UPDATE - 10:30PM SAT
Watching the latest radar and model trends it's becoming very clear that most convective short range models today were wrong. The HRRR , low res NAM, and GFS have been the closest over the last 6-12hrs. I prefer to borrow some of the good ideas from the short range models at apply that to what appears most likely at the moment.
At the moment; a cluster of storms has developed up near the US/CAN border. We are seeing some development near i-94 in ND. The bulk of overnight MCS activity looks to stay further north as opposed to knifing south across Minneapolis/MS River. This opens the door for development across Wisconsin during peak heating.
Instability tomorrow will be very high...above 5,000 j/kg+... on the left side of the instability gradient to be located across Wisconsin. Commonly that is near I-94; but recent model trends show much of southern WI destabilizing rapidly tomorrow also. Here's the thing; there is a massive amount of untouched energy pooled across Iowa and southern WI. If overnight/am convection doesn't dent that; it's going to be fireworks across WI. Rarely do we get this type of energy and rarely does the boundary take on NW/SE configuration in NW flow.
The supercell threat I mentioned earlier is not as concerning right now. I think mainly a damaging wind and large hail threat bowing segments with these storms, though a tornado is not out of the question at boundary intersections or initiation.
I'm hesitant to go all the way to "Severe" for three reasons: 1. We still have tomorrow AM to gain more confidence. 2. It's consistent with messaging so far and don't have to change as much. 3. I think shear is on the weak end. Instability could be ten thousand and all it might do is gust out.
In the end I get the feeling we need to do more about this situation; thus I bumped to severe. But doesn't hurt to wait tonight. It's still possible that an MCS could develop and plow southeastward. I will be up early either way.
One more note: Did not spend much time on Illinois. Main focus is WI.
1:00PM SAT - Video forecast (18 min)
Here are the two MAIN ways things could go on Sunday ...
1. MOST LIKELY - MCS develops over north Dakota/Minnesota and dives south across Wisconsin Sunday morning. Damaging wind gusts are the main threat. This limits day time heating an precludes any afternoon t-storm development. You might still get some strong storms overnight into Monday - depending on the timing - but this would be less certain.
2. LESS LIKELY - All activity fades away this afternoon and convection DOES NOT develop in north Dakota or central MN tonight; or develops further north. MCS pushes through Duluth and northeast Wisconsin. Outflow boundary left from morning complex triggers highly unstable environment Sunday afternoon in central/western WI. Supercell structures lead to damaging wind, very large hail, and tornado threat.
Given the uncertainty at this moment I will hold back a little bit. Either way I expect strong to severe storms Sunday across portions of Wisconsin. It's just when and what exactly that I'm unsure about. Will wait for more information: the 00Z model run and monitor current conditions. I hope to have a more detailed forecast ready later tonight.