The storms, reaching heights of up to 57,000 feet in the atmosphere, were energized by a hot and humid airmass pushing into the Great Lakes (i.e. a strong warm front) and a strong jet stream aloft.
Meteorologist Tom Skilling at the Chicago Weather Center sets the scene:
The storms hit in waves and roared into lakeside counties from southeast Wisconsin into northeast Illinois bringing blinding downpours and hurricane-force gusts clocked as high as 94 mph at Waukegan Harbor and 81 mph just off Chicago’s shoreline at the Harrison-Dever crib.
The storms produced an unforgettable display of lightning according to accounts. As Tom Skilling describes:
More than one observer of Thursday night’s storms ranked them among the most dramatic lightning-producers to occur in this area in recent times. At the height of the storm outbreak, lightning data put the number of cloud to ground strokes at more than 800 in a single 10 minute period.
See a video of lightning striking Chicago’s Willis Tower below:
The storms also generated large hail with many reports of stones reaching one to two inches in diameter. Chicagoans Paul Karpenko and Jennifer Wiley captured the video below of hail pelting their deck and street:
Other storm effects:
*100,000 power outages (Chicago Sun-Times)
* Three 15-foot sailboats capsized on Lake Michigan, throwing six people overboard (NBC Chicago)
* The hail damaged 64 police squad cars (NBC Chicago)
Thursday’s severe weather outbreak was the second in as many weeks. On June 21, a fierce line of storms knocked power out to more than 400,000 customers, produced 80 mph straight-line winds and spawned two tornadoes.
With the storms now beyond the Windy City, extreme heat is moving in. A heat advisory is in effect today from noon to midnight with high temperatures expected to reach the mid-to-upper 90s and heat indices as high as 105.