September 2018 Summary – Eighth Wettest and Second Cloudiest on Record
Blue Hill Observatory
September 2018 Summary:
September got off to a warm start and brought much above normal precipitation. The 24-hour adjusted mean temperature for the month of 64.8 deg F was 3.6 degrees warmer than the 120-year average for September and 3.4 degrees warmer than the 1981-2010 30-year average. The average maximum temperatures was 72.5 deg F, and the average minimum temperature of 58.6 deg F was the second warmest ever observed in September. There were two ninety-degree days, which is two more than average, and the yearly total is up to 14, which is eight more than average. The highest temperature for the month was 92F on the 3rd and 6th, and the reading on the 6th tied the record for the date set in 1900. The lowest temperature was 47F on the 23rd, which was the second warmest monthly minimum temperature ever observed in September, behind only 48F in 1934. A daily record high minimum temperature of 70F was observed on the 4th. A total of 9.25 inches of precipitation was measured, which was more than five inches wetter than the long-term 120-year average and the 30-year mean. September was the eighth wettest on record, and it brought the most precipitation in a single month since 9.96 inches fell in September 2014. The greatest amount in 24 hours was 3.34 inches during thunderstorms on the 18th, which included 1.76 inches in one hour from 10-11 AM EST and 2.75 inches in two hours from 10AM-Noon EST. The measurement of 1.76 inches was the highest rainfall in a single hour since 3.02 inches was observed on June 13, 1998. There were five thunderstorm days during September (three more than average for the month), which brought the annual total up to 21. During September, the average water vapor pressure, a measure of moisture in the air, was 17.6 mb, which was the second highest on record. The mean wind speed was 10.1 mph, which was the lowest ever measured for September, and the prevailing direction was from the NE. A peak gust of 37 mph from the south-southwest occurred on the 21st, from the southwest on the 22nd, and again from the east-northeast on the 24th. September began and ended sunny, though the month generally had much below average sunshine with only 126.8 measured hours of bright sunshine, or 35 percent of possible, which was 22 percent less than the long-term mean of 57 percent. September was the second cloudiest on record behind only 120.7 hours (34 percent) in 1934.
BHO Warmest September Average Minimum Temperature, deg F (1885-2018):
1) 58.8 in 1988
2) 58.6 in 2018
3) 58.5 in 2015
4) 58.2 in 1961
58.2 in 1999
BHO Warmest September Monthly Minimum Temperature, deg F (1885-2018):
1) 48 in 1934
2) 47 in 2018
3) 46 in 1921
46 in 1994
46 in 1999
46 in 2002
46 in 2003
46 in 2008
46 in 2010
46 in 2015
BHO Wettest Septembers, inches (1886-2018):
1) 12.37 in 1999
2) 11.04 in 1933
3) 10.23 in 1896
4) 10.00 in 1961
5) 9.62 in 1987
6) 9.58 in 1888
7) 9.36 in 1918
8) 9.25 in 2018
9) 8.81 in 1907
10) 8.78 in 1996
BHO Lowest September Mean Wind Speed, mph (1885-2018):
1) 10.1 in 2018
2) 10.5 in 2014
3) 10.6 in 2013
4) 10.7 in 2011
10.7 in 2015
BHO Highest September Water Vapor Pressure, mb (1932-2018):
1) 17.9 in 1961
2) 17.6 in 2018
3) 17.5 in 2011
4) 17.3 in 2015
5) 17.2 in 1934
BHO Lowest September Sunshine, hours (1886-2018):
1) 120.7 in 1934 [34%]
2) 126.8 in 2018 [35%]
3) 130.8 in 1889 [36%]
4) 131.1 in 1996 [36%]
5) 133.0 in 1907 [37%]
Mike Iacono
Chief Scientist
Blue Hill Observatory
