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

A thermometer reads the temperature atop an empty Maryland ice cream store freezer on Tuesday. Many grocery stores and restaurants are still waiting for power to return for the first time since last weekend's severe storms. Patrick Semansky/AP Photo hide caption

itoggle caption Patrick Semansky/AP Photo

Fireworks over the National Mall in Washington, D.C., on July 4, 2008. Photo taken from hear the U.S. Marine Corps War Memorial in Arlington, Va. Karen Bleier /AFP/Getty Images hide caption

itoggle caption Karen Bleier /AFP/Getty Images

In Silver Spring, Md., on Monday, Matt MacCartney was one of many workers dismantling fallen trees that took down power lines. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images

Where you're most likely to be in the path of a derecho, and how often. National Oceanic and Atmospheric Administration hide caption

itoggle caption National Oceanic and Atmospheric Administration