wedge is less of a factor...however that will soon change
as moisture approaches from the west associated with the front.
The front itself will be very slow to pass through the region
leading to long periods of -ra/rain and possibly +ra at times.
Visibility will suffer as a result with guidance favoring half
to quarter mile or less at times...before the heaviest rain
works to scavenge any dense fog to at least 1-2sm temporarily.
Otherwise expecting ceilings to remain in the 001-004 range until
late Wednesday morning where IFR will work in from the west with
time...therefore all tafs hint at improvement late in the period.
For the most part expecting winds to slowly veer southeast this afternoon
with a few sites already experiencing such. The frontal passage will yield
further northwest veering on Wednesday...however timing is uncertain thus
kept all sites SW through the period with the exception of kavl
where gusty northwest winds will prevail near the end of the taf cycle.
Outlook...widespread rainfall...unsettled conditions and associated
restrictions will continue into Wednesday before a cold front
crosses the area Wednesday night ushering in dry high pressure for
the rest of the week.
19-01z 01-07z 07-13z 13-18z
kclt medium 69% medium 61% medium 68% medium 63%
kgsp medium 64% medium 67% medium 72% low 53%
kavl high 82% medium 66% medium 63% medium 73%
khky medium 71% medium 74% medium 61% medium 75%
kgmu medium 70% medium 66% medium 66% medium 73%
kand medium 63% medium 73% medium 75% medium 71%
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)
a cold front currently sliding through the Tennessee Valley with push into
the western NC high terrain later this afternoon/evening yielding
heightened Hydro concerns over the next 24 hours. Some areas in the
mountains have already received nearly 1 to 2 inches of rainfall
over the last few days. This antecedent rainfall...in addition to
forecast amounts in excess of 2-3 inches over the next 24 hours will
result in very moist soil conditions favoring runoff and thus rising
levels on area streams and rivers. The peak of the heaviest rainfall
is expected tonight into Wednesday morning...before tapering off
into Wednesday afternoon/evening. Wpc continues to promote a slight
risk of excessive rainfall over the mountains for days 1 and 2.
Due to the slower nature of this event...expecting any stream rises
to be gradual...and thus reaching action/flood stages later in the
forecast cycle possibly when precipitation is beginning to wind
down. A Flood Watch remains in effect through Wednesday afternoon.
Georgia...Flood Watch through Wednesday evening for gaz010-017-018.
Dense fog advisory until 7 PM EST this evening for gaz018-026-
NC...Flood Watch through Wednesday evening for ncz033-035-048>053-
SC...Flood Watch through Wednesday evening for scz001>003.
Dense fog advisory until 7 PM EST this evening for scz004>014-