General studies : November, 1967 / by John C. Ayers, Joseph C. K. Huang.

28

Of definite importance, however, are larger seiches associated
with the passage of large line-squalls from northwest to southeast
across Lake Michigan. These squalls have their fronts oriented
NE-SW and are accompanied by an abrupt increase in barometric
pressure and local high winds. The combination of air pressure
increase and wind pressure depresses the water surface under the
storm front and starts a forced wave running southeastward across
the lake. If the speed of progression of the front is 40 mph or
more, the forced wave is in or near resonance with one of the
natural modes of the southern basin and the wave is reinforced as
it crosses the lake. Bottom friction slows the progress of the
south end of the wave and it arrives on the southeastern shore
with a shape convex toward the southeast. The faster deep-water
north end of the wave appears to strike obliquely along most of
the Michigan shore without causing water-level rises much more
than its own height, but in the southeast corner of the lake the
wave shape is nearly parallel to the shape of the shore and
additional rise of level is caused as the translated water of the
wave is trapped against the shoreline. From the southeastern
corner the wave is then reflected, with additional shallow-water
refraction, westward across the lake to the Chicago region.
The largest of the recent large seiches or storm surges
occurred on 26 June 1954. It caused some damage in Michigan City
and both damage and loss of life in Chicago, consequently it has
been studied in detail and its causes and behavior recognized.
The U. S. Weather Bureau Station at Chicago now issues seiche
predictions routinely when conditions justify.