Severe thunderstorms moved across northern Illinois and southern Wisconsin late Tuesday morning and afternoon producing large, damaging hail. Golf ball sized hail was reported in many locations. These severe storms formed despite the cloudy skies and temperatures in the 50s.

The thunderstorms that moved through Tuesday were all elevated, forming north of a very strong warm front. South of the front temperatures warmed into the upper 80s across central Illinois, while north of the front temperatures were in the upper 40s and low 50s with a brisk east wind.

There was quite a bit of instability in the atmosphere Tuesday, but it was all elevated, meaning it formed above the stable layer of air at the surface. As warm/moist air moved in from the south, it moved up and over the stable surface air. This allowed the rising air parcels to utilize the instability aloft, causing the elevated thunderstorms to form. Surface-based thunderstorms form closer to the surface where there is higher surface heat, humidity, moisture and instability.

Large hail is often a product of elevated thunderstorms. And there can be a lot of it, just like what many experienced Tuesday afternoon. Damaging wind gusts can sometimes occur, but tornadoes are fairly uncommon with elevated thunderstorms. The storms that came through Wednesday morning were more surface-based which is why the wind and tornado threat was more of a concern.