May 24 saw the first 90-degree high temperature of 2018 in the Twin Cities, with May 29th marking the 6th day in a row to reach or exceed 90. How unusual is it to have multiple 90-degree high temperatures in May in the Twin Cities, and does doing so mean we will have a greater-than-average number of 90-degree days during summer?
90-degree readings in the Twin Cities during May have occurred in 48 out of 146 years on record, or about 33% of the time. Only 23 years, however, or about 16%, have had more than one 90-degree day during May. Of those years, May of 1934 was by far the most extreme, with eight daily highs of at least 90 degrees, including a reading of 106 on May 31st.
We find that years with multiple 90-degree days during May have tended to be followed by summers that also have relatively high numbers of 90-degree days. The Twin Cities airport averages about 13 90-degree days per year, and 26% of years historically have had 20 or more such days. For years with at least two 90-degree days during May, however, the average number of 90-degree days for the year jumps to 23.4 (an 80% increase), and 74% of those years go on to have at least 20 90-degree days.
Counts of 90-degree High Temperatures, Twin Cities
May Annual
Year Total Total
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1934 8 34
2018 6 20
1874 5 26
1925 4 20
1987 4 24
1988 4 44
1911 3 16
1921 2 26
1931 2 30
1939 2 23
1949 2 33
1952 2 17
1959 2 29
1964 2 27
1969 2 12
1970 2 26
1972 2 11
1978 2 20
2001 2 20
2006 2 20
2007 2 27
2009 2 6
2010 2 17
2012 2 31
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Average: 23.4