Wet, hot Duluthian summer

July 3, 2012


Jordan Smith
Zenith News

Northland residents will undoubtedly never forget Summer Solstice 2012—when the skies opened up and dropped 8.4 inches of rain, sending millions of gallons of water downhill, tearing up roadways, swallowing cars, and overwhelming storm drains. What they may not remember is the unseasonably warm winter that preceded the worst flood in the region’s history.

“Directly, the warm weather doesn’t have an effect on severe weather,” says Carol Christenson, a meteorologist with the National Oceanic Atmospheric Administration. “Indirectly, it started the severe weather earlier.”

In other words, severe summer weather can’t simply be blamed on warm temperatures six months ago, but the severe weather season, which normally starts in June, actually kicked off well before June this year.

Average statewide rainfall in May was six inches—almost 14 inches in the Twin Cities alone—with rain falling for over half the days in May.

Then a cold front precipitated slow-moving thunderstorms over northern Minnesota and Wisconsin in the days leading up to the June 20 flood.

This rapid deluge of water struck an area already affected by drought conditions during the previous winter, when temperatures were sometimes 20 degrees or more above the seasonal average.

Accumulated snowfall for the 2011-12 season was only 22 inches (a significant portion of that during the “Leap Day Storm” last February), compared to an annual mean accumulated snowfall of over 45 inches, according the Minnesota Climatology Working Group.

Much of Lake Superior’s south shore snowfall comes directly from the lake itself, as opposed to the Twin Ports, which receives most of its winter precipitation from weather systems. As cold air passes over the lake, moisture rises to form clouds, which pick up more moisture as they move inland.

The clouds become heavier as they go, eventually leading to “lake effect snow.” When the clouds move over higher terrain, such as Duluth, snow is “squeezed” from them like water from a wet sponge.

This same principle applies in summer. While temperatures in July might reach into the 80s or 90s, a steady easterly breeze often keeps the thermometer in the 70s, leading to Lake Superior’s nickname, “the Big A.C.”

Another dangerous weather phenomenon is waterspouts—funnel clouds that form above or touch down on water bodies like small tornadoes. In fact, some are tornadoes that form inland and move out over water.

Common in tropical climates, waterspouts remain “fairly rare” on Lake Superior, according to Christenson. However, they have been sighted on inland lakes, including Leech Lake southeast of Bemidji and Lake Winnibigoshish, which spans Cass and Itasca Counties in the Chippewa National Forest.

Most waterspouts are caused by a mass of warmer air rising and displacing the cooler air above it, which warms as it sinks and then rises again. These move slowly and dissipate quickly, but are seen more frequently in places like the Florida Keys.

Waterspouts at land-locked higher altitudes, such as the Great Lakes region, are more often caused by “mesocyclones,” when the low-pressure portion of a severe thunderstorm creates a spinning tube of air.

While these occur less frequently, they move faster and last longer than their tropical counterparts, posing a danger to boats and swimmers.

Contrary to popular belief, storms do not “blow out” over Lake Superior, Christenson says. Some severe weather systems are too strong to be significantly affected by the lake. Others, such as thunderstorms, can even be set in motion by lake winds.

Early in the season, when the lake is cold, it has a stronger effect on fledgling thunderstorm systems. As Lake Superior warms up, its influence on developing storms decreases, but the lake itself does not guarantee any sort of protection from dangerous weather.

This year’s warm winter and subsequent severe weather have had a ripple effect on the local economy and wildlife, according to Leonard Anderson, a retired biology teacher with a Master’s degree in ecology.

Above-average temperatures last winter meant less ice formation over the water. Since water under ice evaporates more slowly, less ice means more evaporation, causing a decrease in water levels.

If Lake Superior were to drop by even a foot, ships passing through the Duluth Harbor will have to reduce the volume of their freight, causing a loss in revenue.

Similarly, the ferry between Bayfield and Madeleine Island, the largest of the Apostle Islands, has been forced to run later into the winter because the lake took longer to freeze. Normally, when the ice becomes thick enough to create a natural “bridge,” vehicles can travel over it and the ferry stops for the season.

Humans aren’t the only ones who rely on the ice bridge. Timberwolves use them to reach Isle Royale, near Michigan’s north shore along the Canadian border. The warmer weather may restrict timberwolf access to the island, leaving the population to inbreed or die out entirely.

Lake Superior’s changing “air shed,” in which Moose Lake has generally been the cutoff between warmer inland conditions and cooler temperatures by the lake, is allowing opossum and other species to steadily encroach as far north as Carlton County, posing a threat to native songbirds, which are prey for the opossum.

Flora is as vulnerable as fauna and doesn’t have the option of forced migration. Apple trees, eastern hemlock, and plants that thrive along the south shore can’t seek out more suitable conditions if average temperatures rise.

Marine life could be thrown out of balance as Minnesota and Wisconsin lakes warm up. The population of inland lake fish, such as walleye, may drop, while those that can adapt to warmer water, such as bass, increase.

Coaster brook trout, highly prized by anglers, live just offshore in Lake Superior. Since this is the only part of the lake shallow enough to really heat up much during summer, this species could be particularly threatened.

Lake Superior’s watershed streams, such as Lester River, Knife River, Baptism River, and Brule River, warm faster than the lake itself, jeopardizing brook trout and steelhead populations.

And rising temperatures tend to be cumulative, says Anderson, because ice acts as a “light surface,” reflecting solar radiation, while liquid water, or “dark surface,” absorbs it. With more absorption this year, it becomes that much harder for ice to form in the future.

But Lake Superior is fickle and Christenson urges caution when prognosticating. “Every single area has challenges to forecasting,” she says.

The Gunflint Trail, for example, has a uniquely rising terrain that makes predictions tricky. “The lake, of course, is a big challenge,” making harder the already difficult task of knowing what the weather may bring.

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