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Area forecast discussion 
National Weather Service La Crosse WI 
312 PM CST Thursday Jan 8 2009 


Short term...tonight through Sunday night 


Forecast concern deals with snow amounts late tonight through Friday 
night. 


Data analysis this afternoon had high pressure centered over MO with 
ridge axis extending northward through Minnesota. Temperatures were sitting 
in the teens despite full sunshine. Otherwise...water vapor imagery 
showing a vigorous middle-level shortwave trough crossing the Montana/Idaho 
rockies at this time. At the surface...fairly strong cyclogenesis 
taking place over north central Montana...with the warm front extending 
southeast into the Central Plains. Isentropic lift ahead of the wave 
and over the warm front was producing quite a bit of cloud across 
the Dakotas into western Iowa. Radar mosaic showing some light returns 
over central South Dakota. Huron South Dakota had 5sm -sn on the 20z ob. This wave will 
be affecting US later tonight through Friday night. More on this to 
follow. 


Short term models showing reasonable agreement through the 
period...lending toward higher forecast confidence. 


The low over north central Montana will propagate into the Central Plains 
tonight with arm of warm air advection/isentropic lift overspreading 
the area from west to east. Taking a look at the model 
soundings/285k isentropic surface...appears enough forcing will 
overwhelm the column to saturation/snow soon after midnight along 
and west of the Mississippi and then northeast of the I-90/94 
corridor after 3 am. As the low swings through central Illinois Friday 
night...the deformation axis snow will persist mainly south of the 
I-94 corridor. High pressure ridge stretching from the northern 
plains into northern Minnesota/far northern WI may provide enough 
subsidence/dry air for a sharp cutoff of the snowfall. Right 
now...it appears that this cutoff will set up across northern Clark 
and Taylor counties. Was also initially concerned with a wintry mix 
across the far south tonight. But now...even the warmest of the models/GFS 
now has this threat remaining south of the area. So...at this 
point...it appears this will be a long-lived -sn event with around 6 
inches possible from tonight through Friday night...mainly along the 
I-90 corridor. Lesser amounts can be expected either side of this 
corridor/axis. As a result...have issued a Winter Weather Advisory 
for late tonight through Friday night for all counties with the 
exception of Taylor County where that aforementioned cutoff of 
snowfall should set up. 


High pressure builds in from the northern plains into the Midwest on 
Saturday for a quiet but chilly day. Highs anticipated to top off 
only in the teens across the area as 925mb air hovers in the -10 to 
-15c range. 


A weak middle-level shortwave trough will drop from Canada into the 
upper Midwest Sunday/Sunday night for a chance of -sn. Forcing not 
overly impressive with this wave...so very light snow expected at 
this time with just a dusting of accumulation. 


Long term...Monday through Thursday 


Latest medium range models remain in good agreement with regards to 
active polar vortex and intrusions of Arctic air into the region. A 
couple middle-level shortwave trough/weak surface low pressure will 
propagate from the northern plains into the upper Midwest/Great 
Lakes region Sunday night through Monday night. These will give the 
area a chance of -sn along with plummeting temperatures as Arctic 
air filters in behind the system. Another system coming down in the 
northwest flow aloft will affect the area with another chance of -sn 
Tuesday night. A reinforcing shot of Arctic air filters in again 
behind this system. European model (ecmwf)/GFS indicating bitterly cold 925mb air 
hovering around -25c Tuesday morning and again Thursday morning. 
This should produce overnight lows in the single digits to tees 
below zero on those morning with daytime highs Tuesday/Wed/Thu 
struggling to make it into the single digits. Another concern will 
be wind chills Tuesday morning in stronger pressure gradient on th 
backside of the low departing into the Great Lakes region. Wind 
chills late Monday night through Tuesday morning could be in the -25 
to -35 range. Will be highlighting the upcoming cold outbreak in the 
extended portion of the severe weather potential statement and include a weather story on our arx 
home Page. 


&& 


Aviation... 


Latest surface analysis indicate elongated surface ridge extending 
from northern Missouri into southern Minnesota. This is providing 
VFR conditions at both rst and lse taf sites and continue into this 
evening. Then...the main concern is next shortwave trough over 
eastern Montana to affect both taf sites after 05z Friday. Latest 
model trends indicating band of snow will move over both rst and lse 
taf sites after 08z. Forecast soundings from both the 08.12z NAM and 
GFS indicates low level atmosphere moistening up after 08z. Have 
introduced IFR conditions at both sites. Best lift associated with 
shortwave trough is expected after 14z Friday at both sites. 
Visibilities of less than one mile is possible at rst and lse taf 
sites after 15z. Due to the inconsistencies where the heaviest band 
of snow is to set up across the area...for now have kept IFR 
conditions through the rest of the taf period. 


&& 


Arx watches/warnings/advisories... 
WI...Winter Weather Advisory from midnight tonight to 6 am CST 
Saturday for wiz032-wiz033-wiz041-wiz053-wiz054-wiz055- 
wiz061. 


Winter Weather Advisory from 3 am Friday to 6 am CST Saturday 
for wiz029-wiz034-wiz042-wiz043-wiz044. 


Minnesota...Winter Weather Advisory from midnight tonight to 6 am CST 
Saturday for mnz079-mnz086-mnz087-mnz088-mnz094-mnz095- 
mnz096. 


Iowa...Winter Weather Advisory from midnight tonight to 6 am CST 
Saturday for iaz008-iaz009-iaz010-iaz011-iaz018-iaz019- 
iaz029-iaz030. 


&& 


$$ 


Short/long term...das 
aviation..........dtj 

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