I went swimming at Oak Street Beach recently after it had stormed overnight. The water was about 200 feet further out from the shore than usual, but after 30 minutes the water returned to its original position. What happened?
Thunderstorms that occurred in the Chicago area overnight produced an air pressure surge that, upon hitting the shore area of the lake, pushed the lake outward somewhat, causing a low wave that traveled outward from the shore. The wave, or water surge, struck the shore opposite from Chicago (on the Michigan side of the lake) and reflected back, causing the water level to increase somewhat at the Chicago shore when it returned. This cycle repeated a few times, resulting in water level fluctuations at the shore.