Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
205 PM CST Friday Dec 13 2013
Short term...(tonight through Saturday night)
issued at 205 PM CST Friday Dec 13 2013
Water vapor loop/latest RUC 500mb analysis shows a broad long wave
trough encompassing the entire Continental U.S.. within this long wave trough
there are two middle-level shortwave troughs that will eventually
impact the area tonight through Saturday evening. Trough number one
was moving east out of Montana/Wyoming toward the northern plains while a more
potent trough number two was moving east out of Colorado/nm into the
Southern Plains. At the surface...this translated to an area of
cyclogenesis over western Kansas with a warm front extending along the
Iowa/MO border into central Illinois. Currently...our local forecast area
was in between the influences of the Southern Plains low and high
pressure over southern Manitoba Canada with east winds and mostly
cloudy conditions resulting. Temperatures this afternoon ranged from
the teens across north central WI...to the lower and middle 20s
Will be watching the Southern Plains and northern plains troughs
converge on the area late tonight into Saturday. Not particularly
strong dynamics associated with these waves. However...the area will
be in long-lived right entrance region of 300mb jet while there is
some weak isentropic upglide noted on the NAM 275k surface.
Also...there is a decent passing lobe of 700-300 pv-advection from
about 12z-18z Saturday. All this is expected to come together to
produce a fairly widespread area of light snow with the peak
centered from about 09z-15z...then tapering off in the
afternoon/Saturday evening as the trough axis passes and subsidence
takes over. Some very light snow/flurries will be possible through
midnight Saturday night before full subsidence kicks in...drying
things out. Appears at this time there is the potential for 1/2 up
to 2 inches of snowfall out of this from tonight through Saturday
evening. Temperature-wise... look for lows tonight in the single
digits and teens...highs on Saturday in the upper teens to middle
20s...and lows Saturday night dropping into the single digits above
and below zero.
Long term...(sunday through friday)
issued at 205 PM CST Friday Dec 13 2013
Sunday looks dry at this time but clouds will be thickening once
again in advance of the next middle-level shortwave to impact the area.
For Sunday night into Monday...warm air advection Wing of this wave
moves in with a good chance for light snow to overspread the area.
Will have to watch closely as model BUFKIT soundings hint at the
potential for some loss of ice in the column. This could lead to
some freezing drizzle issues. Kept precipitation as all snow for now
after coordination/collaboration with other offices...but will
remain Leary and keep a close eye on later model runs for freezing
drizzle potential. Otherwise...snowfall looks to be very light with
less than an inch expected.
Tuesday through Wednesday look to be dry for now as northwest flow
aloft persists across the eastern half of the Continental U.S..
European model (ecmwf)/GFS coming into better agreement with the low pressure system
affecting the area on Thursday. Appears there will be enough warm
air drawn in with this system to produce a rain/snow mix for
portions of the area as temperatures warm into the Lower/Middle 30s
along and south of I-90. Look for the precipitation to Switch Back
to all snow as cold air sweeps in behind the departing low Thursday
There will be a lingering chance for light snow on Friday as a
potent/cold middle-level trough moves toward the upper Mississippi
River from the northern plains. Otherwise...much colder temperatures
look to invade the area with highs dropping back into the teens/20s.
Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1222 PM CST Friday Dec 13 2013
Lower confidence taf forecast this period. Big picture belief is
that there will be some snow at the taf sites for a period of time
from 06-18z...but the question remains of how low the ceilings/visibilities
will get with this system moving through.
Currently an MVFR cloud deck resides just north of the taf sites and
this is slowly moving southwest toward the airfields and will arrive
as the afternoon progresses. This is a stable cloud field and lower
conditions than MVFR are not expected. At 18z...the leading edge to
the clouds were along a kcmy-Kona-ksyn line.
Two systems will affect the area overnight and Saturday...one coming
from the northwest and another moving by to the south. The northwest system should
bring some light snow into the tafs by late tonight however the low-
level lift is somewhat weak...while the middle-upper levels are a bit
more favorable. While the MVFR cloud deck will be in place...just
how much saturation and lift will occur determines the visibility
reduction in the tafs. Have provided a more conservative taf with
IFR visibilities for now. However...customers should monitor later
forecasts if flying Saturday morning. Improving trends are expected
for Saturday afternoon as the systems shift east of the area.