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Area forecast discussion 
National Weather Service La Crosse WI 
351 am CST Thursday Jan 8 2009 


Short term...today through Saturday night 


Forecast concerns this package center on precipitation chances/type/amounts 
Thursday night through Friday night...and plenty of uncertainty therein. 


06z data analysis had a large area of low pressure over New England 
with a trough west into mi/WI. Skinny ridge of high pressure 
extended from saskat to OK with Lee troughing from alb to Colorado. 
Clouds/flurries lingered over central/east WI in the cyclonic 
flow...with clear skies across much of IA/MN. Temperatures not far from 
seasonal normals across most of the region early this morning. 


08.00z NAM/GFS look to have initialized quite well...however this 
does not play out in similar model solutions through the period. 
Solutions reasonably similar through 36hrs...then diverge in the 48- 
72hr period with significant impacts on the sensible weather for 
the area. Dprog/dt of 500mb heights at 08.00z showed NAM/GFS verified 
rather well across noam and trended toward a compromise of their 
earlier runs. Through 36hrs GFS shows better run-to-run consistency 
with the energy moving through the Great Basin. Models continue to 
struggle with the all important piece of energy into the northern 
plains at 36hrs. Between model and run-to-run consistency less than 
one would expect at this time-frame. GFS remains the faster...NAM 
the slower and more supported by European model (ecmwf)/UKMET/can Gem at 36hrs. For 
36-60hrs general trend is slower/stronger with shortwave/energy into 
the middle/upper MS valley with GFS remaining faster/flatter and NAM 
slower/stronger. By 60hrs can-Gem more in GFS Camp while European model (ecmwf)/NAM 
similar and UKMET in the middle. The slower trend of the trough/ 
shortwave exiting east continues in the 60-84hr period. Check of observation 
vs. Model data at 06z showed NAM/GFS reasonable with the surface mass 
fields across the central Continental U.S.. per WV imagery...both models also 
appear quite reasonable with the shortwave details over noam/eastern 
Pacific...thus no model favorite early on. 08.06z NAM maintains 00z 
NAM/European model (ecmwf) timing but not as strong as 00z NAM. Given the limited 
run-to-run consistency and lack of good model consensus already at 
24-36hrs...forecast confidence on the low side this cycle. Favored a 
model/ensemble compromise this cycle...weighted toward the somewhat 
more consistent European model (ecmwf). 


For the short term...narrow ridge of high pressure slides across the 
area today for a quiet...mostly to partly sunny day. 850mb warm 
advection already returns for the afternoon but northwest surface winds 
to keep flow of cool air across the area. Warm advection/clouds 
increase tonight as ridge shifts east and troughing moves into the 
northern plains. Much of the warm advection/fn convergence 
lift GOES into saturation of the initially dry airmass over the 
area. Saturation eventually occurs later tonight with reasonable 
model consensus for -sn to develop over the southwest half of the 
forecast area. Raise probability of precipitation to categorical these areas later tonight. 
Model consensus has trended toward the cooler earlier European model (ecmwf) 
solution of keeping bulk of the 0c or warmer 850mb air southwest of 
the forecast area late tonight/Friday morning...just clipping the 
southwest corner. Still some question as to depth of moisture and 
ice introduction into the clouds across the southwest end of the 
forecast area around 12z Friday...so left mention of -fzdz/-pl there. 
Initial warm advection band of snow lifts north across the forecast 
area late Thursday night/Friday morning. Secondary deformation band of snow 
develops and pivots across the area Friday afternoon/night as the surface- 
700mb low deepens just south of the area. European model (ecmwf)/NAM the more robust 
with this while GFS maintains an open 700mb wave. Depending on which 
model Camp is more correct makes a difference of about half as much 
snow potential with GFS vs. NAM/ECMWF. Model consensus points to a 2 
to 5 inch snow event for the area centered on Friday. Did raise probability of precipitation to 
categorical for much of the area Friday and to likely over the 
southeast half of the forecast area Friday night. Given bulk of this 
looking to be late 2nd period into 4th period and plenty of model 
differences in the details...held off on issuing Winter Weather 
Advisory for now. Day crew will have another model cycle to 
assess...and hopefully 08.12z models converge toward a more common 
solution as leading portion of the trough/shortwave energy reaches 
the West Coast rain-ob network. If the slower/deeper NAM/European model (ecmwf) 
solution ends up more correct may yet need a lingering -sn chance 
into Sat morning but left this dry for now. Weak ridging builds 
across the area for Sat night. Low level warm advection already 
returns by Sun morning as the next shortwave drops toward the area 
and low tracks across southern can. 


Leaning on the model/blend compromise...generally favored a blend 
of guidance highs/lows through the period. Trended toward warmer of 
guidance lows tonight with lows expected this evening then slowly 
rising after midnight as clouds thicken/snow develops. 


Long term...Sunday through Wednesday 


Medium range models continue to show an amplifying flow pattern over 
noam/eastern Pacific in the sun-Wednesday period. Upper Midwest comes under 
increasing northwest/north flow aloft. Plenty of differences remain 
in the timing/strength of shortwaves in this developing flow. Small 
-sn chances sun/Monday reasonable for now. Main impact of this pattern 
shift is a trend toward colder/below normal temperatures as Arctic 
airmasses drop southeast across the region behind the clipper 
systems. With the good between model and run-to-run consistency for 
the strong troughing over central/eastern noam next week...did lower 
temperatures some Monday night/Tue. Temperatures Tuesday night/Wednesday may still be too warm 
but left these alone for now. 


&& 


Aviation...today and tonight 


VFR conditions will prevail today and this evening with a downward 
trend in conditions after 09.06z. Still considerable uncertainty in 
the eventual outcome of the upcoming storm...but current trends 
suggest that the bulk of the precipitation will not occur until 
after 09.12z. As a result plan to trend both taf sites down with 
MVFR ceilings at both sites by 09.12z. Plan to introduce only MVFR 
visibilities late in the period for krst as the new 08.06z NAM now 
suggests the bulk of the precipitation will not arrive until after 
09.12z. Will keep klse dry through 09.12z but both taf sites 
expected to quickly go down to IFR conditions after 09.12z with the 
onset of the heavier snow. Also...little concern for any freezing 
precipitation at either taf sites as a southward shift by the models 
would suggest the potential for this would mainly be across 
northeast Iowa. 


&& 


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


$$ 


Short/long term...rrs 
aviation..........04 

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