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 --------------------------- 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 --------------------------- Average: 23.4