For the second time in a month, the Chicago lakeshore has experienced a seiche. The word is Swiss French and means "to sway back and forth." The results can be deadly for waders along beaches of southern Lake Michigan, who can only see this danger as a frequent fluctuation of lakeshore water. Worldwide, there are various reasons for its creation. But for Lake Michigan, it always results from a thunderstorm or line of storms crossing Lake Michigan. Decaying thunderstorm processes will usually result in a strong pressure rise ahead of the decaying storm. Since water is fluid, this rapid rise in pressure will push a large area of lake water downward and out. This can occur often over the lake. But to arrive at the Chicago lakeshore and start an oscillation going, the pressure rise will need to have occurred over Lake Michigan at a location north of Milwaukee. The resulting outward flowing crest then travels southeast to the west Michigan lakeshore, before rebounding southwest to the Chicago lakeshore.
Lake water rises of 1 to 2 feet are most common but have been known to go much higher. Most prominent in Chicago history is the 1954 seiche that occurred on June 26th. Eight fishermen were swept away in a 10 foot seiche as they worked along the breakwaters. While most seiches appear only as a fluctuation of high and low tides, an actual wave was visible with this monster seiche of 1954. Clues to their presence are only received at the National Weather Service via alert and cognizant observers on the Michigan or Chicago lakeshore. The oscillation frequency between high and low water is usually 20 to 30 minutes. Gravity eventually takes charge and dampens the fluctuation extremes with time. Thus it may take many hours before the seiche subsides completely. This, in turn, depends on the severity of the seiche.