The forecast on this day called for only a 20 percent chance of thunderstorms. It was a typical summer day for Chicago. The main flow was out of the west/southwest. Temperatures were in the lower 80 except for near the lake shore. The phrase everyone who lives in Chicago is familiar with is "cooler by the lake." It is a very common combination of weather for the area. On this day though it resulted in explosive thunderstorm development.
As the lake breeze began making its way inland, it collided with the westerly winds. This caused the air to rise vigorously. Other atmospheric ingredients lined up perfectly to allow this to happen. There was a breakable cap, no subsidence [sinking air] in the area, and the force of the winds colliding was perfect so that the air had nowhere to go but up. More commonly what happens is the lake breeze will be stronger than the main flow and over power it, or vice-versa.
The massive thunderstorms began blowing up shortly after 1pm. They started off in the northern end of the city and gradually developed southward, seemingly on their own outflow boundaries. Because winds aloft were slow in a typical summer fashion, these storms did not move at all, just went up and quickly died.
Below are a couple of radar images saved by Bob Hartig [www.stormhorn.blogspot.com] of the event. I was at work and could not save images myself. There are some interesting things to be seen which I have circled.
In this first image the lake breeze boundary is clearly visible. This feature is very easy to spot on any given day. A storm is already in progress.
In this next image there are a couple things to note. I circled all the wind barbs which indicate the surface flow. Notice how they come in from the west. Behind the lake breeze the wind barbs are coming from the east. Notice how the wind barb within the storm itself has 4 solid lines, this indicates winds greater than 40mph!
Unfortunately these storms went severe warned too late. At the time they were severe and producing severe wind damage across mainly southwestern sections of the city and the nearby suburbs. The following series of photos were all taken within a few blocks of my house.
The view out my front door. Luckily this large limb missed my truck.
Across the steet. Half a tree is down on top of a van.
I decided to hop in my truck and drive around to see what else there was. At the end of the block a local bar had its fence blown down.
Turning onto my block.
A few houses down from the above photo.
Another view of the tree on top of the van.
The house across the street was being remodeled. Its hard to see but the construction sustained heavy damage.
A tree is down just 1 block north of my house.
The falling tree also brought down a powerline.
This was probably the most impressive damage I saw. Likely due to the tree being somewhat rotted.
A damaged car from falling limbs.
Sheet metal.
Ironically it seemed like the most concentrated area of damage was on my block. Other blocks had one or two trees with damage. However more significant damage was done to my former Highschool. The roof of the dugout at St. Laurence Highschool was ripped clean off. Injuring a coach and a player.
Unfortunately a damage survey was never done for the storms. I would have to guess based on their nature that the damage was caused by a wet microburst with winds around 70mph.
The conditions that existed on this day are common. A clear, warm summer day with an inland moving lake breeze. As mentioned before other parameters came into play to cause these storms to quickly form and die. They were short lived, and many people didn't even know it stormed. The storms were extremely localized but powerful.
As for myself, I was downtown during the event. The storms largely missed that area. I was able to sit near the Sears Tower though and listen to repeated lightning strikes. The strikes on the Sears Tower are incredibly loud. Some of the loudest, ground shaking thunder you can hear. While I was not able to experience [or film] the worst part of the storm, it made for a nice afternoon.
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