Do Hot Days in May Foretell a Hot Summer?

May 30th, 2023 saw the first 90-degree high temperature of the year in the Twin Cities, with May 31st doing it again. 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?

Although good hot days in May are pretty common, the Twin Cities has had two or more 90 F high temperatures during May less than a fifth of the time, historically. Those years have usually gone on to have above-average numbers of 90-degree days too.

Including 2023, 90-degree readings in the Twin Cities during May have occurred in 50 out of 151 years on record, or about 33% of the time. Only 26 years, or about 17%, have had two or more 90-degree days during May. Having three or more 90-degree F high temperatures is rare, occurring less than 5% of the time. May 1934 was by far the most extreme, with eight daily highs of at least 90 degrees, including a reading of 106 on May 31st. More recently, May of 2018 had a remarkable run of six straight days, including a 100 F reading on Memorial Day.

Over recent decades, and including the current "normals" period of 1991-2020, the Twin Cities area has averaged 13 days per year with a high temperature of 90 degrees F or higher. But years with two or more 90 F high temperatures during May historically have tended to finish with far more hot days, averaging about 23 for the season. Such a large difference implies that hot years tend to start early. Of the years with at least two 90-degree F days in May in the Twin Cities, 88%, or 22 out of 25, ended the year above the average of thirteen 90 F days, and 72% (18 out of 25) had at least 20 such days.  

In other words, when we have observed multiple hot days in May historically, the year typically has gone on to produce more than the usual number of hot days also. Of course, this is not a prediction! It is only a statement of an apparently strong historical relationship between the number of hot days in May and the number of hot days during the rest of summer.

Below is a table comparing the number of 90 F high temperatures during May in the Twin Cities (for years with at least two), with the total number recorded for the entire year. You can also sort this interactive table to compare the number of 90-degree F days during May (or any month) to the total for the end of the year. (NEW! Do the same thing for Duluth, International Falls, Rochester, and St. Cloud.)

Updated May 31, 2023