• National Weather Service
  • Mean wind speed over the Illinois region

Those 60-80 mph winds that blew through Chicago this morning caused a lot of damage—upwards of 660,000 residents without power in the third severe thunderstorm this month, according to the Washington Post. But the danger didn’t officially end once the storm passed by.

The hurricane-speed winds over Lake Michigan actually dragged a lot of water to the Michigan side of the lake, causing a phenomenon called a seiche, where water in a contained body like a lake or pool sloshes back and forth, pushing abnormally high waves onto either shore as the wave keeps crossing. Think of the sloshing on a trip from stove to sink when you’re draining a pot of boiling pasta, or a very pronounced tide in a body of water that doesn’t experience tides.

Eventually the lake settles back down, but it can be a bumpy ride. According to the Encyclopedia of Chicago, eight people drowned from a particularly vicious seiche in 1954; seven were swept off Montrose Beach and one off North Avenue Bridge. You can see what happens to water levels here.