Severe thunderstorms moved across parts of southern Wisconsin and northern Illinois Tuesday evening, producing hail up to three inches in diameter in some locations, damaging many cars and buildings. Thunderstorms were quick to develop over Wisconsin late Tuesday afternoon ahead of a cold front, producing hail north of Milwaukee and over far southwest Wisconsin. One particular supercell thunderstorm continued to strengthen as it moved into southern Green County before dipping south into Jo Daviess and Stephenson counties. There, golf ball sized hail was reported in Orangeville, as well as a few miles south of Monroe. That storm continued to grow as it moved east, prompting severe thunderstorm warnings further east over Winnebago County. As the storm moved across northern Stephenson County it began to show signs of rotation in the mid-levels. The tornado threat with this particular storm, in this environment, was low but such a strong updraft continued to promote the development of large and damaging hail.
Golf ball to tennis ball sized hail reports continued to come in from near Durand and Davis/Lake Summerset in northwest Winnebago County. As the storm continued to move southeast, another storm developed north of Winnebago and produced golf ball sized hail that covered the ground in Winnebago. Pea to dime sized hail, even moth-ball sized hail, was reported north of Rockford while the strongest part of the storm moved through New Milford. The storm continued to travel to the southeast into northern DeKalb County before moving out of the viewing area a little after 9pm. A few additional showers and storms developed, with one storm producing quarter sized hail in Sycamore a little after 9pm.
To understand why these storms produced such large hail we need to understand how the storm developed itself. All thunderstorms have updrafts and downdrafts. The updraft provides the warm, moist air to fuel the storm while the downdraft is the wind, rain and hail that falls from the storm. Temperatures Tuesday afternoon warmed close to 80 degrees across northern Illinois and southern Wisconsin, building quite a bit of instability in the atmosphere. But that energy was ‘capped’ by warmer air aloft. Think of our atmosphere as a pot of boiling water with a lid on it. The water doesn’t boil as much when the lid is on, but once you remove the lid the water begins to boil rapidly. The same idea applies to our atmosphere. As the cold front moved in from the northwest, it helped ‘remove’ the lid allowing storms to quickly form. We didn’t have all the right ingredients in place for tornado producing thunderstorms, but with the intense rising air through the atmosphere there was rotation present in the mid-levels of the thunderstorm allowing large hail production.
Rising water droplets inside the storm will freeze and collide with other water droplets, ice crystals and hail stones. When the hail stone becomes too heavy for the updraft to hold, the hail stone will fall to the ground. The stronger the updraft, the larger the hail stone is likely to grow. As it remains suspended within the thunderstorm it will continue to collide with other hail stones. This is why large hail stones are rarely ever smooth and have more of a rough look to the outer edges, and sometimes will look like it has layers.
The storms Tuesday evening were definitely intense and damaging but thankfully skies will remain much more quiet during the overnight.