An upper level disturbance developed from a cluster of severe thunderstorms that raked across Kansas and Missouri Thursday night. This disturbance moved northeastward into eastern Iowa by early Friday afternoon and helped ignite severe thunderstorms over eastern Iowa and northern Illinois. One intense supercell thunderstorm formed over eastern Iowa producing tornadoes across eastern Iowa, Jo Davies county in Illinois and into southern Wisconsin. Other severe thunderstorms raked eastward across northern Illinois producing damaging winds and even some large hail. Below is a map of the preliminary severe weather reports from Friday afternoon.
Red "T" indicates reports of tornadoes, blue "W" indicates reports of damaging winds, green "A" indicates reports of severe hail.
Here is a list of severe thunderstorm reports from north central and northeast Illinois.
Below is an image of the Milwaukee doppler radar’s estimated wind speeds around 5000 feet above the ground as the severe thunderstorms were moving into Winnebago county.
The light purple colors are indicative of winds around 70-75 mph blowing toward the northeast (toward Milwaukee’s radar). These winds were detected approximately 5000 feet above the ground and while the winds near the ground were likely not as intense, localized damage was reported in Winnebago county.
Here is the reflectivity imagine from the Milwaukee radar from the same time as the velocity image above.
Notice the slight "bowing" shape the storm is taking on as it approaches southern Winnebago county, and the more pronounced bow shape in southern Wisconsin. Many of the storms Friday afternoon took on this shape, which often times is an indication of the potential for damaging winds.
Another severe thunderstorm developed about an hour later in LaSalle county in north central Illinois. Wind damage was reported from this thunderstorm near Mendota and LaSalle/Peru. Below is an image of the base velocity (winds blowing toward the radar) from the Chicago doppler radar.
The light purple colors are 60-70 mph winds approximately 2000 feet above the ground blowing toward the Chicago doppler radar located in Romeoville, IL. At this time these thunderstorms were likely producing damaging winds in northeast LaSalle county and went on to produce additional damage in Kendall, Kane, and DuPage counties. Here is the corresponding reflectivity image taken at the same time as the image above.
The dark red colors south of DeKalb and in northeast LaSalle county are indications of very heavy rain and possibly some small hail.