‘Dat was some pretty darn big hail we had last weekend, hey? It was especially dramatic for those of us that didn’t bike quite fast enough to make it home before the storm started (spoiler alert – we lived). Then again, the wind and hail was nothing compared to that other storm that happened that one time way back when, right?
If you’ve lived in Minnesota or Wisconsin for more than a year, you know that unpredictable weather is one thing you can count on around here. The temperature can swing more than 50° from one day to the next, and it’s entirely possible to have hyper-local storms that dump rain and hail in one area, without so much as a drop of rain in a nearby community only a few miles away. For as long as people have been tracking the weather in our region, there have been blizzards and thunderstorms, drought and floods, tornadoes and heat waves. Despite this predictable unpredictability, however, scientists have noticed that typical weather patterns in the upper Midwest are not entirely the same as they used to be.
During his keynote talk for the St. Croix Summit on April 29, Minnesota Climatologist Mark Seeley talked about the kinds of weather information researchers gather and the changes they’ve seen over time. For example, though Minnesota has always suffered from severe thunderstorms and occasional “mega rain events,” these kinds of storms have actually become more frequent in recent years. During the first 140 years that people were recording weather information in Minnesota, there were seven mega-storms – an average of one every 20 years. In the past 15 years, there have already been five mega-storms – an average of one every three years! Most of us can remember these large storms vividly because of the damage they caused; they include events such as the flooding in Duluth, June 19-20, 2012 and the storm that dumped 15-17 inches of rain on southeast Minnesota, August 18-19, 2007, flooding the Whitewater River.
In general, we are getting more of our rainfall from thunderstorms nowadays, and less from plain ol’ boring, drizzly rain. According to Seeley, 75% of the precipitation in southeast and eastern Minnesota in 2004 and 2009 came from thunderstorms. In between these storms, there are often extended periods without any rain. As a result, many communities find themselves in the unusual position of battling floods in localized areas, while still under drought conditions as a whole.
Last Sunday’s storm in Stillwater demonstrated how easily that can happen. During the worst of the hailstorm, I watched torrents of icy stormwater rushing down one of the city streets toward Lake McKusick. The water was moving so fast that most of it swept right past the storm drains, flooding McKusick Rd. at the bottom of the hill. The next day, as I was sweeping leaves and twigs up from our sidewalk and driveway, though, I noticed that our gardens were still bone dry.
City planners and engineers use rainfall data from the National Oceanic and Atmospheric Administration (NOAA) to determine how big to make stormwater pipes and ponds to prevent flooding in our communities. Until recently, the standards were based on weather data collected during the first half of the 1900’s, including the 1930’s Dust Bowl Era. When the standards were updated in 2013 to include rainfall data since 1961, engineers realized that the average 50 and 100-year storms had increased by 20-25% in many parts of Minnesota. As a result, the new 100-year rain event is more like what we used to call a 200-year storm. These very large storms quickly overwhelm older stormwater pipes and ponds and, because the ground gets too saturated to absorb more rain, there is usually lots of runoff. The best ways communities can protect themselves from flooding damage due to increasingly frequent large storms are to preserve wetlands and open space in flood prone areas, and create larger easements and safe overflow routes for stormwater ponds and ditches.
There’s a reason FiveThirtyEight Science just ranked Minneapolis as one of the top three major U.S. cities with the most unpredictable weather. As the saying goes around here, “If you don’t like the weather, wait five minutes and it will change.” To protect yourself from hail damage as severe storms become increasingly frequent, consider my advice. Start carrying a bike helmet with you at all times, just in case a storm rolls in.