May 2025 Summary – Eighth Wettest on Record

Blue Hill Observatory May 2025 Summary:

May 2025 was warmer and much wetter than average with much less sunshine than expected. The approximated 24-hour mean temperature for the month of 57.4 deg F was 1.9 degrees warmer than the 1891-2020 130-year average for May, and it was 0.7 degrees warmer than the 1991-2020 30-year average. The average maximum temperature of 67.0 deg F was 0.2 degrees warmer than the 30-year normal, and the average minimum temperature of 49.3 deg F was 1.1 degrees warmer than the 30-year normal. It was generally a warm month with a notable cool period from the 20th-25th, which included a slow-moving, late-season nor’easter. The warmest temperature during May was 84F on the 3rd, and the coldest temperature was 41F on the 23rd. The mean water vapor pressure, which is an absolute measure of water vapor in the air, was 12.8 mb, which corresponds to a mean dew point of about 51F. This amount was above the long-term mean water vapor pressure for May of 10.5 mb, which equals a mean dew point of about 46F. The total precipitation was 7.62 inches, which was 3.80 inches more than the 30-year normal, and the month was the eighth wettest May on record. The largest storm of the month was a late-season, coastal nor’easter that brought a storm total of 3.74 inches of rain on the 22nd-23rd including a thunderstorm on the 22nd. The greatest amount in 24 hours was 3.73 inches on the 22nd-23rd. There were five thunderstorm days during May, which was two more than average. There was no additional snowfall during May, and the final 2024-25 seasonal snowfall was 35.8 inches, which was 33.9 inches below the 30-year normal. The minimum sea-level pressure during the month was 29.04 inches on the 31st, which tied as the lowest sea-level pressure ever observed in May at Blue Hill. The maximum sea-level pressure was 30.39 inches on the 12th. The mean wind speed was 11.6 mph, and the prevailing wind direction was from the south-southwest. The highest wind gust was 46 mph from the northeast on the 22nd and from the west on the 31st. May bright sunshine was well below average with 175.1 hours, or 40 percent of possible, which was twelve percent less than the long-term average for the month.

BHO Highest May Precipitation, inches (1885-2025):

1) 10.10 in 2006
2) 9.16 in 1954
3) 9.14 in 1984
4) 8.65 in 2005
5) 8.18 in 1998
6) 7.91 in 1967
7) 7.81 in 1990
8) 7.62 in 2025
9) 7.55 in 1920
10) 6.80 in 2002

BHO Lowest May Sea-Level Pressure, mb (1885-2025):

1) 29.04 on 8 May 1967
   29.04 on 31 May 2025
3) 29.07 on 1 May 2023
4) 29.12 in May 1945
5) 29.15 in May 1938
6) 29.18 on 17 May 1916
7) 29.19 on 19 May 1976
8) 29.22 on 18 May 1985

Mike Iacono
Chief Scientist
Blue Hill Observatory