That's the rainfall radar from Weather Underground. The relevant warning:
729 PM CDT Mon Aug 4 2008
The National Weather Service in Chicago has issued a
* Tornado Warning for...
northwestern Cook County in northeast Illinois...
northwestern DuPage County in northeast Illinois...
central Kane County in northeast Illinois...
* until 800 PM CDT...
* at 724 PM... National Weather Service radar indicated strong
rotation in a severe thunderstorm near Elburn... moving east at 53
mph.
* The tornado will be near...
Wayne... DuPage Airport... and West Chicago by 735 PM...
Bartlett... and 6 miles south of Streamwood by 740 PM...
Take cover now! If no basement is available... move to an interior
room on the lowest floor. Mobile homes and vehicles should be
abandoned for more substantial shelter. Avoid windows!
Lat... Lon 4203 8818 4186 8817 4187 8848 4202 8849
time... Mot... loc 0029z 271deg 46kt 4194 8835
Halbach
doomsey reports the sirens are on in Elmhurst, and Liz in Elmwood Park, though 
kgarner says they're quiet in Bartlett. Power's flickering, lightning and thunder are near-constant. It sounds like they're getting a real fun pounding out there. The Cub game has been suspended, a 6pm CDT start for ESPN. It's moving east at 50 MPH, gusts to 80.Ah, the fun I miss living on the West Coast. A little 5.4 quake last week, and we're still sunny, clear, and 75ish. Though it's kinda humid...
Also:
Statement as of 8:01 PM CDT on August 04, 2008
... A Tornado Warning remains in effect until 815 PM CDT for
northeastern dupage and central Cook counties...
At 758 PM... trained weather spotters reported a funnel cloud. This
tornado was located near Franklin Park... moving east at 47 mph.
Do not use your car to try to outrun a tornado. Cars are easily
tossed around by tornado winds. If you are caught in the path of a
tornado... leave the car and go to a strong building. If no safe
structure is nearby... seek shelter in a ditch or low spot and cover
your head.
Lat... Lon 4203 8784 4189 8784 4190 8803 4199 8802
time... Mot... loc 0101z 268deg 41kt 4195 8786
I love the panicy-sounding-but-accurate advisements to take shelter. You just don't get that here.
