A man walks across the washed-out Wagonwheel Gap Road in Boulder, Colo., on Monday. Mark Leffingwell /Reuters/Landov hide caption

itoggle caption Mark Leffingwell /Reuters/Landov

John Hoffenberg watches as floodwaters flow down a street in Boulder, Colo., on Sunday. Mark Leffingwell/Reuters/Landov hide caption

itoggle caption Mark Leffingwell/Reuters/Landov

People cool off Wednesday in a pool in Shanghai, where temperatures reached an all-time record: 105.4 degrees. Frank Langfitt/NPR hide caption

itoggle caption Frank Langfitt/NPR

Excessive heat warnings will continue for much of the Desert Southwest through Monday. Here, Maria Wieser of Italy drinks water while sightseeing in Death Valley National Park on Friday. Chris Carlson/AP hide caption

itoggle caption Chris Carlson/AP

Quite a show in Chicago: Lightning struck the Willis Tower (formerly the Sears Tower, at right) on Wednesday as the storm system moved through. Scott Olson/Getty Images hide caption

itoggle caption Scott Olson/Getty Images

A map shows the chance of severe thunderstorms Wednesday evening, with the National Weather Service predicting strong winds and storms moving eastward to the mid-Atlantic Thursday. NOAA hide caption

itoggle caption NOAA

The funnel of this tornadic thunderstorm came close to the ground near South Haven, Kansas, on Sunday. Gene Blevins /Reuters /Landov hide caption

itoggle caption Gene Blevins /Reuters /Landov