Catastrophic flooding has been making headlines to our south recently from Missouri to Kentucky, where up to a foot of rain has fallen in some places within just a couple days. But Minnesota is not immune to similarly extreme events.
Our short-term memory may be clouded by two consecutive drought summers, but recall this spring was very wet across much of Minnesota and we’ve had an upward trajectory in precipitation overall.
When things come together just right, the consequences can be catastrophic.
According to research from the University of Minnesota’s Climate Adaptation Partnership, "mega-rain" events, which are defined by 6+ inches of precipitation, have doubled since 2000 compared to the 27 years prior (1973-1999).
One of the most recent extreme rain events in Minnesota was a 48-hour period between Sept. 10-12, where parts of southwestern Minnesota had 10 inches of rain. Extreme rainfall rates wound up flooding parts of Interstate 90, as shown in the feature video of this story.
Will there be a mega rain event in MN Saturday?
The European model is forecasting precipitable water (total water in the atmospheric column available for a thunderstorm or system) values in the top 1% for the time of the year.
It’s entirely possible the Twin Cities area is missed by the heavy rainfall that takes shape Saturday into Saturday night, but a mega-rain could fall somewhere in southern Minnesota, western Wisconsin or northern Iowa. It’s only a matter of time before any of us experience it again.
As our climate continues to warm, we can expect more volatile precipitation events. Below are different scenarios for the number of days (per 100 years) where rainfall exceeds 4 inches based on future emissions levels.
Why is it happening? Warmer climate
The atmosphere is supercharged, especially in the summer months due to both warmer temperatures, which create more energy, and increased water vapor. Warmer air holds more water (think of why we have humid days in summer versus fall or winter). For every one degree (F) increase, that’s a whopping 4% increase in water content.
Water doesn’t just spread out evenly in the atmosphere. When a storm system, hurricane, or in the case of Minnesota, a frontal boundary develops, it pools that extra water into smaller areas, which in turn sets the stage for extreme precipitation.
Annual precipitation has increased 15% for many parts of Minnesota, including the Twin Cities, over the past 147 years.
A big contributor is an increase in downpours and heavy rainfall events. Minnesota, Missouri and Kentucky (as well as Wisconsin) have seen a 27% to 42% increase in heavy rainfall events, which are defined by top 1% events. In other words, the number of events that would typically happen just once every 100 years.
Below is a chart showing the increase in the number of days with varying rates of heavy rainfall, be it a half inch, one inch or two inches of rain in Minneapolis-St. Paul. All of them have become more frequent.
Minnesota will still have droughts, like this year, last year and in 2010-2012. Those are weather features, but the overall climate trend is for extreme rains when we get them.
2019 set a new all time record for precipitation at 43.17 inches, more than a foot above the modern normal in the Twin Cities. The previous record was just three years earlier in 2016 at 40.32 inches. So Minnesotans need to help in the global effort to reduce future catastrophic climate scenarios but also adapt to a world with more extremes.
BMTN Note: Weather events in isolation can't always be pinned on climate change, but the broader trend of increasingly severe weather and record-breaking extremes seen in Minnesota and across the globe can be attributed directly to the rapidly warming climate caused by human activity. The IPCC has warned that Earth is "firmly on track toward an unlivable world," and says greenhouse gas emissions must be halved by 2030 in order to limit warming to 1.5C, which would prevent the most catastrophic effects on humankind. You can read more here.