Tomorrow commemorates the 20th anniversary of the Plainfield, Illinois, tornado. It is the only F-5 intensity tornado ever to occur in the month of August and it struck without warning. Because of the lack of warning, 29 people died and 353 were injured even though it occurred in mid-afternoon.
Why was there no warning? There were a number of factors but the most important was the geographic separation between the radar in Marsailles, IL and the warning meteorologists in Rosemont. The image above is from the Marsailles radar while the giant tornado was on the ground. The (likely) hook echo is not seen in this image because it is in the ground clutter (I have outlined the likely shape of the tornadic thunderstorm in pink below).
The radar “technician” in Marsailles did not know to tilt the antenna up to a higher angle to eliminate the clutter and make the potential tornado visible. Because the meteorologists in Chicago could not see the hook and because there were no spotter reports of a tornado, they didn’t issue a warning. A later investigation indicated that some law enforcement agencies knew about the tornado but didn’t report it to the NWS because they assumed the NWS already knew.
After this and other fiascos (Big Thompson Canyon, Delta 191) with a large loss of life due in part to the separation of the radar and warning responsibility, the National Weather Service underwent a reorganization in the mid-1990′s the eliminated this problem. I tell this story in detail in Warnings: The True Story of How Science Tamed the Weather.
The image below, from the NWS in Chicago, shows the telltale brown grass caused by the high winds and scouring in a major tornado.