864 
fxus63 karx 072053 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
252 PM CST Wednesday Jan 7 2009 


Short term...tonight through Saturday 


Fairly active weather to close out the week as mean upper trough 
over the Great Lakes region is replaced by even a larger one by 
early next week. Main concern in short term is precipitation threat 
from next system that will start to impact area on Thursday night. 


First short wave trough exiting area this afternoon with some 
lingering light snow around. Accumulations have been under a half 
inch but snow should come to abrupt end by early evening. May have 
to linger minor snow threat a few hours but otherwise look for 
colder temperatures and quiet weather into Thursday as ridge builds 
in. Have lowered temperatures a bit based on trends upstream. 


Impulse approaching northwest U.S. Will be next concern. By Thursday 
night this short wave energy is expected to cross the northern 
rockies. In response warm air advection and isentropic lift start 
increasing ahead of wave and will likely start to saturate airmass 
west of forecast area. Layer frontogenesis also noted 600 mb and 
above from central Minnesota into western Wisconsin by 12z Friday so 
will likely see snow break out in earnest and hence have raised snow 
threat considerably. Q-g convergence noted not only in a deep layer 
but fairly long lived area well into Friday afternoon also supports 
categorical probability of precipitation. 


There remains some uncertainty how event will unfold given run to 
run guidance and surface low track differences. Perhaps biggest 
change from earlier forecasts is trend for frontogenesis a bit 
further south...setting up a deformation area that extends across 
southeast Minnesota into central Wisconsin north of the surface low 
across Iowa. 07.12z NAM had been the furthest north with this 
solution...more in agreement with 07.09z sref members. Other 07.12z 
guidance had layed this out more directly across forecast area and 
now 07.18z NAM is headed that way. This starts to gain some 
confidence in a more south solution...which also agrees better with 
HPC winter forecasts. With higher probability of precipitation now Thursday night and Friday... 
have also increased snow amounts which could certainly be into 
advisory criteria. In fact...depending on future model runs and 
location of deformation axis may be looking at some higher amounts 
which will have to be watched. 


Further south solution also plays into precipitation type forecast 
concerns. Earlier thinking was we could see some mixed precipitation 
in region as warm layer from 750 to 850 mb nosed up from the south. 
Perhaps colder air moving in behind current short wave trough is 
lending toward colder solution for late week but trends have been 
colder and colder. Closely examination of wet bulb temperatures... 
BUFKIT soundings and Cobb output suggest far less mixed 
precipitation so have also trended forecast that direction. Will 
leave small threat for sleet or freezing drizzle in northeast Iowa 
and southwest Wisconsin given proximty to warm layer and location of 
surface low. A few 07.09z sref members still hint at possible mixed 
precipitation to the south as well. Depending on track could see 
less high cloud ice and shot at drizzle near surface low with large 
scale lift saturating layer as well. 


Deformation axis location could mean lingering snow into Friday 
evening...especially in Wisconsin so have adjusted probability of precipitation that 
direction as well. Otherwise short wave should exit and snow should 
end to start weekend. Weak impulses in northwest flow could bring 
some snow showers but generally Saturday looks quiet. 


Long term...Sunday through Wednesday 


Later forecast periods will be dominated by northwest flow. Mean 
upper ridge expected to build over western states setting up 
stronger and stronger plunge of Arctic air into Great Lakes region. 


Strengthening short wave trough in northwest flow will drop into the 
Midwest Sunday into Monday but definite timing differences in medium 
range guidance. 07.12z GFS brings system across area while European model (ecmwf) 
tends to split the energy. None the less small probability of precipitation seem reasonable 
during that time frame with potential for another light snow event. 
West Coast ridge really builds after this with strong and fast 
moving short wave dropping out of central Canada as Arctic air 
plummets south by Monday night. 


Story by Tuesday will be very cold temperatures. Forecast database 
has been adjusted downward by some 10 to 15 degrees and that may 
still be too warm. Too early to tell if this will be more of a 
glancing blow or not but confidence is growing with these colder and 
colder solutions. Weak impulses dropping around polar vortex could 
also spawn flurries from time to time middle week in brunt of cold air 
but too noisy at this point for details. 


&& 


Aviation...tonight and Thursday 


Cyclonic flow will continue to be found across the upper Mississippi 
River valley through at least middle evening...and then a ridge of high 
pressure will build across the region. As a result...flurries and 
MVFR ceilings /1500 to 2500 feet/ should last through 08.03z and 
then skies should clear. Looking upstream...much of this area is 
generally void of clouds. VFR conditions will be seen through 
Thursday and then ceilings will rapidly diminish on Thursday evening 
as strong warm advection and frontogenesis develops across the 
region. There may be even some snow after 09.03z across southeast 
Minnesota and northeast Iowa. 


&& 


Arx watches/warnings/advisories... 
WI...none. 
Minnesota...none. 
Iowa...none. 
&& 


$$ 


Short/long term... Shea 
aviation.......... boyne