Midwest/Lakes Severe Setup: June 22, 2016

A threat for severe thunderstorms with large hail, destructive winds and a few tornadoes is taking aim at portions of the Midwest and Great Lakes late this afternoon into tonight. Chicago is very close to the bulls-eye of this threat area, but other portions of northern Illinois and surrounding areas are included as well. The setup is fairly complex, however with the ingredients in play, there is a high likelihood that severe, damaging thunderstorms will affect at least portions of the region.

• Greatest threat area: Northern Illinois, far southern Wisconsin, far southwestern Lower Michigan and northern Indiana. (Including Chicago, Joliet, Rockford and South Bend)
• Main threats: Large to very large (bigger than golf ball) hail, isolated tornadoes and destructive winds, which may become increasingly widespread overnight.
• Timing: First supercell thunderstorms develop over northern Illinois/southern Wisconsin between 5 and 8 p.m. Thunderstorms become more numerous into the night and spread eastand southeast, merging into clusters and perhaps one or two more expansive thunderstorm systems. The tornado threat will be greatest before 9 p.m., while hail may continue overnight and non-tornado damaging winds become more widespread after 9 p.m. (Rough time estimates, as this could change somewhat)

The convection allowing models (CAMs) are in decent agreement with the threat zone, but show an array of different scenarios with how convection evolves. There is good agreement with timing, as most model solutions, including various ensembles, begin to develop storms in the late afternoon, between roughly 5 and 7 p.m. The trend may be slightly later and with the HRRR (often the most reliable high resolution model) leaning closer to 7 p.m. or so, confidence increases then. Capping due to warm mid-level temperatures should prohibit thunderstorm develop through the early and mid afternoon. Due to the somewhat delayed start, thunderstorm may be limited to Wisconsin and Illinois at the onset, with the surface low expected to be in the eastern Iowa vicinity by 5 p.m.

Convective evolution will probably take one of two routes:

1. A few isolated supercells develop late in the afternoon, possibly merging into clusters overnight.
2. Initial supercells quickly merge into one or more MCSs, perhaps forming a more significant system (derecho).

Based on the magnitude of shear expected and considerable directional shear (veering of winds with height), a more discrete mode (isolated supercells) appears to be favored. Into the night, even if a derecho does not form, there will likely still be corridors of destructive winds with thunderstorm clusters and/or MCSs. The overnight wind threat appears greatest over northern Indiana.

At least 2-3 supercell tornadoes appear likely, centered over northern Illinois, far southeastern Wisconsin and northwestern Indiana. One or more strong tornadoes will also be probable (60/40). Embedded tornadoes can continue into the night within convective clusters/systems.