The storm was already gone.
Sky clearing, wind dropping, the lakefront back to something people would walk on.
Then the water started moving on its own, up and down, over and over, in bright daylight.
Nothing shook. No earthquake, no landslide, nothing under the lakebed at all.
The thing that made it had passed overhead an hour earlier and kept going.
So how does weather alone do that?
A pressure jump that keeps pace with its own wave
A squall line carries a sharp step in air pressure at its leading edge.
The step is small, a few thousandths of an atmosphere, and it presses the water surface down by roughly an inch.
An inch is nothing. What matters is that the dent travels.
Water in a basin also carries long waves at a speed set only by depth, and over about 200 feet of water that speed works out near 55 miles an hour.
A squall line crosses a lake at close to that.
When the two match, the storm stops passing over the dent and starts pushing it, mile after mile, adding energy to the same bump the whole way.
That is the resonance, and it is why the wave that arrives can be a hundred times the inch the pressure alone accounts for.
The basin that concentrates it
Lake Michigan is roughly 300 miles from top to bottom and about 118 miles across at its widest, reaching 925 feet at its deepest.
The southern end is the shallow end, and it is also a closed corner.
A wave that runs south down the lake has nowhere left to go.
It shoals as the bottom rises, which slows it and stacks it higher, then reflects off the shoreline and comes back.
Harbors make it worse rather than better, because a rectangular basin with a narrow mouth has a natural period of its own and will ring like a struck glass when something hits it at the right frequency.
The city sitting in that corner has the most instrumented shoreline on the lake.
What the instruments caught on the day
On July 27 of this year, a line of storms tore southeast across the Chicago metro area.
It produced seven confirmed tornadoes, two of them rated EF1, the stronger carrying peak winds of 110 miles an hour along an 11.4 mile path.
A gust of 104 miles an hour was measured at an airfield near Monee, among the strongest ever recorded in Illinois. Hail reached 3 inches. Rainfall peaked near 3.95 inches.
Then the water responded.
Gauges on the Portage Burns Waterway in Indiana and at the Chicago Lock swung by up to 2.7 feet over periods of 20 to 30 minutes.
Waukegan Harbor recorded rapid level changes and currents strong enough to drag buoys and debris around.
Forecasters wrote it up as a possible meteotsunami, and two weeks later a second event put a lakeshore flood warning on the same waterfront.
Why this is not the rare event it sounds like
The word tsunami does the damage here.
A survey of two decades of six minute gauge records across the Great Lakes counted about 106 a year, and 51 of them on Lake Michigan alone.
The single biggest source anywhere in the system is Calumet Harbor, on Chicago’s own south lakefront, at 29 events a year.
The size fits the same pattern. A swing you would expect roughly once a year at these stations is about 2.7 feet, and the ten year figure is closer to 4.3.
So the number in the record is an ordinary year’s event on the most active shoreline on the lakes, which is not a small thing to say.
What is genuinely missing is prediction, because the generation mechanism is still described in the literature as poorly understood and there is no operational warning product for it, unlike the forecasting that moves large vessels and the current knowledge sailors built over centuries.
The reason a few feet is the dangerous number
Nothing about 2.7 feet threatens a building.
It threatens people, and only because of when it arrives.
The storm has passed, the beach has refilled, and the wave shows up under a clearing sky with no warning of any kind, pulling swimmers off a shelf or sweeping a pier as a surge rather than a breaking wave.
Piers are where this has done its worst, and the pattern is always the same: outflow first, then the return.
The counting is the useful part of the story, because none of this existed as a category until gauges sampled fast enough to see it, as the official event summary shows.
A Lake Michigan meteotsunami was a mystery for decades and is now a line in a database, as the survey paper made possible.
Knowing how often it happens is not the same as knowing when.
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