September 2023 Summary – Warmer and Wetter Than Average

Post-tropical cyclone and former Hurricane Lee passes east of New England on the morning of 16 September 2023. Image courtesy of the College of DuPage NEXLAB (weather.cod.edu).

Blue Hill Observatory September 2023 Summary: 

September 2023 was generally warmer and wetter than average with less sunshine than usual. The approximated 24-hour mean temperature for the month of 64.6 deg F was 3.1 degrees warmer than the 1891-2020 130-year average for September, and it was 1.5 degrees warmer than the 1991-2020 30-year average. The average maximum temperature of 72.5 deg F was 0.2 degrees cooler than the 30-year normal. The average minimum temperature of 58.4 deg F was the third warmest on record and 2.8 degrees warmer than the 30-year normal. The highest temperature was 92F on the 7th, which tied the record for the date previously set in 2007 and 2015. The lowest temperature was 46F on the 27th, which tied as the third warmest monthly minimum temperature on record for September. The total precipitation was 6.77 inches, which was 2.71 inches more than the 30-year normal and extended the generally wet pattern during the summer months. The heaviest rainfall events brought 1.92 inches in less than 24 hours on the 13th and 1.56 inches on the 18th. Those dates were also two of the six thunderstorm days during the month, which was four more than average, and the annual total increased to 28. There were impacts from two tropical cyclones including former Hurricane Lee, which brought 0.21 inches of rain and a peak wind gust to 44 mph from the west-northwest as it passed to the east as a post-tropical cyclone on the 16th and former Tropical Storm Ophelia, which brought 1.10 inches of rain and a peak wind gust to 38 mph from the east-northeast as it passed to the south a post-tropical system on the 23rd-25th. The average water vapor pressure, which is an absolute measure of water vapor in the air, tied for the second highest on record for September at 17.6 mb. The mean wind speed for September was 10.2 mph, which was the second lowest on record for the month, and the prevailing wind direction was from the east-northeast. The highest wind gust was 44 mph from the west-northwest on the 16th. September bright sunshine was below average with 190.9 hours, or 53 percent of possible, which was four percent less than the long-term average for the month.

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

1) 58.6 in 2018
2) 58.5 in 2015
3) 58.4 in 2023
4) 58.2 in 1961
   58.2 in 1999
6) 57.9 in 2016
7) 57.8 in 2005

BHO September Highest Average Water Vapor Pressure, mb (1932-2023):

1) 17.9 in 1961
2) 17.6 in 2018
   17.6 in 2023
4) 17.5 in 2011
5) 17.4 in 2021
6) 17.3 in 2015
7) 17.2 in 1934

BHO September Lowest Average Wind Speed, mph (1885-2023):

1) 10.1 in 2018
2) 10.2 in 2023
3) 10.3 in 2022
4) 10.5 in 2014
5) 10.6 in 2013

Mike Iacono
Chief Scientist
Blue Hill Observatory