521PM CDT: This squall line has produced severe wind damage over a 270 mile path. Given the large swath of damage, it can be considered a derecho.
511PM CDT: The storms are severe again as they cross through northern Indiana and parts of extreme southwestern Michigan. There are two distinct lines now -- the one that went through IA/IL earlier, and a new one that formed as storms out ahead of the line merged into it. It's going to be a battle of the lines to see which one dominates the overall movement of the squall. Northern Indiana and southern Michigan need to be on alert for 70-80 MPH winds. The radar is pegging 85-90 MPH winds in the storms just east of Valparaiso.
415PM CDT: Here comes the line into Chicago. It's not severe right now, but it'll have gusty winds, lots of rain, and just as much lightning. There may be a cool shelf cloud out along the leading edge of the storms, but don't stick around to watch it too long.
A major line of severe thunderstorm is moving through eastern Iowa right now, and if it manages to hold together with the same intensity it has right now, it will tear through parts of northern Illinois and southern Wisconsin with winds of 60-70 MPH or higher over the next few hours. 75 MPH winds were reported by a trained weather spotter as the line of storms moved through Cedar Rapids, IA about an hour ago.
Here's a rough estimate of the timing of the line of storms based on their movement as of about 120PM CDT:
This, of course, is provided that the line holds together and continues moving at the same speed in the same direction. Things can and do change rapidly with severe weather, so if you or anyone you know lives in northern Illinois or southern Wisconsin, start paying attention to the weather and spread the word.
I will continue to update this diary as a liveblog as long as time allows. I have some things to do this afternoon, so there may be large gaps between updates. You can keep track of the storms on your own using Wunderground's excellent radar website, as well as watching your local National Weather Service office, as well as the Storm Prediction Center's website.
146PM CDT The line may start to weaken a bit as some storms have formed out ahead of the main line, which should serve to kill off some of the instability being fed into the front of the storm. There are still warnings in effect for ~60 MPH winds. The storms, regardless of winds, are producing an impressive amount of lightning:
125PM CDT: Here's a close up of the worst part of the line approaching the Davenport area:
And here's the velocity (wind) image, showing winds of 55-65 knots about a thousand feet above ground level along the line of storms. These winds can easily translate to the surface (if they're not doing so already), hence the warnings.