Gale Warning

NOAA has issued a gale warning for Lake Huron from the Mackinac Bridge to Presque Isle Light.  For the next several hours we are supposed to have winds up to 35 knots (40 mph), with gusts to 45 knots (52 mph).  That's for 5 nautical miles offshore, so the winds onshore may be less.  Still, those winds will generate waves up to 18 feet.  These conditions should prevail through much of tomorrow, and then begin to subside as Hurricane Sandy blows itself out.

USCG Gale Warning Flags

This morning the experts on the Weather Channel said that winds out of the north could generate waves up to 25 feet on southern Lake Michigan, roughly from Chicago east to South Haven.  The result could be significant beach erosion in this area.

Small contained bodies of water, like the Great Lakes, do odd things when the wind blows hard enough, long enough.  In this kind of storm, we might see a seiche on one or more of the lakes.  A seiche (pronounced saysh) is a standing wave which moves back and forth, like a pendulum, through a body of water.  You yourself can create a seiche when you get into your bathtub too quickly, resulting in a wet floor.  Lake Erie is especially prone to such waves, which can result in significant differences (up to 16 feet) from one end of the lake to the other.

This is a fascinating phenomenon — as long as you aren't standing in its way.  On June 26, 1954, a 10-foot seiche suddenly struck the Chicago waterfront, sweeping away and drowning eight fishermen.

For a good visual illustration, see the earthguide.diagram page:

http://earthguide.ucsd.edu/earthguide/diagrams/waves/swf/wave_seiche.html

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