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fxus63 karx 041017 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
417 am CST Thursday Dec 4 2008 


Short term...today through Sunday 


Main concerns for this forecast package include snow chances Friday 
night/Saturday morning as well as temperatures throughout the 
period. 


09z water vapor imagery indicating several weak short wave troughs 
passing through the middle-level air flow into the upper Mississippi 
River valley region. Snow showers moved across southern Minnesota 
ahead of this energy through the overnight hours. 


Generally good agreement between the models through this time 
frame...especially as newest NAM runs latching on to cold 
frontal snow Friday night. 


Main short wave energy will pass across the forecast area this 
morning...which should weaken the ongoing area of snow as it moves 
east. Middle and upper levels of the atmosphere will be fairly dry 
through the day...however the cold pool will be overhead with 850mb 
temperatures at -15 to -20c according to 00z radiosonde observations...bringing 1000- 
850mb lapse rates to between 7 and 9 c/km. So believe scattered 
instability snow showers will develop through the afternoon hours. 


Next system to affect the forecast area moves in Friday night. With 
good agreement on this system between the 04.00z and 04.06z models... 
continued the trend of rising snow chances for the Friday 
night/Saturday morning time frame. BUFKIT soundings from the NAM 
indicating depths of 5-7k feet of saturation in the dendritic layer 
overnight and Cobb data from both GFS and NAM runs suggesting average 
ratios of 15:1 across the forecast area. So although higher ratio 
snows may occur...the system will move through the area fairly 
quickly...therefore snow accumulations should be in the range of 1-3 
inches. 


Temperatures will remain below normal through the period. Several 
rounds of cold air will push into the area about every 2 days or 
so...as short wave troughs quickly move through the northwest flow 
into the region. There will be times of moderate warm air advection 
between systems...but this will be brief...therefore not allowing 
much for moderation of temperatures. 


Long term...Sunday night through Thursday 


Main concern during this time frame deals with snow chances in 
Monday/Tuesday time frame in accordance with cold frontal passage. 
Models have been consistently producing precipitation...however do 
have differences in how things will play out. Basis on these 
differences depend on the strength of the trough digging into the 
central portion of the Continental U.S. As the 04.00z GFS is more amplified 
than 03.12z European model (ecmwf)/Canadian runs. This leads more of an elongated 
precipitation pattern from the GFS...while the other models keep the 
majority of the snow on the northern side of the low center. Due to 
the uncertainty won't do much for changes from ongoing forecast... 
leaving room for further evaluation in upcoming shifts. 


&& 


Aviation... 


Weak short wave trough over southern Minnesota expected to slide 
southeast past the area this morning. This wave may bring a short 
period of MVFR ceilings and visibilities with some snow this 
morning. Once this wave move by...cyclonic flow with steep low level 
lapse rates will help to produce afternoon snow showers. Too 
difficult to determine whether these will affect either taf site and 
plan to address with vicinity snow showers. Afternoon ceilings will 
be variable between MVFR and VFR conditions. The clouds are expected 
to scatter out this evening with VFR conditions for the remainder of 
the night. 


&& 


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


$$ 


Short/long term...Brooks 
aviation...04