Shown above is a large, wedge tornado that touched down near Rochelle, Illinois, on April 9, 2015. This was the most powerful tornado observed in this area of northern Illinois since at least 1950. For a portion of its approximately 18 mi (30 km) path, this twister was rated EF4 -- devastating winds in excess of 200 mph (320 kmph). It killed two people and injured more than twenty. The tornado developed in a large, supercell thunderstorm that formed when a strong, low-pressure system plowed into moist, unstable air. I was approximately 1.5 mi (3.8 km) or so southeast of the tornado when I captured this picture.