Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
255 am EST Sat Dec 21 2013
high pressure will move off the Carolina coast today as a cold front
approaches from the Mississippi Valley. Expect this front to cross
the mountains Sunday...then the foothills and Piedmont Sunday night
and Monday. Cold Canadian high pressure will build in from the
northwest through middle week.
Near term /through tonight/...
at 230 am EST Saturday...an upper trough will progress from The
Rockies to the plains today and tonight...while a ridge downstream
amplifies and moves off the East Coast. A shortwave exiting the
trough axis will reach the southern Appalachians by Sunday morning.
A slow moving cold front will move from the MS River Valley today to
the central and southern Appalachians by early Sunday morning. Moist
Gulf inflow ahead of this boundary will move upslope into the south
facing Blue Ridge Escarpment today...supporting increasing probability of precipitation.
Precipitation will spread east across the area as isentropic upglide
sets up. The greatest quantitative precipitation forecast will be along the Blue Ridge where NC
borders SC and Georgia tonight. Modest instability will offset increasing
shear to limit storm organization. The best overlap of severe
weather parameters will be in SC...GA...and SW NC. High temperatures
today will run over 5 degrees above normal in warm advection under
the departing upper ridge. Minimum temperatures tonight will run
over 25 degrees above normal as warm advection continues under
thickening low cloud cover.
Short term /Sunday through Monday night/...
as of 240 am EST Saturday....the models continue to bring a band of
prefrontal precipitation into the mountains Sunday morning. While
the band should start out as a line of severe storms this afternoon
and tonight...but the time it enters the mountains it/S expected to
be mainly moderate to heavy rain with embedded showery elements. The
GFS generates over 2 inches of rain over the southwest and central
NC mountains and the mountains of NE Georgia and western SC by the the afternoon hours
on Sunday. This is certainly possible...and is something we will
need to keep a close eye on. These areas take quite a bit of rain to
flood...and the thinking now is that this will not be sufficient to
cause flood or flash flood problems.
During the course of the day on Sunday the band of precipitation should
should move out across the foothills and Piedmont. As the first
short wave lifts into southern Canada...and the next wave rounds the
base of the long wave over the Southern Plains...the synoptic scale
forcing will weaken some during the afternoon hours. The low level wind
field will also weaken...as the strongest kinematics lift north of
the region. The boundary parallel flow...weak lapse rates and generally
saturated atmosphere Don/T look particularly conducive to severe
weather. However...the shear will still be sufficient to support a
few severe storms...especially where there is any amount of heating
ahead of the line. Most likely this will include the western Piedmont of
NC and SC during the middle to late afternoon hours. Maximum temperatures will run
around 20 degrees or more above normal ahead of the line.
By Sunday evening the next wave will start to eject. This will
rapidly strengthen the 250 mb jet over the Ohio Valley and Upper Middle
west. Low level ageostrophic convergence will strengthen over the western
Carolinas and NE Georgia as this happens...likely resulting in a band of
heavy rain somewhere along or near the I-85 corridor. Precipitable water/S during
this time are projected on the gefs to be around 3 Standard
deviations above normal. Lapse rates also steepen just a little
during the evening hours with the NAM BUFKIT soundings showing
slightly elevated cape values increasing to over 500 k/kg across the
Piedmont. It/S hard to say exactly where the heaviest rain will
fall as the forcing will extend all the way from SW Georgia to southeast Virginia.
However...a couple of inches of rain is quite possible late Sunday
and Sunday night...with locally higher amounts. This could easily
result in urban and some small stream flooding. I/ll play this up in
the severe weather potential statement...though it/S a little too early for flood or flash watches
at this time.
Yet another wave could bring a resurgence of the precipitation over the
foothill and Piedmont zones during the day on Monday. However...the
highest precipitable water/S should be east of the forecast area by this time...resulting in
lower quantitative precipitation forecast amounts. Finally...developing northwest flow may bring snow
showers to the northern and central NC mountains Monday night...especially
along the Tennessee line. With 800 mb temperatures falling quickly to -10 to 12 degree c
and the very moist upstream ground conditions...we could see some
rather vigorous snow showers overnight.
Long term /Tuesday through Friday/...
as of 130 am EST Saturday...according to the GFS we should be either
dry or nearly dry through the entire medium range. The European model (ecmwf) and
Canadian are not as optimistic. At the start of the day 4 at 12z
Tuesday...high pressure centered near St Louis and some light northwest
flow snow from the virginias to the Great Lakes. Quick drying takes
place and around 12z Wednesday an elongated zone of high pressure
will extend along The Spine of the Appalachians from the Carolinas
to New England. On the GFS...this high pressure lingers over the
Carolinas into Friday and is then replaced by another cold high from
Alberta area of Canada into the weekend. If any precipitation occurs on the
GFS it would be late in the weekend if at all. The 12z Friday European model (ecmwf)
brings a closed off 500mb low across the Great Lakes with a deep
trough on Thursday as surface low pressure develop over or near
North Florida. These lows on the European model (ecmwf) move NE just off the Atlantic
coast Friday with drying cold high pressure coming toward our area
from the plains. The Canadian is like previous GFS runs in
developing a hefty low over the north Gulf on Thursday and moving it
NE toward our area. Temperatures will be below normal through the
entire medium range with the coldest day on Wednesday with lows in
the 20s and highs in the 40s which is 6 to 8 degrees below normal.
Aviation /08z Saturday through Wednesday/...
at kclt...a blend of model guidance supports MVFR visibility by
daybreak...and at least an MVFR ceiling in the morning. Guidance hints
at a brief IFR ceiling..but confidence is too low to carry on at this
time. The ceiling improves to low VFR by afternoon. Then guidance
suggest a low IFR ceiling late this evening. With such a dire collapse
of the ceiling so late in the forecast...it will not be carried at this
time. Light SW winds will prevail overnight...increasing and
becoming gusty during the day...staying up through the evening as
the pressure gradient tightens ahead of a front. MVFR visibility will
accompany showers in the evening.
Elsewhere...guidance suggest low VFR ceilings decreasing to MVFR after
dawn...with perhaps some IFR in SC. The IFR scenario is not favored
at this time due to low confidence. Ceilings return to low VFR at NC
sites this afternoon. Guidance supports a rapid collapse through
MVFR to IFR in the foothills this evening...but with confidence
low... MVFR will be favored. At kavl and MVFR ceiling is expected in the
evening. Light SW winds will increase and become gusty today ...
will stay up tonight in a tight pressure gradient ahead of a front.
Increasing coverage of showers will support MVFR visibility this evening.
Low level wind shear is expected this evening at kavl and kand as
low level flow strengthens.
Outlook...moisture will continue to increase through the
weekend...with restrictions likely from Sat night through Sunday in
clouds and precipitation. A cold front will pass through Sun night into
Monday...so conditions will improve late Monday through middle week.
08-14z 14-20z 20-02z 02-08z
kclt high 95% high 86% high 98% high 83%
kgsp high 87% high 98% high 94% high 87%
kavl high 84% high 80% medium 78% high 81%
khky high 91% high 84% medium 71% high 82%
kgmu high 80% high 94% high 96% high 85%
kand high 82% high 94% high 88% 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)
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