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Seiches: Hidden Dangers On Lakes and Bays

December 21, 2021
By WeatherBug Sr. Meteorologist, Chad Merrill
When you think of severe weather that occurs on the Great Lakes, thunderstorms, waterspouts and lake-effect snow probably come to mind. There is another hidden danger not connected to any form of precipitation that can occur without notice, but signs of its development are predictable: seiches

An energized child taking a bath and moving erratically in the water creates a sloshing motion back and forth. What is observed is called a seiche with water splashing up against one side of a bathtub while at the opposite end of the tub, water levels are much lower. The water levels finally come back in balance when the child becomes still. 

The same concept applies on a much larger scale to what happens occasionally on a large lake or bay. Strong wind created from a rapid change in atmospheric pressure pushes water from one end of a body of water to another. Typically, a seasonably strong weather system of this magnitude occurs late in the fall, through winter and into early spring. It can also occur if a tropical system moves parallel to an elongated bay or lake in the late summer or early fall. 

A constant sustained wind will push water across an open lake and pile it up on the opposite side. A great example of where a seiche is most likely to occur is Lake Erie. The lake is oriented southwest to northeast. An approaching seasonably strong cold front, which is usually preceded by a gusty southwest wind, can push water from Toledo, Ohio (southwest location on the lake) to Buffalo, N.Y. (far northeast point on the lake). The longer the southwest fetch occurs, the bigger the water level difference will be between Toledo, Ohio, and Buffalo, N.Y. 

Usually, strong cold fronts that produce a big pressure and temperature difference move at such a clip that the southwest flow ahead of them doesn’t have time to produce a major seiche before the wind switches direction to the northwest. There have been exceptions to the rule. A strong southwest wind helped power a 22-foot seiche that breached a 14-foot-high sea wall in 1844. The seiche killed 78 people and dammed the ice on Niagara Falls to the point where it stopped flowing. In 2008, waves of 12 to 16 feet flooded Buffalo, N.Y. In most recent memory, a strong westerly wind ahead of a cold front removed the five feet of water on Lake Erie near Toledo and raised the level near Buffalo, N.Y., from 4.8 feet to 7.65 feet. This level in Buffalo was just shy of the Flood Stage threshold, which is 8 feet. 

Lake Ponchartrain in Louisiana is another hot spot for seiches. Lines of gusty thunderstorms that move west to east during the summer across the lake can lower the level near Interstate 55 and exaggerate the level under Interstate 10. 

The example given earlier of a child playing erratically in a bathtub and creating mini seiches occurs on a very short time scale. Only seconds go by between the lowest level on one side of the tub and the highest level on the opposite side as the sloshing occurs. Seiches that occur on lakes and bays often have a four-to-seven-hour window between the lowest point where the water is being removed and the highest point on the opposite side of the lake where it is being “piled up” by strong winds. 

Meteorologists monitoring the advancement of cold fronts and squall lines across large lakes and bays also make sure to follow the water levels to predict when a seiche may occur. While there is no such alert as a Seiche Watch or Warning, if flooding is expected on the lake or bay, a Coastal Flood Advisory can be issued to alert residents. A Small Craft Advisory would be issued when meteorologists expect sustained winds to exceed 20 mph over the lake or bay or if waves four feet or higher could occur. 
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Story Image: High sediment water is seen entering Lake Ontario from Lake Erie via the Niagara River following a seiche in late November 2015. A strong southwest wind ahead of a cold front carried sediment from western Lake Erie to the northeast corner of the lake. (Courtesy of Jesse Allen, NASA Earth Observatory and Suomi National Polar-orbiting Partnership).