The white dots on the graph above show the HLA (high-low average) for each month since January 2010, while the purple lines show the trend for September.
(Copyright © 2017 by Wil C. Fry. Some rights reserved.)
Following August’s lead, September 2017 was the coolest of all the Septembers since we moved here in 2010. With an HLA of 80.6°F, it barely dropped below 2012’s mark. It is notable that September is the least-variable of the 12 months, only varying 3.2° from warmest to coolest. Unlike August, September is not on a general cooling trend; it warms and cools regularly.
This was the first September in eight years with zero 100-degree days. It’s only the second time that September saw fewer 100-degree days than did June. If we’ve seen the last 100-degree day of this year, then it’s the earliest end since we moved here, and the shortest span from first to last — June 23 through Aug. 19, only 57 days.
Eighteen days hit 90°F or higher, which is a tie for the fewest among the past eight Septembers.
98°F was the high for the month, the lowest September high since we moved here, and the fourth time that September didn’t beat June’s high.
62°F was our low, which is warmer than average. This follows a several-year trend of our lows getting warmer while our highs flatten out.
We received 1.15 inches of rainfall in September, the third-driest September. While September still averages more rain than August, it’s getting close. The past five Augusts average more rain than the past five Septembers.
Six days saw precipitation, which is exactly average for September.
Earlier this year, due to the surprising warmth, I began to suspect 2017 would surpass 2012 as our hottest year of all time. It is still possible — 2012’s summer wasn’t overly remarkable for its heat; the record came from the winter, fall, and spring months. The surprisingly cool August puts a serious dent in 2017’s chances, but September was almost identical to 2012’s.
Four of 2012’s monthly HLA records still stand (January, April, May, and December). In order for 2017 to overtake 2012 at this point, we need to be above average temperature for at least two of the next three months, which is easily possible. October has been on a strong warming trend the past eight years, November could go either way, and December looks due for another warm one too.
Rainwise, we have already received more rain than two entire years (2011 and 2014), and if current trends continue, we’ll beat all but 2015. More than any of the past seven years, 2017 has seen the rain spread evenly throughout most months, with few long dry spells.