Unusually warm conditions have dominated the last 18+ months in Minnesota, but how does this warm episode stack up historically?
To answer this question, we examined monthly average temperatures for the state of Minnesota going back to 1895, and compared those values to the averages from the entire 1895-2016 record. We looked at warm streaks, which are runs of consecutive months that are warmer than the long-term averages, and also warm periods, which we defined as regimes of predominantly warm conditions with no more than two consecutive months that were cooler than average. We note that we could have come up with countless other definitions.
For historical comparisons, we simply counted the total months involved in each streak or period, computed the average temperature departures, and also calculated the "total accumulated warmth" (TAW), which is the sum of all temperature departures through the duration of each episode.
Whichever way we look at it, the current warm episode is indeed making history. As Table 1 shows, the 18-month streak (through February 2017), is the longest on record and also has the greatest accumulated warmth, with a TAW of 99.3 F. Based on average monthly departures, the streaks from 1986 to 1987 and 1999-2000 were more intense, but those streaks were also less than half the duration of the current one. It is safe to say that Minnesota has never seen a warm streak like the one presently underway.
Table 1. Minnesota warm streaks, ranked by duration.
Duration Avg. Monthly
Date Range (Months) Departure(F) TAW (F)
-------------------------------------------------------
Sep 2015 - Feb 2017* 18 5.5 99.3
Jul 2011 - Sep 2012 15 5.3 79.4
Apr 1941 - Apr 1942 13 3.5 45.8
Jul 1998 - Jul 1999 13 3.5 45.3
Sep 2005 - Aug 2006 12 4.4 53.1
Dec 1986 - Jul 1987 8 7.1 57.0
Dec 1920 - Jul 1921 8 4.0 32.1
Sep 2004 - Apr 2005 8 3.9 31.3
Nov 1999 - May 2000 7 6.5 45.2
Sep 1994 - Mar 1995 7 3.9 27.5
Mar 1988 - Sep 1988 7 3.5 24.8
May 2007 - Nov 2007 7 2.6 18.0
*ongoingOf the warm periods, which include gaps of one or two cool months, the most significant episode stretched from September 2004 through November 2007, when 33 of 39 months were warm and when the total accumulated warmth through those 39 months was 125.2 degrees F. The present warm episode currently ranks #2, possibly still gaining ground, depending on whether it can avoid three consecutive cool months.
Table 2. Minnesota warm periods, ranked by total accumulated warmth (TAW).
Duration Warm Avg. Monthly
Date Range (Months) Months Departure(F) TAW (F)
------------------------------------------------------------------
Sep 2004 - Nov 2007 39 33 3.2 125.2
Jun 2014 - Feb 2017* 33 28 3.4 110.0
Jan 1986 - Jan 1989 37 28 2.6 96.4
Jul 1997 - Jul 1999 23 21 4.0 91.6
Nov 1999 - Feb 2002 28 21 3.3 91.5
Jul 2011 - Feb 2013 20 19 4.4 88.8
Feb 1930 - Aug 1932 31 24 2.6 80.2
Sep 1940 - Apr 1942 20 17 2.6 51.9
Sep 1920 - Nov 1922 27 20 1.8 48.1
Jul 1989 - Aug 1991 26 17 1.7 43.4
Feb 1952 - Feb 1954 25 16 1.5 38.2
Oct 1962 - Jul 1964 22 16 1.7 36.8
*ongoing
Of the warm periods, which include gaps of one or two