Minnesota ain't what it used to be, that’s for sure.
Most reasonable people were astonished and appalled at the Dec. 15 tornado outbreak. Even if you like mild weather and thunderstorms, you want it to occur in the season that it should. Most took the historic severe weather outbreak as yet another sign of a planet and climate in peril.
So how much truth is there to that? Can we blame every extreme event on climate change?
Rapid Attribution Studies have become a great and necessary method to determine the climate influence on specific weather events. While there is no such study yet on this never before seen severe weather event, I’m almost certain there will be. These deep dives look at the data and compile probability statistics.
To give you an idea what these studies can tell us, let’s look at the Pacific Northwest heat wave this past June. Climate scientists determined the extraordinary heat was a once in 1,000 year event – so not something we can expect *every* summer, just yet.
But, climate change has made this event 150 times more likely. That’s to say, before manmade climate change, that kind of scorching heat would happen 1 in 150,000 years (i.e. IMPOSSIBLE without our influence).
Now to our Dec. 15 severe weather.
It wasn’t just the tornadoes that were off the charts. The temperatures were indeed record breaking, but we’ve been that warm this late twice before since 1873. Those days, however, were your typical winter warm ups without fanfare. Still, it appears the warm airmass to that extent was a once in a decades occurrence.
The low pressure in the storm system was exceptionally low, perhaps a once in a century or more event for the time of year in this region. The most insane ingredient to Wednesday’s madness was the moisture content. Precipitable water (or water in the atmospheric column, not just surface) was four or five sigma, in statistical terms.
Four or five sigma means it was an event that occurs just once over a period of thousands of years. Since this is based partly on model data, I won't be specific, but if you know anything about probabilities, the odds of all of the rare occurrences (low pressure, warmth, moisture content) coming together at once gives a setup like Dec. 15 in this region an even smaller probability.
So back to the climate change aspect. How does just 2-4 degrees (°F) warming have such an outsized effect? Again, we have to turn to statistics. The visual aids will help, but this is complicated stuff. I'll lay it out scientifically and then put it in laymen terms.
In statistics we talk about standard deviations: what’s the normalized variation from the average? One standard deviation (1 sigma) within the average covers 68% of data (the middle "Average" section of the chart below). Perfectly normal at this point.
Two standard deviations (2 sigma) covers 95% of data. Still here, 95% of all of our weather is pretty normal, though some days will feature colder than normal or warmer than normal temperatures. But as you can see in the chart below, an increase of 0.5 standard deviations (a few degrees) moves the 1 in 100 occurrence (2.5 sigma) to 2 sigma (1 in 22 years).
In laymen terms, 2-4 degrees of warming leads to what were once extraordinarily rare weather events happening exponentially. And the warmer the planet gets, the more these extremely rare events happen.
Let’s take mid-July for an example. The average high in Minneapolis is 83. The standard deviation is 6-7 degrees, meaning high temperatures basically sit between 76-90 degrees. More accurately, the high temp in mid-July in Minneapolis will be between 76-90 degrees 68% of the time, and between 69-97 degrees 95% of the time.
But the planet warming 2-4 degrees (°F) means we more frequently escape that 95% normal, which results in more extreme weather. In fact, just a few degrees change makes extreme heat 4.5 times more likely and the cool end of extremes 3-4 times less likely.
This is why we see way more high temperature records compared to any cold records and why temps in the negative 20s used to happen three times per winter and now only happen once every 2-3 years.
Hitting 58 degrees in Minneapolis and 69 in La Crosse on Dec. 15 represents an extreme weather event, and when Mother Nature decided to squeeze out record high moisture content and an incredibly powerful low pressure system at the same time, the result is a tornado outbreak 10 days before Christmas and five days after 10-20 inches of snow fell in the same areas that were rocked by severe storms.
And that, folks, is how you get tornadoes in Minnesota in December for the first time since at least viking times, or perhaps even the first time ever.