999 
fxus63 karx 212038 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
338 PM CDT Thursday Aug 21 2008 


Short term...tonight through Saturday night 


Precipitation chances from tonight through Saturday are the 
primary concerns for this part of the forecast. 


At 3 PM...a 500 mb short wave trough was moving north into 
southern Iowa and northern Illinois. This system was spreading 
light to moderate rain northward into the forecast area. The 
21.12z models are in fairly good agreement that this system will 
move north into our area this evening...and exit the region toward 
sunrise on Friday. As this system continues to move northward... 
this gradually weakens. As a result...I placed the highest 
precipitation probabilities in the forecast for this evening...and 
then went with just chance probabilities after midnight. With the 
best forcing located across Wisconsin...I went with my higher 
chance probabilities across western Wisconsin. 


On Friday...the models are somewhat caught between system. A much 
weaker upper level system will be moving northward through the 
western Great Lakes...and a much stronger 500 mb short wave moving 
into western Ontario and western Minnesota. As a result...I have 
just have slight chance to chance precipitation probabilities into 
the forecast. 


On Friday night and Saturday...the models are consistently showing 
that a positively tilted 500 mb wave will moving through Ontario and 
northern Great Lakes. Much of the synoptic forcing looks to be 
north of the area. As a result...the precipitation amounts and 
chances are starting to look less than what they were a couple of 
days ago. I was really tempted to lower the precipitation 
probabilities into the chance range. However for consistency sake 
and there still being a chance that the models may revert back 
to their original solution...I held off making too many changes to 
these time periods. 


Long term...Sunday through Thursday 


The 21.12z medium range models are in fairly good agreement that a 
500 mb ridge will be in control of the weather through Tuesday night... 
and then they start to carve out 500 mb trough across the plains and 
the upper Mississippi River valley. The European model (ecmwf) is much faster than 
its 21.00z solution and also the GFS and Gem. The generation of 
this trough makes a lot of sense when one looks at the various 
strong tropical forcing features are located around the Globe. 
The models typically do not handle these situations that well...so 
this leads to much uncertainty in this part of the forecast. At 
this time...it looks like the best probability of precipitation 
looks to be from Wednesday into Thursday. In addition there is quite 
a bit of uncertainty where the remnants of Tropical Storm Fay will 
end up next week. The Gem suggest that this moisture may end up in our 
area on Thursday. Meanwhile many of the other medium range models suggest 
the Tennessee and Ohio valleys will be affected anywhere from 
Wednesday into Thursday. 


&& 


Aviation...tonight through Friday morning 


An upper level disturbance over the Central Plains will continue to 
move northward across the upper Midwest and Great Lakes region this 
afternoon and into tonight. Latest mosaic radar indicated a shield 
of precipitation extended from southeastern Minnesota to northern 
Minnesota. Visibilities underneath precipitation were ranging from 
2sm to 5sm and ceilings varied from below a 1000 feet to 5000 feet. 
Limited instability associated with the upper level 
disturbance...have no mention of thunder at both taf sites this 
afternoon and into tonight. With the abundant low level moisture and 
upper level disturbance tracking north into both taf sites. Began 
lowering ceilings and visibilities to MVFR conditions by 20z 
Thursday. Latest RUC and NAM indicate inversion developing over the 
region late this evening. With decent amount of low level 
moisture...have lowered ceilings to IFR conditions at both lse and 
rst taf sites by 04z and 05z Friday. A tight pressure gradient 
develops over the area Friday morning. Have introduced wind gusts up 
to 23 knots at rst after 15z. 


&& 


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


$$ 


Aviation - dtj 
short and long term - boyne