Bouts of chilly air during May 2017 put the brakes on the longest known warm spell in Minnesota history.
May started with snow showers and temperatures 15-20 degrees below average. It took almost two weeks of mild and even warm weather to undo the temperature deficits from May 1 and 2. The eventual warmth of mid-May gave way to a run of cool weather, which proved to be the deciding factor. Even though most locations had 17 or 18 warm days, compared to just 13 or 14 cool ones, the depth of the cold was great enough to finish below-normal at virtually all stations in Minnesota.
The slightly cool month ends an incredible run of warmth. For 20 straight months, September 2015 through April 2017, stations throughout southern and central Minnesota remained warmer than 1981-2010 normals. The second longest streak in history came just five years earlier, in 2011 and 2012, when many stations logged 16 straight warm months.
If the climate were truly stable, then any month would have equal chances of being warmer or cooler than normal. Using basic probability theory, the chance of 20 straight warm months is about one-ten-thousandth of one percent, and even the chance of 16 straight warm months is still between one and two one-thousandths of a percent--highly unlikely! Indeed, the whole of Minnesota has been warming rapidly since about 1970, making above-normal temperatures more likely than below-normal ones, even as the baseline "normal" values are updated to reflect the warmer conditions.
It is impossible to know when the next warm streak will begin, and we will require over a year-and-a-half of continued warmth after that first warm month before we can talk about challenging this impressive record!
Below is some information about the temperature characteristics in the Twin Cities since September 2015.
Twin Cities Average Monthly Temperatures
Avg Dept. from Month Year Temp 1981-2010 Normal --------------------------------------------- September 2015 67.9 + 5.9 October 2015 52.1 + 3.2 November 2015 41.3 + 7.6 December 2015 30.2 +10.5 January 2016 17.6 + 2.0 February 2016 24.9 + 4.0 March 2016 41.3 + 8.5 April 2016 48.1 + 0.6 May 2016 61.3 + 2.2 June 2016 71.3 + 2.4 July 2016 75.2 + 1.4 August 2016 73.4 + 2.2 September 2016 66.1 + 4.1 October 2016 52.9 + 4.0 November 2016 44.1 +10.4 December 2016 20.9 + 1.2 January 2017 20.9 + 5.3 February 2017 31.2 +10.3 March 2017 34.0 + 1.2 April 2017 50.5 + 3.0 May 2017 58.5 - 0.6 Streak Ended