Tornados striking urban areas are even worse, because homes are packed more tightly together. This system caused some real damage yesterday in Minneapolis:

A huge storm system spawned at least three tornado touchdowns as it moved northeast across the Twin Cities metro area Sunday afternoon, killing one person in particularly hard-hit north Minneapolis and critically injuring two others among 29 needing medical attention.

Minneapolis Mayor R.T. Rybak and Police Chief Tim Dolan said early Sunday night that a large section of north Minneapolis -- roughy 4 square miles -- was being put under a curfew to help emergency personnel move around and to combat potential looting of damaged homes and businesses.

[...] Rybak and City Council Member Barbara Johnson got an aerial view from a helicopter. The mayor described the damage as “widespread and significant.” City officials said at least 100 homes were damaged, some of them totaled.

The same system hit a hospital in Joplin, Missouri:

Damage was widespread across the south side of Joplin. John Campbell, operations director for the Missouri State Emergency Management Agency, confirmed fatalities have been reported, but he did not yet have an exact number or specifics.

Phone communications in and out of the city of about 50,000 people about 160 miles south of Kansas City were largely cut off.

Jasper County Emergency Management Director Keith Stammer said the St. John's Regional Medical Center on the city's south side took a "direct hit." Witnesses said windows were blown out on the top floors of the hospital.