The not-so-cold, hard facts of warming Septembers in Minnesota

The coolest air mass since May has settled into Minnesota and we’ll see our coldest overnight in the Twin Cities since May 25. That was the last time the mercury dipped below the 50-degree mark. On average, the first night in the 40s comes around Sept. 5, so this is about on target (in our modern climate: 1991-2020 average).

Historically, of course, it would have come more than a week earlier on average, around Aug. 28. Hibbing, on the Iron Range, could see frost tonight. That would come just a few days earlier than the Sept. 10 average. Their last frost came June 10, which is a week and a half later than normal. Much of northeast Minnesota is facing a frost advisory Friday night, in fact.

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While our summer ended up being close to normal for a rare change, Septembers are anything but normal anymore. September is the fastest-warming month in the warm season. So, one cool night does not a September make.

September is warming at a pace of more than a degree every decade. The only other months that match or beat it are December and January (on a statewide basis). In just five decades, September averages have warmed nearly two standard deviations. I’ve discussed this before: June and September are rapidly warming while July and August are warming at a very slow pace. Essentially what’s happening in the Minnesota climate, for now at least, is our summers are getting considerably longer rather than hotter extremes. At the same time, winters are shrinking.

Consider that only eight of the last 24 Septembers have been cooler than the modern average (1991-2020). If we look at the historic average (1895-1924), only four of last 24 have been cooler than that.

Often times there are misinformation trolls or knowingly ignorant people leaving comments that they (and everyone) should know better. For example...

Ups and downs are normal weather. Even in our undeniably warmer Septembers there will be a cool one now and then. 2020 was a chilly September. Most others have been much warmer. It’s the trends, that’s what affects biodiversity. Same with winters. We will certainly have another winter like 2013-2014 again eventually, but those are more and more infrequent if not downright rare.

As for the ‘chemtrails’ comment, it’s called condensation. It’s water, not a chemical, Karen. If we’re to be an innovative, entrepreneurial nation we need to have better science skills than a preschool child.

How about the rest of this September?

This September hopefully will not be as ridiculously hot as last year. It was the hottest September on record in the Twin Cities. On a statewide basis it was narrowly the second hottest, only trailing 2015. Recall we had five of our 33 days in the 90s just in September last year. All the forecast models have this September keeping with the trends as a warmer than normal month. The European model has us 5 to 7 degrees above normal across the state over the next 30 days.

If those temperature anomalies pan out, it would be the warmest September on a statewide basis and probably come into the top three for the Twin Cities. Other models are more in the +2 to +5 degree range. As usual in the world of weather, time will tell.

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 .