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

Where is spring? Icy branches partially blocked a city street Wednesday in Sioux Falls, S.D. More wintry weather is expected there Thursday. Dirk Lammers /ASSOCIATED PRESS hide caption

itoggle caption Dirk Lammers /ASSOCIATED PRESS

That's the coast of the U.S. on the left, the tip of Greenland at the top center and the coast of Europe on the top right. Meanwhile, the storm's tail extends down into the Caribbean. NOAA Satellite and Information Service hide caption

itoggle caption NOAA Satellite and Information Service