Seiche — Central Chippewa County, Michigan

2025-06-21 · Central Chippewa County, Michigan

Event narrative

Significant water level changes measured by water level equipment at Southwest Pier at the mouth of the St. Mary's River shipping channel, upstream of the Soo Locks. An amplitude of 6.67-ft of change from trough to peak is the largest in 20 years of seiche/meteotsunami data at this location. This resulted in delay of marine traffic through the Soo Locks until water level fluctuations had dampened enough for safe navigation of the shipping channel on the upstream end of the Soo Locks. Water level fluctuations were also noted on the Sault Ste. Marie, MI power canal, resulting in impacts to the Cloverland Hydroelectric Plant at the end of the power canal downstream of the Soo Locks. Additional periodic water level fluctuations were noted around Whitefish Bay and the upper St. Mary's River for the next several days until finally settling back down to normal.

Wider weather episode

An unseasonably hot and humid air mass beneath a strong upper ridge in place across the Midwest led to the development of storms late on the evening of the 20th across the Northern Plains. These congealed into a thunderstorm complex over the Upper Midwest, tracking eastward along the perimeter of the ridge, into the Upper Great Lakes on the morning of the 21st. A NW-SE oriented broken line of storms developed along a warm advection axis ahead of the initial line of storms; this produced hail up to the size of golf balls across parts of northern Lower Michigan, including the Traverse City metro area. Meanwhile, a strong mesolow developed at the heart of the slowly decaying thunderstorm complex, ultimately driving a meteotsunami across the length of Lake Superior, resulting in significant water level changes of as much as 2-4ft on Whitefish Bay and the Upper St. Mary's River during the morning hours. This resulted in delays in freighter traffic through the Soo Locks, the waterway through which nearly 100 percent of America's domestic iron ore passes. Periodic water level fluctuations up to 2 feet remained present on the eastern end of Lake Superior through the next couple days, and water levels did not settle back down to normal for several days through the end of June.

Source: NOAA Storm Events Database, event_id 1289450. Narrative written by the NWS forecast office that issued the report.