Standing on the beach on a warm, sunny day, last night’s storm forgotten, you look out at Lake Michigan.
There’s something out there – a wave forming across the whole horizon. Around you, other beachgoers are still swimming in the surf. Bad idea. Now’s the time to high-tail it out of the water.
A big meteotsunami is a relatively rare phenomenon – one occurs roughly every three years on Lake Michigan – but it hits without warning on otherwise clear-skied days and can cause injury and drownings.
“The big issue is rip currents,” says Tim Halbach, the warning coordination meteorologist with the National Weather Service office in Sullivan. After the wave hits the shore, the water reverses course, sending the current rushing back out. “The force of that would be strong enough to pull someone out into the lake and keep them there until the current dies.”
This all starts with a wind storm, most often in late spring and early summer. If a squall from the northwest moves fast enough through Milwaukee and over Lake Michigan, it’ll push a wave all the way across to the Michigan shoreline. “When you get that buildup in the height of a wave, it wants to bounce back,” Halbach says.
The wave reverses course o Michigan and rolls back toward Wisconsin. Within roughly one to five hours, it’ll hit the western shore. Low-level meteotsunamis here might be only a foot or two. Bigger ones will reach over 4 feet, though they can get much worse, especially farther south where the lake is shallower. The most catastrophic one on record hit Chicago on June 26, 1954, when a 10-foot wave surge killed eight people.
If a storm indicates the possibility of a meteotsunami – one sign is barometric pressure rising substantially around the storm – the weather service can post a warning, but there’s currently no real-time meteotsunami warning system. So if you’re on the beach on a clear, sunny afternoon after a big morning storm, keep your eyes on the water.