May 2020 was 2.1 degrees below average, and followed an April that was also cooler than average (-2.7 degrees). The last time back-to-back months had below average temperatures was in March and April 2018. This was the chilliest May since 2008. What made the month cooler than average was the 10-day period between May 5-14, which was nine degrees colder than average. (The month's other 21 days were slightly above average.)
The brunt of the extended cold period mentioned above was May 9, when the high/low was just 48°/34° (nineteen degrees below average). The day's low reading was the coldest reported in May since 1891. Previous to this May, the last one with a reading in the 30s was in 1978 (38°). Lows were in the 30s for three consecutive days, May 8-10 - just the fourth May with a cold streak of this length, joining 1891, 1923 and 1947. Besides the chilly air, snow flurries fell before daybreak, tying May 9, 1977 for the latest date with a trace of snow.
May was also a dry month. With just 1.65" rain measured, this was the driest May since 2005 (and fourth driest since 1970, and 22nd driest among all Mays). This amount was 60% below average (a deficit of two-and-a-half inches). The wettest day was the Saturday of Memorial Day weekend when 0.65" was measured, with most of this amount pouring down between 11 AM and noon. This was the rainiest Memorial Day Saturday since 1982.
Before the unseasonable Arctic outbreak of May 5-14, May began on the mild side, with the first four days four degrees above average. 5/1 snapped a 16-day streak of cooler than average temperatures; 5/2's high of 73° was the first reading in the 70s in more than six weeks; and May 3 saw the year's first reading of 80°+ - the first time since 2014 that a year's first 80 didn't occur in April.
May's warmest reading, 84°, came six days after 5/9's frosty reading of 34°. This was the first May since 2008 to have no readings of 85° or warmer. (A typical May's warmest reading is in the upper 80s.) The warmest mean temperature was on 5/29 and 5/30, both with a high/low of 81°/67° (eight degrees above average).
Finally, there were a dozen days with lows cooler than 50°, double the average amount (since 2000) and the most in May since 2008.
Here are recaps of the previous five Mays: