May 2021 Summary – Warmer and Wetter Than Average

Blue Hill Observatory May 2021 Summary:

May was generally warmer and wetter than average. The 24-hour adjusted mean temperature for the month of 57.7 deg F was 2.2 degrees warmer than the 1891-2020 130-year average for May, and it was 1.0 degrees warmer than the 1981-2010 30-year average. The average maximum temperature of 68.1 deg F was 1.7 degrees warmer than the 130-year mean, and the average minimum temperature of 48.0 deg F was 1.5 degrees warmer than the 130-year mean. The highest temperature observed all month was 87F on the 22nd and 23rd, and the lowest temperature was 38F on the 1st. The month ended with a pair of cold rain storms over more than three days from the 28th to 31st, which included 63 consecutive hours of temperatures colder than 50F. This was the longest such period since there was a comparable period of 63 hours of temperatures below 50F on May 31st to June 3rd in 2015. The daily high temperature on the 30th of 46F was a new record low maximum for the date, breaking the previous record of 51F set in 1996. This reading was also the coldest high temperature this late in the season since a high of 46F was observed on June 2nd, 1946. May total precipitation was 5.60 inches, which was nearly two inches more than the long-term 130-year average and an inch and a half more than the 1981-2010 30-year mean. The largest rainfall of the month occurred during the last four days when 3.57 inches were observed during a pair of cold rain storms on May 28th to 31st. The first of these storms brought the greatest precipitation in 24 hours during May of 2.50 inches on the 28th-29th. An additional 1.07 inches of rain fell on the 30th-31st. No frozen precipitation was observed during May, and the final 2020-2021 seasonal snowfall was 76.2 inches, which was about 14 inches more than the long-term average. A thunderstorm day was observed on the 26th, which was two fewer than average for May, and it was the third thunderstorm day of the year. The mean wind speed for the month was 11.4 mph, and the prevailing wind direction was from the west-northwest. The monthly peak wind gust was 44 mph from the east-northeast during the rain storm on the 29th. May sunshine was above average with 241.3 hours, which was 55 percent of the possible bright sunshine and three percent more than the long-term average.

Blue Hill Observatory Spring (Mar-May) 2021 Summary:

Spring 2021 was generally very warm and sunnier than average. The 24-hour adjusted mean temperature for March-May of 48.0 deg F was 3.1 degrees warmer than the 1891-2020 130-year average for spring, and it was 1.9 degrees warmer than the 1981-2010 30-year average. Spring 2021 was the eighth warmest on record. The average maximum temperature of 58.5 deg F was 3.5 degrees warmer than the 130-year mean, which tied for the eighth warmest on record, and the average minimum temperature of 38.3 deg F was 1.6 degrees warmer than the 130-year mean. Spring 2021 precipitation was 12.66 inches, which was slightly more than the 130-year mean, but it was an inch and a half less than the 1981-2010 30-year average. Total snowfall during spring was 3.8 inches, which was more than 11 inches less than the long-term average. The mean wind speed for spring was 12.7 mph, which tied for the fifth lowest on record, and the prevailing direction was from the west-northwest. The peak wind gust during the season was 69 mph from the west-northwest on March 1st. Spring was much sunnier than average with a three-month total of 691.7 hours, which was 58 percent of the possible bright sunshine, making the season the sixth sunniest spring on record.

BHO Warmest Spring (March-May) 24-Hour Mean Temperature, deg F (1885-2021):

1) 51.3 in 2010
2) 50.9 in 2012
3) 49.4 in 1991
4) 49.0 in 1977
5) 48.4 in 1998
6) 48.2 in 1976
7) 48.1 in 1945
8) 48.0 in 2021
9) 47.9 in 1903
10) 47.8 in 2016

BHO Warmest Spring (March-May) Average Daily Maximum Temperature, deg F (1885-2021):

1) 61.4 in 2010
2) 61.1 in 2012
3) 60.5 in 1991
4) 60.3 in 1977
5) 59.6 in 1945
6) 59.0 in 1976
7) 58.8 in 1903
8) 58.5 in 1921
   58.5 in 2021
10) 58.4 in 1946

BHO Highest Spring (March-May) Sunshine, hours [percent] (1885-2021):

1) 750.6 in 1977 [63%]
2) 715.4 in 1964 [60%]
3) 709.6 in 1963 [60%]
4) 694.2 in 1911 [59%]
5) 693.8 in 1962 [59%]
6) 691.7 in 2021 [58%]
7) 689.3 in 1969 [58%]
8) 684.3 in 1934 [58%]

BHO Lowest Spring (Mar-May) Mean Wind Speed, mph (1885-2021):

1) 12.0 in 2012
2) 12.3 in 2016
3) 12.6 in 1995
   12.6 in 2018
5) 12.7 in 2019
   12.7 in 2021

Mike Iacono
Chief Scientist
Blue Hill Observatory