Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1229 am EST Sat Dec 20 2014
a plume of moisture will rotate across the region tonight into
Saturday with dry high pressure returning on Sunday. An area of low
pressure will bring rain chances back by Monday.
Near term /through today/...
0515 UTC update...winds were updated from the NAM and adjmav. Sky
cover was increased per satellite imagery.
915 PM update...very light precipitation is occurring over al/GA...where
radar mosaic indicates elevated stratiform returns. Upper divergence
appears to be the main driver for this activity. Said divergence
spreads over our western zones a little after midnight. Revised probability of precipitation to
match the expected onset. 00z NAM not looking as wedgy tmrw
morning...evidently a result of less quantitative precipitation forecast. Adjusted temperatures through daybreak
per latest observation and short term consensus product. Observation trends indicate
the high cloudiness is doing perhaps an even better job of limiting
cooling than expected and the new min temperatures are slightly warmer...in
line with expectation for a weaker wedge. This reduced the coverage
of wintry weather even more.
630 PM update...18z NAM/GFS seem to have trended a bit more in favor
of cad development vs their counterparts from 12z. This is
particularly true of the NAM. Ptype is still a tricky forecast. I do
feel fairly confident that over the mountains/foothills the best forcing
from the shortwave will depart before the low level warm advection
creates enough of a warm nose to cause a changeover to freezing rain or
sleet. I did regenerate the weather types based on the 18z NAM and on
slightly colder surface temperatures...and as one might expect a bit more
wintry precipitation resulted. However model quantitative precipitation forecast is still light and temperatures
do warm enough through the morning to change most areas back over to rain
fairly quickly. Snow accums remain under 1 inch...with just a few
hundredths of ice at most on ridgetops. We will continue to let the
Special Weather Statement speak for the travel impacts.
As of 230 PM...a fast-moving middle level shortwave will track across the
Tennessee Valley tonight...then cross the County warning forecast area early Saturday within quasi-
zonal flow aloft. Meanwhile...a 1025-1028 mb high pressure system will
remain across the Great Lakes to the St Lawrence Valley. A weak surface wave
is expected to ripple across the Gulf of Mexico...with a brief shot of
deeper layer relative humidity crossing the area just ahead of the middle level trough.
The result is an increase in clouds through this evening...with some
light precipitation developing overnight through Saturday morning. Models
seems to have settled on a very light quantitative precipitation forecast event...with generally
just a few hundredths to a tenth of an inch in the mountains and lower
Piedmont...and a trace to few hundredths elsewhere. The main
challenge still looks to be p-type in the mountains and adjacent NC
foothills...where temperatures will be supportive of wintry mix. Forecast soundings
show moistening profiles from the top down...with wetbulb effects
creating mostly a rain/snow event. However...with weak Omega and
marginal ice nuclei activation...there may be spotty freezing rain
and sleet mixed in as well...especially at onset of precipitation...probably after
midnight. Using a 50/50 blend of NAM/GFS with the top-down
approach...I do get a mixed bag of precipitation during the wee morning
hours...but then transitioning to mostly snow by daybreak. A light
coating of ice/sleet/snow will be possible...especially above 3500 feet from
Haywood County northeast. This isn/T enough to warrant a Winter Weather
Advisory...so we will highlight wintry mix and slick spots on
roadways in a Special Weather Statement (sps). Outside the
mountains...mainly just sprinkles to a few hundredths of light rain
expected...primarily across NE Georgia and the upstate.
Saturday...as the middle level wave exits...taking the moisture and lift
with it...probability of precipitation quickly taper off. Forecast soundings show the surface layer
warming...resulting in any lingering light precipitation changing back to
rain. Cold air damming looks to be weak or event more of a
"look-alike" event...as precipitation just too light to hold a wedge
in...given parent high weak and well to the NE of the favored area.
Temperatures will be tricky...but expecting plenty of cloud cover...so I
still undercut MOS by a couple categories...generally middle-upper 30s
above 3500 feet...and middle-upper 40s below 3500 feet...including Piedmont.
If clouds scatter out...then temperatures could bust by a couple categories.
Short term /tonight through Monday/...
as of 230 PM...Sunday should begin dry as short wave ridging builds over
the southern appalachian region. However...short range guidance
indicates that 290k isentropic lift will begin to increase from the
south Sunday afternoon. The combination of increasing
moisture...southeast low level flow...and increasing upglide should
result scattered light rainfall by late afternoon across the region. The
building cloud cover and scattered rain should limit heating...with highs
ranging from the u40s to low 50s.
Sunday night...cold air damming appears to increase across the
foothills and Piedmont. Short range models indicate increasing NE
surface winds with 4 mb NE-SW pressure gradient. Isentropic lift is
expected to peak east of the mountains around dawn on Monday. The period
from 6z-15z should see the greatest rainfall rates...generally
around 0.25/3 hours. Sunday night temperatures are forecast to remain
in the u30s to l40s. However...areas across the northern NC mountains could
dip to freezing during the pre dawn hours. Spotty freezing rain is possible
across portions of the northern mountains...little to no ice accumulations.
For Monday...cold air damming will rule the weather. Shallow
isentropic lift should continue through the day...supporting light
rainfall through the daylight hours. Steady NE winds and thick
clouds will limit daytime heating. I will forecast highs rising over
morning lows by 5 degrees or less. This approach will place forecast
highs between the u30s to l40s...perfectly placed cooler than warm
bias guidance during cad.
Long term /Monday night through Friday/...
as of 220 PM Friday...the transition to a highly amplified...yet
somewhat progressive upper air pattern will be complete by the
beginning of the medium range period...with a deep cyclone/upper
trough expected to translate across the eastern Continental U.S. Early in the
period. Light rain will be possible Monday night through Tuesday in
weak/moist isentropic/upslope lift regime. Rain chances will then
increase substantially Tuesday night/early Wednesday...as cold front and
potent short wave trough approach the southeast. A consensus of
global model solutions would see surface low pressure pass west of
the Appalachians. Thus...p-type should be all-liquid. There is a low
chance for thunder across the Piedmont Wednesday morning...although if
anything the latest guidance suggests less of a potential for this.
Rapidly dropping snow levels may result in a transition to a
northwest flow snow event across the mountains on Wednesday.
However...a strong speed maximum upstream of the upper trough axis will
keep low level flow more westerly than northwest through much of Wednesday.
Meanwhile...moisture will also retreat during this time...as the
upper trough axis progresses rather quickly off the East Coast.
Therefore...any northwest flow snow event would be quite low-impact.
The remainder of the medium range will see a return to a less
amplified pattern across the Continental U.S....spelling moderating temperatures by
day 7...after several days of cool weather in cloudy/periodic in-
situ cold air damming/low upper heights regime.
Aviation /06z Saturday through Wednesday/...
at kclt...ceiling will lower to low VFR overnight as low level moisture
and precipitation arrive from the SW. Guidance takes the ceiling down to
MVFR in the afternoon...leaving it there until clouds and erode and
precipitation ends late Saturday evening in the wake of a departing
coastal low. A guidance blend supports MVFR visibility Saturday evening.
NE winds will persist.
Elsewhere...low VFR ceilings and light precipitation are expected before
dawn as low level moisture overspreads the area...with snow and
freezing rain possible at kavl a few hours either side of
daybreak...and snow at khky around the same time. By morning
guidance takes the kavl ceiling down to MVFR...with SC sites following
by midday. Guidance suggests a brief low IFR ceiling at kavl early
Friday evening. Ceilings erode in the foothills Saturday evening as
clouds erode behind a departing coastal low...while kavl improves to
Outlook...lingering low level moisture may keep some fog and low
stratus around Saturday night through Sunday. A wetter system will then
track into the area for Monday likely bringing widespread clouds and
precipitation restrictions. Yet another storm system is expected to cross
the area late Tuesday or early Wednesday...with another round of
05-11z 11-17z 17-23z 23-00z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 100% high 100% high 100%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 100% 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)