Catastrophic flooding has been making the headlines to our south recently from Missouri through Kentucky- where up to a foot of rain has fallen in some places within just a couple days, but Minnesota is not immune to such 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.
Overall annual precipitation increasing by 15% for many parts of Minnesota, including the
Twin Cities.
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, defined by top 1% events.
Below is a chart showing the increase in the number of days with 0.50” or more of precipitation (green), 1.00” or more (blue), and 2.00” or more precipitation days. It’s easy to see how our annual precipitation has increased from 26” to 30” between just even 1 or 2 more days of 2”+ or a week’s worth more of 0.50”+ rainfalls.
The reason for this increase is a warmer climate. The atmosphere is super-charged, especially in the warm season due to both warmer temperatures = more energy, but also more water vapor. Warmer air holds more water (think of why we have humid days in summer versus fall or winter: it’s temperature dependent). 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 either. When a storm system, hurricane, or in the case of Minnesota, a frontal boundary develops, it ‘pools’ that extra water into smaller areas creating the conditions for extreme precipitation.
Take Saturday for example, 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.
When things come together ‘just right’ (or just wrong, depending on your perspective), the consequences can be catastrophic. According to research from the University of Minnesota’s Climate Adaptation Partnership, ‘MEGA-RAINS,’ defined by 6” or more of precipitation, have doubled since the year 2000, compared to the 27 years previously (1973-1999).
If 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.
In the near term, 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 or western Wisconsin, northern Iowa. It’s only a matter of time before any of us experience it again.
Minnesota will still have droughts, such as we have had this year and last 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 amount 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 of heavier rains and drought.
Comments