It's late (1:35 AM CDT as I start this) but I thought I'd post a few thoughts regarding tomorrow's intriguing setup for southern WI, northern IL, northern IN, and southern Lower MI.
The past several runs of the GFS and NAM have been consistent in creating a potentially robust severe weather environment across this region during the afternoon and evening hours on Tuesday. Several key factors are still in the air, but as we approach this potential event, we can try to shed some clarity on them.
1) Ongoing convection. Currently, there are two areas of convection ongoing upstream from the threat area. One is up in Minnesota and the other is in Iowa. Storm motion vectors and the theta-e gradient demarcating the moist, unstable airmass in place across the plains and lower Great Lakes from the more stable, drier airmass to the northeast and along which nocturnal complexes typically move, suggest that the complex in Minnesota, should it hold together, will likely slide across northern and eastern WI and across Lake Michigan into western MI as it begins to weaken tomorrow morning. Released from this MCS should be a decent outflow boundary that will likely skirt across the lake into eastern WI/northeastern IL/northwestern IN during the morning hours. By the time it reaches this point, item #2 will come into significant play.
2) Strong low-level wind shear and surface wind flow. The 12z NAM this morning had 35kt of wind at 925mb over NE IL at 21z Tuesday afternoon. That is simply incredible. Typically by August we are in the heart of the summer doldrums, where the jet stream weakens, overall atmospheric flow weakens, and thus we have overall just less wind in the atmosphere. The few times we get substantial low-level wind shear it is usually caused by the nocturnal low-level jet, a feature more common in the plains. There is certainly nothing nocturnal, however, about 21z (4 PM CDT). Additionally, sfc flow is progged to be up to 20kt! This will aid in providing healthy inflow into a northwest flow storm event, especially given that the sfc flow is from the SW. Additionally, that strong SW sfc flow is going to likely halt the progression of the outflow released by the morning convection. This should lead to the boundary stalling and slowly washing out, allowing for it to retain its vorticity-enhancing characteristics while storm-detrimental characteristics.
3) Strong instability. This comes with both pros and cons. It's going to be hot. Like, mid-90s hot. And it's going to be humid, likely extremely humid. Dew points often pool ahead of boundaries in the summer, and indeed both the GFS and the NAM pool dewpoints ahead of the front, with some areas progged to have 80°F or higher dewpoints tomorrow afternoon. This may be overzealous in spots, but with corn and soybeans really taking off this time of year, the potential to realize this is certainly there. This will lead to an extremely unstable airmass, with MLCAPE of 3500-5000J/kg in areas where cloud debris from the morning convection doesn't significantly hinder heating. Due to intense heating, however, there is the potential that LCL heights may rise up to 1250m, which is a potential negative to tornadoes. Wind-wise, however, higher LCLs would tend to actually increase the threat for downbursts/microbursts and significant wind damage.
4) Deep-layer shear. Deep-layer (0-6km) shear of 35-40kt is progged to be in play tomorrow. Additionally, these shear vectors run fairly perpendicular to the front. All this means that the deep-layer shear will likely be supportive of supercell structures for a time Tuesday afternoon/evening before storms congeal into an MCS.
Overall, the bottom line is this. It's impossible to point out the exactly location of greatest threat tomorrow. That will depend on where the current activity ends up. However, wherever the greatest threat is, it appears there will be a potential for supercells with hail, wind, and even tornadoes. There may even be an enhanced tornado threat *if* a supercell can favorably interact with the remnant outflow from the morning convection. Finally, even after an MCS forms, unseasonably strong low-level shear values dictate that mesovortex formation potential and subsequent tornado potential cannot be ignored.
