September 2017 Summary – Eighth Warmest on Record

Blue Hill Observatory

September 2017 Summary:

September was notable for being generally warm, and it was drier than normal until the last day of the month. The 24-hour mean temperature for the month of 65.1 deg F was 3.9 degrees warmer than the 120-year average for September and 3.0 degrees warmer than the 1981-2010 30-year average. September was the eighth warmest on record and the warmest since a mean of 65.2 deg F in 2016. The high temperature of 86F occurred on the 25th and 27th during several days of very warm and humid weather the last week of the month, and the reading on the 27th set a new record for the date surpassing the previous record of 84F set in 1998 and 2014. The coldest temperature during September was 44F on the 30th following very heavy rain that day. A total of 5.50 inches of precipitation was measured during September, which was about one and a half inches more than the long-term 120-year average and the 30-year mean. The greatest amount in 24 hours was 2.33 inches (almost half of the monthly total) during strong thunderstorms on the 30th that also brought small hail to the Observatory. This was the last of four thunderstorm days during September, which brought the annual total up to 17. Through September, there has been at least one thunderstorm day observed in each month so far this year, which has only happened once before (in 1996) during the entire Blue Hill climate record. The mean wind speed for September was 10.8 mph, which was the fifth lowest on record for the month, and the prevailing direction was from the north-northeast. Hurricane Jose approached New England from the south on the 19th-20th and then lingered offshore for several days as it brought a total of 1.60 inches of rainfall from the 18th to 23rd. This storm also brought the peak wind gust for the month of 47 mph from the north-northeast on the 21st. September bright sunshine was close to average with 200.6 hours, or 56 percent of possible, which was one percent less than the long term mean.

BHO Warmest September 24-Hour Mean Temperature, deg F (1885-2017):

1) 67.2 in 2015
2) 66.1 in 1961
3) 65.5 in 2005
4) 65.4 in 1999
65.4 in 2010
6) 65.3 in 1930
7) 65.2 in 2016
8) 65.1 in 2017
9) 65.0 in 2002
10) 64.9 in 1983

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

1) 10.5 in 2014
2) 10.6 in 2013
3) 10.7 in 2011
   10.7 in 2015
5) 10.8 in 2017
6) 10.9 in 1917
   10.9 in 2003

Mike Iacono
Chief Scientist
Blue Hill Observatory