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fxus63 karx 230855 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
255 am CST sun Nov 23 2008 


Short term...through Tuesday night 


Primary forecast concern was associated with snow probabilities and 
amounts tonight into Monday morning. 


Early this morning...low level southerly flow was established 
across the upper Mississippi Valley. Moist boundary layer evident 
per low clouds and/or fog as seen in 11-3.9 micron satellite imagery. 
Further west...surface cold front was advancing into the northern 
plains. Water vapor imagery showed vigorous short wave tracking 
across Idaho...while another wave was seen near Lake Winnipeg area. 


23.00z deterministic NCEP and European models...along with 23.03z 
sref data...had overall good clustering of upper level and surface 
features through 26... morning. Most noticeable 
difference in the European and NCEP data suite was that NAM/WRF had 
slower progression of middle level wave moving across the upper 
Mississippi Valley tonight into Monday morning. In spite of this... 
confidence was high that the forecast area should receive a light 
snowfall event tonight into Monday morning...followed by dry weather 
Tuesday into Wednesday. 


Tonight into Monday morning...vigorous middle level wave expected to 
intensify as it tracks eastward into the upper Mississippi Valley. 
This was shown well by 500-300mb pvu juxtaposed with increasing 
strength of q-g convergence signal. As a result...robust upward 
vertical motions noted in minus 12c-minus 18c snow growth zone... 
in addition to frontogenesis and negative epv signal. All of this 
points to potential for bursts of snowfall and thus intense rates. 
Based on deterministic forcing signals and quantitative precipitation forecast probabilities in 
23.03z sref...precipitation probabilities were increased tonight. 


Utilizing top-down approach with BUFKIT forecast soundings suggest 
precipitation type should initially be rain and snow this evening. 
Then...as hydrometeors fall through dry sub-cloud layer... 
precipitation should change over to all snow as environment cools and 
saturates. This will especially be true when precipitation rate 
increases with onset of strongest forcing. 


With regard to snowfall amounts...interesting to note 23.00z Cobb 
data in the NAM/WRF had higher amounts than GFS...which was based on 
NAM/WRF slower progression of middle level wave. Opted to utilize a 
compromise approach at this time using dprog/dt of Cobb data. 
Therefore...utilized 12 to 1 water to snow ratio between 24.06z- 
24.12z when potential for dendritic growth should be at a maximum. 


This suggests potential for 1-3 inches of total snowfall in the 
forecast area tonight into early Monday morning. Anticipating 
highest amounts across parts of central and southwest WI in proximity 
to strongest forcing. Subsequent forecasts may need to adjust quantitative precipitation forecast and 
thus snowfall amounts as further evaluation of deterministic and 
probabilistic data occurs. 


Remainder of Monday and into Monday night...yet another middle level 
wave is expected to dive southward...bringing with it sufficient 
forcing associated with 700-500mb q-g convergence and 500-300mb pv 
advection. In addition...low level lapse rates expected to steepen 
to 8-9c/km...all of which should aid in supporting chance of snow 
showers. Current data base handles this well and no changes made. 


Long term...Wednesday through Saturday 


Deterministic and probabilistic models in the 23.00z data suite 
continued to have good consensus through much of the medium range. 
Therefore...forecast confidence remains high for quite weather 
pattern through the end of the week. Current data base appears to 
handle this scenario well...thus no changes were made to the 
extended outlook. 


&& 


Aviation...today through tonight 


During the last 2 to 4 hours...the MVFR ceilings have cleared out 
across much of the forecast area. The only place still have ceilings 
between 2000 and 2500 feet is across Clark and Taylor counties in 
north central Wisconsin. With southerly winds advecting in drier air 
from the south...I suspect that these areas will lose this lower 
deck somewhere between 23.09z and 23.12z. 


Generally VFR ceilings and visibilities are expected today...and 
then the ceilings and visibilities will rapidly drop this evening as 
a 500 mb short wave trough approaches the area. This is in response 
to this system producing a deep layer of strong Omega and 
frontogenetic forcing between 700 and 500 mb. At this time...it 
looks like the precipitation will develop quicker than the current 
tafs indicate. I am thinking that the precipitation will develop 
across southeast Minnesota and northeast Iowa between 24.00z and 
24... between 24.03z and 24.07z across western Wisconsin. 


The precipitation should initially develop as rain...and then 
quickly transition to snow as both evaporative cooling and strong 
vertical velocities rapidly cool the atmospheric column. The 23.21z 
Storm Prediction Center sref show that 10 to 30 percent of the members have at least 100 
mb of dendritic growth. As a result...we will likely see the 
visibilities quickly fall to a half mile or less once the snow 
develop. There was even some hints that we could see some slantwise 
convection across northeast Iowa and southwest Wisconsin between 
24.03z and 24.06z. This could also enhance the precipitation rates. 


&& 


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


$$ 


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