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Area forecast discussion 
National Weather Service La Crosse WI 
310 am CDT sun Jul 6 2008 


Short term...through Tuesday night 


Primary forecast concerns remain focused on rain chances today 
through Monday night...along with potential for locally heavy quantitative precipitation forecast 
amounts in this time frame. 


Early morning surface analysis had cold front extending from 
northwest Minnesota into eastern South Dakota and western Nebraska. 
Scattered convection occurring along and ahead of front in return 
southerly flow along instability axis associated with 850mb moisture 
transport. Water vapor imagery indicated short wave tracking across 
Ontario province of southern Canada. Water vapor also indicated 
subtropical feed occurring from Mexico into the Central Plains. 


06.00z NCEP and European deterministic models were in fairly good 
agreement with regard to surface and upper level features through 
09.12z and this was corroborated by latest sref data. As a result... 
forecast confidence was high that the forecast area should receive 
periods of rainfall today into Monday night. Of note...06.00z GFS 
appears to be suffering from convective feedback and thus its quantitative precipitation forecast 
depiction was suspect. 


Of special note...potential for locally heavy rain appears to still 
exist based on deterministic and probabilistic data suite. Even 
though confidence was not as high as one would prefer in pinpointing 
where heavy rain potential will be...favored time frame continues to 
focus mainly on Monday night. 


Today...surface front expected to continue moving eastward into 
northwest WI...while trailing portion of should begin to stall across 
southern Minnesota. Ribbon of MLCAPE 1500-2500j/kg and plume of ml dew 
points in middle to upper 60s should encounter front during afternoon 
and evening. Even so...lack of strong forcing signals suggest 
scattered convection at best...which was corroborated by high 
resolution arxmesowrf. Therefore...maintained rain probabilities in 
high-end chance category. 


Severe storm potential today appears to be low-end per Storm Prediction Center day one 
outlook. This based on wind shear of 20-25kts...which should be 
mainly confined to lowest 3km and unidirectional according to BUFKIT 
forecast soundings. Multicell storm Mode still anticipated...which 
should be outflow dominated. In addition...forecast soundings 
continued to suggest wet bulb zero and freezing levels will be quite 
high Sunday afternoon and evening...about 10k-13k feet respectively. 


Tonight...focus of low level Theta-E convergence shifts across 
Iowa...where highest instability also resides. Nocturnal low level 
jet expected to impinge on this area during the late night hours. 
Corfidi vectors suggest back building and thus repeat of convection 
over same areas may occur. Resulting impact on the forecast area 
should be highest probability of heaviest quantitative precipitation forecast across parts of 
northeast Iowa and southwest corner of WI. Based on this...increased 
rain probabilities into likely category between 07.06z-07.12z in 
those locations. 


Monday into Monday night...front expected to extend in an east-west 
orientation as it parallels middle and upper level flow. Deterministic 
and probabilistic data continued to support another upper level and 
surface wave to advance across the upper Mississippi Valley Monday 
night. Due to forcing signals associated with dynamical and 
kinematic processes...forecast confidence was high enough to increase 
rain probabilities into likely category across the entire forecast 
area during the time frame 08.00z-08.12z. 


An east-west oriented front always a concern...especially when 
combined with precipitable water values of nearly 2 inches... 
strong 850mb moisture transport impinging upon front...and warm cloud 
depth up to around 4km. All of these parameters suggest efficient 
rain producing storms should occur...especially Monday night. There 
could still be potential for back building and therefore possibility 
for convection to repeat over same areas. 


Even though antecedent conditions have been dry and 1hr and 3hr 
flash flood guidance values were 2.5-4.0 inches...anticipated 
multiple rounds of thunderstorms could cause localized runoff 
problems. Thus...plan to issue a hydrologic statement to heighten 
awareness of locally heavy rain potential. 


Long term...Wednesday through Saturday 


Forecast confidence was high that the forecast area should have a 
dry period at midweek. This was supported by deterministic and 
probabilistic data suite...which suggests surface high pressure 
settles across the upper Mississippi Valley. Current data base 
appears very reasonable in this regard...so no changes were made in 
this portion of the extended outlook. 


There were significant differences between 06.00z GFS and European model (ecmwf) 
associated with progression of next surface and upper level wave 
advancing into the northern plains toward the end of the week. This 
lends considerably lower forecast confidence during this time frame. 
Mref suggests rain probabilities still at a minimum late in the week. 
Low-end rain chances Friday night into Saturday...as in current data 
base...appear reasonable at this time. Thus...no changes were made 
in this portion of the extended outlook until consensus improves. 


&& 


Aviation... 
the 06.00z models are in general agreement that a cold front will 
move into the area this afternoon...and then the front stalls out 
across the region as it becomes parallel with the upper level flow. 
Several upper level systems transition east across the region. The 
first one moves through this morning...and the second one moves 
through after 06z tonight. Even though there is general agreement 
with these...the models differ on their precipitation placement and 
timing. This is likely a result of the varying precipitation schemes 
used in them and somewhat weak forcing. Due to this there is much 
uncertainty in the thunderstorm probabilities. The 05.21z Storm Prediction Center sref 
would suggest that the best probabilities will be across northeast 
Iowa and southeast Minnesota early this morning and after 21z this 
afternoon...and south of the taf sites tonight. I will try and use 
these to better define these chances. While convection is very 
unclear...ceilings look to be generally above 3000 feet through this 
period. 


&& 


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


&& 


$$ 


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

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