Weather Watch

Just like off SLO County coast, Great Lakes can generate some big waves — just not that big

The Great Lakes (Lake Erie, Huron, Michigan, Ontario and Superior) make up the most extensive freshwater system on the surface of the Earth. In fact, 20 percent of the world’s supply of surface freshwater is contained in the Great Lakes.

These five lakes have a combined total shoreline length, including islands, of nearly 11,000 miles — a lot, especially when compared to California’s 840-mile long coastline.

Lake Superior is the deepest at 1,330 feet, followed by Lake Michigan at 923 feet at its lowest spot.

Water drains from Superior, Michigan, Huron and Erie into Lake Ontario by way of Niagara Falls then into the St. Lawrence River (seaway), which eventually flows to the Atlantic Ocean. Through canals and man-made modifications, the Great Lakes Basin is linked to the Mississippi River by way of Chicago and Calumet rivers.

These thousands of miles of inland waterways give the United States a significant cost advantage in the transportation of agricultural products and other types of cargo. To navigate and help to keep ships safe, hundreds of lighthouses with Fresnel lenses were built along the shores of the Great Lakes.

The lighthouses along the Great Lakes, much like the Piedras Blancas, Point San Luis and Point Conception light stations, have their own light characteristics, like the number of flashes per minute or color, which provides a secure navigational tool.

Lake Michigan alone has over 100 lighthouses compared to around 30 light stations that stand tall on the California coast.

Last week, my family and I traveled to Naval Station Great Lakes, Illinois, on the shore of Lake Michigan to watch our daughter’s graduation ceremony from U.S. Navy Basic Training. The day before Chloe’s graduation, we had some time to do some sightseeing, so we headed north to Milwaukee, Wisconsin.

There we saw a high-speed ferry named “Lake Express” that transports people and their vehicles from Milwaukee across Lake Michigan to Muskegon, Michigan, in just about 2 1/2 hours, a fraction of the time it would take to drive south to Gary, Indiana then back up north.

During summer, the Lake Express makes up to six crossings each day, decreasing to four trips per day during the spring and fall. During the summer, the winds are often calm, the surface of the lake is like glass. However, between late October through mid-April, they suspend operations due to increasing winds, waves and the formation of ice. The comfort of their passengers is paramount; consequently, they closely monitor weather and lake conditions.

They decided to cancel the ferry crossings on Sept. 27, the day after we visited, because of increasing winds, waves and the threat of severe thunderstorms on the lake.

Gale’s from the north can turn these tranquil lakes to raging freshwater seas. In late September 2011, an autumn storm with 40 to 50 mph northerly winds with an occasional gust to 60 mph blew across Lake Michigan and generated 23-foot waves with a 10 second period at the South Michigan NOAA lake buoy located 43 nautical miles east southeast of Milwaukee.

On Oct. 24, 2017, the NOAA lake buoys recorded 29-foot high short-period waves on Lake Superior north of Marquette, Michigan. These are the highest waves ever reported on the Great Lakes.

To put that in perspective, the highest swell measured at the Diablo Canyon Power Plant waverider buoy was 21.3 feet with a 17-second period. The Diablo Canyon station has been collecting wave data since June 1983.

On Nov. 10, 1975, Edmund Fitzgerald, an American Great Lakes freighter sank in a storm on Lake Superior with the loss of the entire crew of 29. Buoys that measured wave height on the Great Lakes weren’t installed until 1979, so we’re not sure how massive the waves were. Perhaps Gordon Lightfoot’s song said it best: “When the gales of November came slashin; when afternoon came, it was freezin’ rain in the face of a hurricane west wind.”

As any captain will tell you, the longer period and less steep swells have exponentially more energy than shorter period waves, but the tighter interval and much steeper seas are more challenging to operate in.

I couldn’t find waves with periods over 11 seconds recorded by the buoys In the Great Lakes. That’s unlike California’s coastline, where swells with 20, 25 or even 30 second periods from massive mid-latitude storms in the North Pacific during winter, or from large southern hemisphere storms during summer arrive along our beaches.

Even though the Great Lakes are massive, they just don’t have the vast expanse to allow longer period waves to develop. As the winds blow and seas grow, they can reach only a 7-to-1 ratio of wavelength to wave height. In other words, a wave with a 7-foot length can reach only one foot in height before it breaks. When the wave breaks, longer wavelengths develop, allowing the seas to increase in height.

Depending on the wind speed and duration, and the length of the fetch, this cycle continues, allowing for even longer periods and higher amplitudes. Waves in the North Pacific Ocean can reach over 100 feet in height with periods over 25 seconds.

These lake buoys, like the wave-rider buoy at Diablo Canyon Power Plant, measure “significant wave height,” which is defined as the average height of the waves in the top third of the wave record. That turns out to be very close to what an experienced mariner — an “old salt” — would perceive the wave heights to be.

I’ll be speaking at History Center gala

I will be speaking at “The History Center of San Luis Obispo County’s Annual Gala” on Oct. 25 at the historic King David Lodge in San Luis Obispo, and you’re invited! The theme is “Echoes of the Pecho Coast.” There will be a five-course dinner catered by Debbie Collins of Del Monte Cafe. For tickets, please visit www.historycenterslo.org/gala.html.

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