Storms erupted to life over sections of the Chicago area on Thursday, June 28th 2012 producing widespread damaging hail up to, and probably over 2.00″ in diameter and frequent cloud to ground lightning. This all happened on a day where the official temperature at Midway reached 100 degrees, and in the early stages of a developing drought for all of Illinois and much of the midwest. Area rain has been largely scarce for the past few months, and the storms did in fact bring some much needed rain, but only for a tiny fraction of areas that desperately need it.
As the afternoon temps crept up to 100 degrees, so did the humidity. Dewpoints reaching the mid 70s were common across much of the area. This in turn lead to extreme instability across the area. A strong cap was in place which was expected to thwart thunderstorm development across much of the midwest. There was a potential trigger for storms with a cold front sliding south through Wisconsin as well as a lake breeze from Lake Michigan. Each element by themselves was not enough to overcome the cap. However, where the two met, enough forcing and lift was generated to allow updrafts to punch through the cap. Given the extreme levels of instability caused by the hot and humid conditions, these storms were able to quickly tap that energy and explode to life.
Storms quickly went severe after only 3 scans on radar, with tops reaching 56,000 feet in a matter of minutes. That is incredibly fast thunderstorm development. The tall cloud tops meant the storms were able to pull the precip well above the freezing layer, and with wind shear values above 40kts to help promote rotating updrafts, conditions were perfect for the development of large hail.
Below are some radar shots and pictures.
In this image you can see the first storm going up where the 2 boundaries meet. The cold front is the line extending west from where the storm is, and the lake breeze is the line extending southeast. The lake breeze was moving East to West, and the cold front was moving Southeast. This is what some refer to as a “mesoscale accident.” Very localized features were able to overcome the cap. Had it not been for the lake breeze aiding in lift, these storms would not have happened. The red box indicates a severe thunderstorm warning for ping pong ball size hail.
The initial storm above weakened, but the boundaries kept moving, sparking new development along their way.
In just 20 minutes the storms had exploded. Large golfball size hail began falling at this time.
Shortly after, hail around 2.00″ fell in Midlothian, IL where my mom lives. These are some of the large stones from their backyard. Photo credit to Cheryl/Alan Gonzalez.
Large hail pummeled a number of locations in the southern suburbs. This photo of a vehicle dented by hail comes from Denise Grzyb, taken in Crestwood.
Storms began congealing into a linear complex as they moved into Northwest Indiana. A severe thunderstorm watch was issued by the NWS for these areas. The storms continued to produce large hail as well as damaging winds around 60mph in these areas.
Back at home near Midway Aiport, they left behind some good structure on their backside and a wicked display of positive CG lightning with some incredible thunder.
Here is some video footage shot by Nick Stewart in Evergreen Park, IL of a transformer exploding from a lightning strike.
As mentioned, the storms did bring some much needed rain to the area, but only a small portion. Below is the doppler estimated rain. Every bit helps, but the area needs much more.
In conclusion, on a day that saw a very small chance for thunderstorm development, storms were able to form due to some very localized mechanisms. These types of events are quite difficult to see coming. Similar conditions exist quite often in the summer months and many times nothing ever happens. The forecast called for only a 20-40% chance for storms, but IF storms did form they were expected to go severe. It is always important to be aware of the weather whenever thunderstorms are in the forecast. As you can see from the storms reports, the local Chicago area was really the only area in the country that experienced severe storms, and it was all due to local climatology where a daily lake breeze met with a cold front in a hot and humid airmass.
Live updates about the event were provided on the Aerostorms facebook page and hopefully helped provide some people with an advanced notice. Check out the page here: http://www.facebook.com/TornadoChasing
Thanks to those who contributed content for this summary! Until next time, stay weather savvy my friends!