May 2017 Summary – Cooler and Wetter than Average

Blue Hill Observatory

May 2017 Summary:

May was generally cool with a few very warm days and was slightly wetter than average. The 24-hour mean temperature for the month of 54.9 deg F was 0.4 degrees cooler than the 120-year average for May and more than one degree cooler than the 1981-2010 30-year average. The month was the coldest May since 2008, and it got off to a very cold start. The average high temperature for the first fifteen days of the month was only 56.8F, which was the sixth coldest on record for that period. For the 1st to the 15th, the high temperature of 65F was the lowest for the first 15 days of May since records began in 1885. The average maximum temperature for the month of 63.5F was about three degrees cooler than average, and the average minimum temperature of 47.1F was less than a degree cooler than average. Despite the generally cool weather, there was a brief period of warm temperatures from the 16th to the 19th. This period included the highest temperature of the month of 96F on the 18th, which was not only a daily record that surpassed the previous record for the date of 88F set in 1906, 1936 and 1986, it was also a new monthly temperature record that surpassed the previous record for any day in May of 94F set on May 26th, 2010. The high temperature of 90F on the 17th was also a daily record that surpassed a reading of 88F set in 1974 and 1977. The coldest temperature during May was 36F on the 9th. A total of 4.53 inches of precipitation was observed, which was about three-quarters of an inch more than the long-term 120-year average and a little under a half inch more than the 30-year mean. The greatest amount in 24 hours was 1.67 inches on the 13th-14th. There were two thunderstorm days during May on the 18th and 25th. The mean wind speed was 11.1 mph, which was the second lowest on record for May, and the prevailing direction was from the south. The peak gust was 46 mph from the south-southeast on the 6th. May bright sunshine was much below average with 159.5 hours or 36 percent of possible, which was 16 percent less than the long-term mean. This sunshine duration was the seventh lowest on record for May.

BHO Coldest 1st-15th May Average Maximum Temperature, deg F (1885-2017):

1) 48.6 in 1917
2) 55.3 in 1885
3) 56.0 in 1962
4) 56.1 in 1921
5) 56.5 in 1954
6) 56.8 in 2017
7) 56.9 in 1920
8) 57.0 in 1907

BHO Coldest 1st-15th May Maximum Temperature, deg F (1885-2017):

1) 65 in 2017
2) 66 in 1917
3) 68 in 1962
68 in 1967
5) 69 in 1966
6) 70 in 1898
70 in 1928

BHO Lowest May Mean Wind Speed, mph (1885-2017):

1) 10.0 in 2012
2) 11.1 in 2017
3) 11.2 in 1982
   11.2 in 2003
5) 11.3 in 2013
6) 11.4 in 1999
   11.4 in 2014

BHO Lowest May Bright Sunshine, hours (1885-2016):

1) 136.1 in 1888 [31%] 2) 147.4 in 2005 [34%] 3) 150.8 in 1927 [35%] 4) 152.4 in 1983 [35%] 5) 155.0 in 2003 [35%] 6) 155.4 in 1948 [36%] 7) 159.5 in 2017 [36%]
8) 168.5 in 2011 [39%] 9) 169.7 in 1917 [39%] 10) 170.7 in 1892 [39%]

Spring (Mar-May) 2017 Summary:

Spring 2017 featured very changeable weather with a warm April, a cool March and May and more rainfall than average. The 24-hour mean temperature for Spring of 45.5 deg F was 0.7 degrees warmer than the 120-year average and 0.6 degrees cooler than the 1981-2010 30-year average. The season started with the unusual occurrence of March being colder than February, which was followed by the third warmest April on record. May was generally cooler than average, yet a new monthly record high temperature of 96F was set during a brief warmup during the middle of the month. The season was wet with a total of 16.06 inches of precipitation, which was almost four inches more than the long-term mean and about two inches more than the 30-year mean. The total spring snowfall of 14.1 inches was slightly less than average, and the 2016-2017 snowfall season ended with a total of 66.4 inches, a few inches more than average. Spring bright sunshine was 46 percent of possible, which was four percent less than average.


Mike Iacono
Chief Scientist
Blue Hill Observatory