Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
1111 PM CST Wednesday Dec 4 2013
Short term...(this evening through Thursday morning)
issued at 316 PM CST Wednesday Dec 4 2013
Main forecast concern is on departing weather system...followed
by the Arctic air mass surge.
As of 3 PM Wednesday...surface analysis showed 998 mb low
pressure centered near Decorah Iowa...with a warm front situated
from near Prairie Du Chien Wisconsin eastward into southern
Wisconsin. Finally...the leading edge of the cold air was along
the I-35 corridor in Minnesota and Iowa.
The surface low is expected to lift northeast to northern Wisconsin
later this evening...then over Lake Superior by midnight. The cold
front should make a clean sweep across the entire forecast area by
03z. The surge of warmer air ahead of the surface low is keeping
winter precipitation threat low at this time but still plenty of
1/4 sm visibilities or lower. Locations will have to wait for the
cold frontal passage for improved visibilities...and areas east of
the Mississippi River and north of I-90 in Wisconsin will likely
hang onto the reduced visibilities the longest.
Ahead of the cold front...strong low level lift and saturation
combined with very little ice saturation aloft will likely lead to
drizzle. High res models such as the hrrr capture this fairly well.
Behind the cold front...temperatures quickly fall into the upper 20s
to near 30 degrees. Although moisture is trapped in the low-level
inversion...the lift begins to drop off but there is likely to be
a period of freezing drizzle. As even colder air works in
aloft...saturation in the ice growth layer returns so precipitation
could end as snow flurries or snow showers...especially across
central Minnesota...into north central Wisconsin before tapering
off altogether toward midnight. As such...the Winter Weather
Advisory for Taylor/Clark counties will remain in place but likely
could be ended toward midnight.
For Thursday morning...good consensus that plenty of moisture
remains trapped in the low levels...mainly centered around 900 mb.
Lift is very weak but the surge of Arctic air should put the dgz
closer to 900 mb. This supports very light but occasional to periods
of snow flurries across north central Wisconsin...becoming more
scattered closer to Rochester and La Crosse.
Long term...(thursday through wednesday)
issued at 316 PM CST Wednesday Dec 4 2013
Highlights in the long term are Arctic air mass surges and
accumulating snowfall potential Sunday night.
The current storm system deepens and quickly departs northeast of
the Great Lakes region Thursday while the western two-thirds of the
Continental U.S. Remains under the influence of the upper level longwave
trough. As well advertised previously...the Arctic air surge
begins in earnest Thursday and Friday as noted by 850 mb
temperatures falling from an average of -8c 12z Thursday...to -17c
by 18z Friday. 850 mb standardized anomalies fall to 2 to 2.5
degrees below normal...which is not record cold territory but
certainly unseasonable for this time of year. The fresh snowpack
across central Minnesota into north central Wisconsin is likely to
keep temperatures on the lower side of guidance as well. Latest
snow depth readings show anywhere from a few inches from south central
Minnesota...to almost 10 inches in Taylor County. Forecast
temperatures were nudged down slightly by a few degrees from
A noticeably colder day on tap Thursday...with highs only in the
teens to lower 20s. Stiff northwest winds will make for wind chill
readings in the single digits above zero at times for much of the
day. Even colder temperatures are expected Friday and
Saturday...mainly single digits to teens. For reference...normal
highs for this time of year are in the upper 20s to lower 30s and
lows in the teens. As far as low temperatures go...several
mornings of sub zero readings expected for parts of southeast
Minnesota into north central Wisconsin while the rest of the area
should fall into the single digits above zero.
A Wind Chill Advisory is likely needed for parts of the area Friday
night into Saturday as the surface high pressure drops south to our
west. This keeps a 10 to 15 miles per hour wind during the overnight across the
area...resulting in wind chill readings in the 15 to 25 degree below
zero range north of Interstate 90.
For the upcoming weekend...Saturday looks to be another unseasonably
cold but dry day with highs in the single digits to teens.
However...on Sunday with the main upper level trough remaining in
place across the western Continental U.S....another piece of upper level energy
digs into the southwest U.S. Which is expected to allow a surface
low to develop in the Lee side of The Rockies. Previous model
cycles...and the 04.12z guidance continue to show very good
agreement this far out which boosts confidence during this period.
The surface low is expected to be near central Oklahoma 18z
Sunday...then move toward central Illinois by 06z Monday. On the
northwest side...an inverted surface trough coupled with the upper
level energy is likely to provide plenty of lift to support snow
across much of the forecast area. Precipitation chances were boosted
to likely across northeast Iowa into western and southern
Wisconsin...and into the 40 to 50 percent range elsewhere. Still too
early to speculate on snowfall amounts...but impactful weather is
looking likely Sunday afternoon into Monday morning for portions
of the area.
In the wake of the late weekend system...another surge of Arctic air
will usher in even colder air with 850 mb temperatures in the -20 to
-25c range. Aided by a possible fresh snowpack...widespread morning
temperatures in the single digits below zero are likely with teens
below zero possible in spots.
Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1111 PM CST Wednesday Dec 4 2013
Winds have increased Post a cold front...and will stay strong and
gusty from the west/northwest through Thursday evening. Don/T see a
low level wind shear threat at this time.
Ceilings are problematic and confidence is not high in when the MVFR
/to IFR/ cloud deck will climb/clear. BUFKIT soundings and low level
moisture fields continue to keep some saturation under an inversion
through the better part of Thursday. Soundings do suggest some
improvement on Thursday...with some increase in ceiling heights...but likely
staying MVFR. These 1500-2500 kft ceilings could hang around into early
Friday morning. Latest fog/stratus satellite imagery points to
clearing over western Minnesota...but VAD/profiler winds are west to
northwest...suggesting this clearing will stay north of the
krst/klse. Going to stay pessimistic with the MVFR ceilings for now and
hold them through much of Thursday...bringing VFR in for the evening