Weather

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Area forecast discussion 
National Weather Service La Crosse WI 
235 am CST Friday Nov 21 2008 


Short term...through Sunday night 


Primary forecast concern was associated with low-end precipitation 
chances late tonight into Saturday and Sunday into Sunday night. 


Morning surface analysis had high pressure building into the upper 
Mississippi Valley. Air mass filtering into the region one of the 
coldest and driest of the season...with surface temperatures and dew 
points 10f-16f and 5f-10f respectively. 11-3.9 micron satellite 
imagery indicated stratocumulus and flurries...mainly affecting north 
central WI. Water vapor imagery showed short wave tracking across 
northern rockies...while strong eastern Pacific 300mb jet noted. 


Deterministic NCEP and European 21.00z data suite continued to have 
good agreement on overall upper level and surface features...which 
was corroborated by 21.03z sref. As a result...forecast confidence 
was fairly high in details depicted in current data base. 
Therefore...rather benign weather conditions expected to continue 
through the weekend...with low-end precipitation chances at most. 


Tonight into Saturday...moderate to strong q-g convergence focused 
across Iowa...as middle level wave tracks southeastward. However...this 
forcing appears to be out of phase with deepest relative humidity. As 
a result...light quantitative precipitation forecast in the 21.00z NCEP and European models...as well 
as NCEP high resolution arw4 and nmm4...diminishes as middle level wave 
tracks further southeastward. This was corroborated by 21.03z sref. 
Opted to maintain low-end snow chance 22.09z-22.18z over southwest 
portion of the forecast area in proximity to forcing signal. 


Sunday and Sunday night...surface cold front passage anticipated 
Sunday afternoon and evening. Weak low level Theta-E convergence 
noted along front...along with q-g convergence not phasing with 
highest relative humidity until system has passed east of the 
forecast area. Therefore...appears probability of measurable 
precipitation...albeit a low-end chance...should be wake of system 
where colder and more unstable air resides. No changes were made to 
the probabilities for scattered snow showers in this time frame. 


Long term...Monday through Thursday 


21.00z deterministic GFS and European model (ecmwf) continued to indicate an 
amplified 500mb flow pattern developing over the Continental U.S.. while there 
were some differences in the model details...overall scenario 
suggests a large trough lingering across the northeast part of the 
country. Meanwhile...mean 500mb ridge becomes established in the 
center part of the nation...while short waves emanating from the 
eastern Pacific push into the western U.S. 


Based on decent consensus among deterministic and probabilistic data 
suites...forecast confidence was good through the extended outlook. 
Thus...except for possibility of snow showers on Monday...data base 
continues to support a dry weather pattern through Thanksgiving day. 
No changes were made to the extended outlook. 


&& 


Aviation...today and tonight 


High pressure will provide generally clear skies through middle 
afternoon today...and then the clouds will gradually increase 
after 21.21z as a 500 mb short wave trough approaches the region. 
The ceilings will gradually lower through tonight. By 22.08z the 
ceilings will likely be in the 6k to 8k foot range and remain in 
that range through at least 22.12z. There may be enough saturation 
and lift for a slight chance of snow in southeast Minnesota and 
northeast Iowa after 22.09z. Visibilities through tonight are 
expected to remain in the VFR range. 


&& 


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


$$ 


Short/long term...Thompson 
aviation..........boyne