Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1237 am EST sun Dec 22 2013
a cold front will move slowly across the region Sunday into Monday.
Cold Canadian high pressure will then build in from the northwest
Near term /through today/...
0530 UTC update...winds were updated with a blend of the latest NAM
and adjmav...favoring the former heavily to capture higher winds
speeds on Mountain Ridge tops. Probability of precipitation were adjusted per radar
trends...and quantitative precipitation forecast adjusted accordingly.
As of 950 PM...recent satellite images indicated that partial clear
has occurred across the Piedmont. It appears that cloud cover will
increase over the next couple of hours. I will update the forecast
to adjust cloud cover and timing. In addition...surface observation across the
Piedmont indicated that south winds have developed gusts in the
teens to middle 20s. As a cold front approaches from the
west...pressure gradient across the western Carolinas should
increase...maintaining gusty winds. The wind gust potential will be
the greatest across the high terrain...highlighted with a Wind
Advisory until noon Sunday. Otherwise...the pop and temperatures
appears in good overall shape.
As of 710 PM...radar continued to indicate light rain showers to drizzle
across the mountains and areas along and west of I-26. I will update probability of precipitation
to reflect recent radar trends. South winds continue to support very
mild temperatures across the region...with Greenwood...to
Laurens...to Chester observing temperatures around 70 degrees. I
will align the temperature forecast to observations...with most of the
region forecast to remain in the 60s overnight.
As of 245 PM...vigorous negatively-tilted shortwave can be seen on
water vapor loop pivoting NE through Texas this afternoon...with impressive
baroclinic Leaf signature extending from Mexico to the Canadian
Maritimes. The amplified pattern has allowed an anomalously warm and
moist air mass to spread in across the region...with middle-upper 70 degree
temperatures across the lower Piedmont. Models generally agree on moist upglide
and southwesterly upslope showers to increase in coverage this evening across the
western third of the County warning forecast area...while a large band of showers and thunderstorms
will track slowly eastward across the Ohio/Tennessee valleys overnight. The
consensus is for a strong to severe line of convection to be
knocking on the door of the Smoky Mountains by daybreak Sunday.
However...despite a 50-60 knots 850 mb jet...instability will be fairly
weak. So still expecting the line to weaken as it enters and crosses
the NC mountains Sunday morning (spc day 1 outlook has eastern edge of slight
risk just to our west). Otherwise...it will be a record to near
record warm overnight..with temperatures only dropping into the middle 50s to
lower 60s (above normal highs).
On Sunday...decent model agreement on a slow-moving band of precipitation
associated with a stalling cold front. Convection will likely lose
its severe potential late morning to midday...but precipitable waters of 1.5 to
1.75" (300% of normal)...with strong low level convergence and southwesterly
upslope should result in moderate-heavy rain rates. Much of the SW-facing
slopes of the SW and central NC mountains could easily see 2-3" of rain
by sunset Sunday evening. Another concern with the frontal band is
that the low level jet will still be 50+ kts. Therefore a damaging wind threat
will likely exist...even with weakening low-topped convection. Given
strong gradient winds overnight and the wind threat once the
convection arrives...will issue a Wind Advisory for the Tennessee border
counties (and the balsams in jackson) for elev above 3500 feet for
tonight through Sunday morning. Granted most strong gusts will be
localized and at elevations above 4500 feet or so.
As the afternoon wears on...guidance does show some instability across the
Piedmont ahead of the convective line...especially along I-85 and southeast. Bulk
shear of 45-60 kts with convective available potential energy of 400-600 j/kg may be enough for the
line to attain a quasi-linear convective system (qlcs). We are still
in a 5% see text for Sunday...and will continue to highlight in severe weather potential statement.
Also...decent rain rates will continue into the Piedmont...while
tapering off somewhat across the mountains temperatures will be tricky...especially
southeast of I-85 and along the I-77 corridor...where even a Little
Break in the low stratus could allow temperatures to spike higher than
current forecast. Even with the expected clouds and precipitation...kclt may
break the record high again...getting into the low 70s.
Short term /tonight through Tuesday/...
as of 230 PM EST Saturday...the short range continues to look very
busy. A broad swath of moderate to occasionally heavy showers will
be ongoing at 00z Monday across the forecast area. Lingering weak
instability over the Piedmont of the Carolinas suggests that thunder
will remain a possibility through the evening hours. The main
problem with the forecast is the eastward progress of the cold
front...which will bisect the forecast area at the start of the short
range period. It seems reasonable that the front will make little
progress through Sunday night as the upper trough axis remains stuck
over the plains with another short wave diving down into its base.
This will keep a few waves of low pressure moving along the front
which will hold the precipitation over the NC mountains through most of the
night...so the pop was increased there. Rainfall amounts are a
concern. The consensus is that the NAM is overdone with its heavier
rain axis over the mountains after 00z Monday...so the forecast is more in
line with the GFS and wpc with heavier amounts east of the Blue Ridge.
After considering antecedent conditions...qpf...and uncertainty with
the location of the heavier rain axis...prefer to wait one more
cycle before issuing a watch.
The trailing short wave will lift out of the upper trough on Monday
which will finally give the front an eastward kick. Precipitation chances
gradually shift eastward through the day Monday leaving behind some
higher pop in the usual areas on the Tennessee border as the wind comes
around to the northwest and cold advection ensues. A high chance pop was
kept through the night on the Tennessee border for northwest flow upslope snow
showers. The model quantitative precipitation forecast is low or non-existent...so for now the
potential only looks like it would be about an inch. Kept a chance
going after sunrise on Tuesday given the model presentation of
lingering 850 mb moisture. The snow showers should taper off Tuesday
morning as the 850mb moisture finally dries up. The rest of the day
on Tuesday looks seasonally cool.
Long term /Tuesday night through Saturday/...
as of 1201 PM EST Saturday...under developing zonal flow aloft and
low level ridging...will plan on continuing the dry but chilly forecast for
Christmas evening and day with temperatures a couple of categories below
climatology. The upper pattern is still prognosticated to amplify through
Thursday along with the approach of a weakly forced and moisture
starved cold front. The 21/12 UTC op GFS continues to limit response
with the passage of this feature and will plan on continuing with the
going dry and slightly milder forecast. Will bank on aforementioned surface
front translating to the coast by Friday with simply dry and
seasonably cool conditions in it/S wake atop the County warning forecast area into next
Aviation /06z Sunday through Thursday/...
at kclt and elsewhere...a cold front will approach from the
west...and move slowly across the area today and tonight. Model
guidance supports widespread restrictive ceilings and visibility as moisture
moves in ahead of the front. Showers will increase in coverage...
with IFR and possibly LIFR ceilings around daybreak. Bases may lift to
MVFR by midday...before falling to IFR in the evening. An
increasing pressure gradient will support gusts from the south.
The heaviest rain showers/thunderstorms and rain are expected Sunday afternoon.
Outlook...the front will sag southeast of the area on Monday...so
conditions will improve late Monday through middle week.
05-11z 11-17z 17-23z 23-05z
kclt high 98% high 84% high 100% high 90%
kgsp high 100% high 84% high 83% medium 77%
kavl high 94% medium 74% high 87% high 87%
khky high 100% high 84% high 81% medium 65%
kgmu high 93% medium 78% medium 74% medium 71%
kand high 92% medium 79% high 80% medium 67%
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)
record high temperatures for today Dec 21st...
Avl...68 in 1970
clt...71 in 1998
gsp...70 in 1956
Record high temperatures for Sunday Dec 22nd...
Avl...71 in 1970
clt...72 in 1889
gsp...70 in 1970
Record high minimum temperatures for Sunday Dec 22nd...
Avl...56 in 1923
clt...60 in 2011
gsp...61 in 2011
NC...Wind Advisory until noon EST today for ncz033-048>052-058-059.