December 2023 Summary – Second Warmest on Record

Blue Hill Observatory December 2023 Summary: 

December was exceptionally warm with more precipitation but less snowfall and sunshine than expected. The approximated 24-hour mean temperature for the month of 38.3 deg F was 8.2 degree warmer than the 1891-2020 130-year average for December, and it was 5.5 degrees warmer than the 1991-2020 30-year average. The average maximum temperature of 45.6 deg F was 5.4 degrees warmer than the 30-year normal, and it was the third warmest on record. The average minimum temperature of 31.8 deg F was 6.9 degrees warmer than the 30-year normal, and it was the second warmest on record. The December average minimum temperature matched the average minimum observed in November. Only once before has this been achieved at Blue Hill, in 1996, when the December average minimum temperature that year, 30.5F, slightly exceeded 30.1F in November. The warmest days in December reached 61F on the 11th and 18th, which was a new daily record for the 18th surpassing 59F in 1928. The lowest temperature was 16F on the 22nd, and this reading was the warmest December monthly minimum since 23F, the record for the month, was observed in 2015. The total precipitation was 7.33 inches, which was 1.94 inches more than the 30-year normal. Much of the monthly rainfall, 2.94 inches, fell during the largest storm on the 17th-18th, which also brought record warmth, wide-spread power outages, and a daily record wind gust to 90 mph. Nearly all of the rainfall, 2.93 inches, fell within 24 hours on the 17th-18th. Another major warm, rainstorm brought 2.62 inches of precipitation, with periods of heavy rain, in less than 24 hours on the 10th-11th. The only frozen precipitation was 0.3 inches of snow on the 6th, which was 12.3 inches below normal for December and the ninth lowest on record for the month. This snowfall ended a period of 266 days without measurable snowfall, which was the sixth longest on record. There were no thunderstorm days, which was typical for the month. Mean water vapor pressure for December was 6.7 mb, which was the second highest on record. The lowest sea-level pressure was 28.96 inches during the storm on the 18th, which was the lowest in December since 28.82 inches was recorded on December 27th, 2010.  The mean wind speed was 11.2 mph, which was the lowest on record for December, and the prevailing wind direction was from the west-northwest. The highest wind gust was 90 mph from the south-southeast during the rainfall on the 18th. This wind gust surpassed the previous record for the date of 76 mph set in 1978. It was the strongest wind gust at Blue Hill since 91 mph was observed on January 27th, 1996, and it was one of the strongest gusts on the summit of Great Blue Hill since 1980. December sunshine was below average with 119.7 hours of bright sunshine, or 44 percent of possible, which was two percent less than the long-term average for the month.

BHO Highest December Approximated 24-Hour Mean Temperature, deg F (1885-2023):

1) 42.9 in 2015
2) 38.3 in 2023
3) 38.1 in 2006
4) 37.5 in 2011
5) 37.3 in 2001
6) 37.2 in 1891
7) 36.9 in 1998

BHO Highest December Average Maximum Temperature, deg F (1885-2023):

1) 50.5 in 2015
2) 45.9 in 1891
3) 45.6 in 2023
4) 45.5 in 2006
5) 45.4 in 2011
6) 45.3 in 1990
7) 45.1 in 1953

BHO Highest December Average Minimum Temperature, deg F (1885-2023):

1) 36.9 in 2015
2) 31.8 in 2023
3) 31.2 in 2006
4) 30.8 in 2001
5) 30.5 in 1996
6) 30.1 in 2012
7) 29.7 in 2021

BHO Lowest December Snowfall, inches (1885-2023):

1) T in 1899
   T in 1953
   T in 1999
   T in 2011
5) 0.1 in 1943
   0.1 in 2018
7) 0.2 in 1936
   0.2 in 1957
9) 0.3 in 2023

BHO Longest Periods Between Measurable Snowfall Days (1886-2023):

1) 304 days, 15 March 1999 – 13 January 2000
2) 287 days, 17 March – 29 December, 1941
3) 276 days, 28 March – 29 December, 2015
4) 270 days, 6 March – 1 December, 1902
5) 269 days, 22 March – 16 December, 1973
6) 266 days, 15 March – 6 December, 2023
7) 265 days, 12 March – 2 December, 1983
8) 264 days, 13 March – 2 December, 1966
9) 262 days, 12 March – 29 November, 1948
   262 days, 14 April 1953 – 1 January 1954

BHO Highest December Mean Water Vapor Pressure, mb (1885-2023):

1) 7.8 in 2015
2) 6.7 in 2023
3) 6.2 in 1996
4) 6.0 in 1973
    6.0 in 1982
    6.0 in 2012

BHO Highest Wind Gusts Since 1980, mph (1980-2023):

1) 100 SSE on 27 September 1985
2) 95 S on 11 January 1980
   95 S on 11 February 1981
   95 ENE on 29 March 1984
5) 91 SSE on 27 January 1996
6) 90 SSE on 18 December 2023
7) 89 SSE on 28 December 1983
8) 87 SSE on 19 January 1996

BHO Lowest December Mean Wind Speed, mph (1885-2023):

1) 11.2 in 2023
2) 11.8 in 2015
3) 12.0 in 2017
4) 12.1 in 2021
5) 12.2 in 2014
6) 12.3 in 2013
7) 12.4 in 2018

Mike Iacono
Chief Scientist
Blue Hill Observatory