000 
fxus63 karx 060132 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
832 PM CDT Sat Jul 5 2008 


Update... 
have gotten a chance to hash through numerous hi-res WRF/RUC 
datasets for the overnight/Sunday morning time frame...and it 
certainly will be interesting how things play out after about 07z. 
Currently...MUCAPE axis /3000-5000 j/kg/ per 05.23z 13km RUC 
running from eastern Kansas into eastern South Dakota immediately ahead of surface trough 
attached to rather impressive Manitoba middle tropospheric wave. Low 
to midlevel flow /using 850-800mb layer as proxy/ currently 
crossing this deep cape axis at about 45 degree angle in western 
Iowa/western Minnesota...with elevated echo festering in far wcntl Minnesota. Same 
13km RUC run waits until about 07z and then turns 30kt flow in 
this layer more westerly as wave moves through with nearly an orthogonal 
Crossing of the cape axis as it slowly moves east. Hence...in 07z- 
10z time frame see radical increase in precipitable water and subsequent moisture 
transport/isentropic ascent toward western forecast area. Models 
peculiarly dry...especially RUC/NAM...in northwestern Iowa/scntl Minnesota later tonight...and 
after being very robust at 12z with quantitative precipitation forecast /05.12z NAM/ off to the 
west. This most likley tied to lack of overall convergence as low 
level jet actually in two segments...with min in convergence until 
later tonight when southern segement /neb and Iowa/ focuses better 
convergence into southern Minnesota/northern Iowa. 05.15z sref certainly offering 
support for convective development...and think this the way to go. 
Hence...expect scattered- broken band of storms to develop over northwestern/ncntl 
Iowa..or move in from South Dakota if development occurs upstream later 
tonight. Given very weak middle level flow...and more dominant low 
level jet...storms will have strong urge to sink more south than 
move east...building into higher instability feed to the south 
/see current Nebraska storm cluster as example/. That should help 
mitigate a more rapid ewrd progression. Irregardless...increased 
probability of precipitation further in western forecast area later tonight /already timed 
nicely by previous arx forecaster/ and also just a bit into Sunday morning as 
remnants and continued destabilization aloft may allow storms to 
continue into Sunday morning...though confidence tapers quickly 
once the low level jet begins to mix out/weaken. 


Binau 


&& 


Previous discussion... /issued 318 PM CDT Sat Jul 5 2008/ 


Short term...through Monday night 


Main forecast concern is with the rain chances through the period. 


Surface analysis this afternoon shows high pressure over Lower 
Lake Michigan. Weak trough of low pressure/dry line extends from 
North Dakota into the Nebraska Panhandle. 


The 05.12z models present similar ideas but with wide differences 
on where and when rain will occur. This leads to a low confidence 
forecast through the period and precludes any major changes to 
the current database. 


Models continue to show the return flow setting up over the area 
tonight on the back side of the departing surface high and ahead 
of the low level trough. The NAM shows decent warm air 
advection of 2 to 8c/12 hours over the area. The main axis of the 
low level jet/moisture transport will be across Iowa and Minnesota 
with the western sections of the forecast area on the eastern edge 
of the moisture transport gradient. By 12z Sunday the NAM does 
show some turning to the low level jet to help focus the moisture 
transport into the area. A weak short wave trough is expected to 
move across the region tonight to combine with 2 to 4 microbars/S 
of upglide on the 305k isentropic surface to produce some decent 
lift. The NAM and GFS both develop rain in this pattern with a 
possible mesoscale convective system developing over northern Iowa into southern 
Minnesota. Expecting the mesoscale convective system to stay west of the area but lighter 
rain ahead of it could spread into the area late tonight and have 
raised probability of precipitation and spread them farther east. For now have stayed with 
chance probability of precipitation but noted that the 05.15z srefs are now up to a 60 
probability across the far western sections. 


The trough of low pressure is expected to become a cold front and 
move into central Minnesota and northwest Iowa Sunday afternoon as 
another short wave trough comes across the upper Midwest. Should 
see scattered thunderstorms develop along the front during the 
afternoon but coverage not expected to be that great. Low level 
frontogenesis...fn vector and moisture convergence not that great 
along the front and the short wave trough is not expected to be 
very strong either. Prospects for severe weather not looking that 
great as only about 20 to 25 knots of 0-3km shear is expected to 
be present. Decent instability will be in place with 1000-1500 
j/kg of ml cape expected but with high wet bulb zero and freezing 
level heights main threat should be isolated wind gusts and 
locally heavy rain. 


The front should become parallel to the upper level flow and stall 
out over the area Sunday night. The short wave trough will be moving 
away from the area and low level jet/moisture transport looks to 
be focused into the front across northern Illinois into southeast 
Wisconsin. There will still be a chance for some rain across 
mainly the southern sections of the region. 


Rain chances will start to increase again Monday afternoon into 
Monday night as the most significant short wave of the period is 
expected to come across the upper Midwest. This should lead to 
cyclogenesis along the front Monday afternoon over Kansas or 
Nebraska with this low tracking along the front Monday night. 
Differences on the track of the low with the NAM being farther 
north than the GFS while the 05.15z sref would favor the more 
northern track of the NAM. The low level jet will intensify ahead 
of this low and should be focused on the forecast area by 06z 
Tuesday. An mesoscale convective system is expected to develop in this pattern and have 
focused the heavy rain threat along the Interstate 90 corridor. 
Mesoscale convective system expected to be transitory in nature which should preclude a 
lot of training from occurring and should limit the flooding 
threat. Storms Monday night should be could rain producers though 
as the warm cloud depth looks to increase to just over 4 km. Still 
some severe threat Monday afternoon and evening as well. Still not 
a lot of shear with 20 to 25 knots in the 0-3km layer but with 
some cooling aloft with the approach of the upper level 
trough...wet bulb zero and freezing level heights come down enough 
for the possibility of more hail along with some isolated wind 
gusts. 


Long term...Tuesday through Saturday 


Medium range models show very good agreement through the period. The 
stationary front will finally get shoved east and south of the area 
Tuesday night into Wednesday as the upper level trough shifts east 
over the eastern part of North America. This will allow ridging 
aloft and at the surface to build over the upper Midwest. As a 
result...maintained the low chance probability of precipitation into Tuesday evening and 
then went with a cooler and dry forecast for Wednesday through 
Friday. The ridging will start to slide east of the area for late in 
the period allowing the next trough to start crossing over The 
Rockies. The 05.00z European model (ecmwf) suggest the possibility of some activity 
developing on the backside of the departing surface high in the 
return flow for Friday night and Saturday and have some low probability of precipitation in 
for the possibility. 


Aviation... 


VFR conditions expected through the period. From Sunday 08z and 
beyond there will be an increase in middle-level clouds with ceilings 
in the 6-8kft range. There will also be enough elevated instability 
around to produce a few rain showers/ts especially west of the Mississippi 
River...but again...ceilings and visibility expected to remain VFR. 
Otherwise...southerly surface airflow in the 10-20kt range will be 
seen at both krst and klse taf sites through the period. 


&& 


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


$$ 


Short/long term...04 
aviation...das 

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