The numbers are in and confirm the obvious: It was one of the wettest Septembers on record.
But it wasn’t just unusually wet — it was also abnormally cloudy, warm and humid. This muggy and moist combination yielded Washington’s swampiest September.
Here are some of the numbers that bear this out:
September’s 9.73 inches of rain ranked as the fifth most on record, edging ahead of super-wet 2011 to make the top five. Rainfall was 6.01 inches above normal.
It rained so frequently that it turned into a challenge to find time to spend outside.
So it might come as a surprise to Washingtonians who were holed up indoors that the month also ranked as the fifth warmest on record, tied with 2010.
The month’s average temperature of 75.5 was 4.4 degrees higher than normal. Fifty percent of the 10 warmest Septembers have occurred since 2000:
No previous September on record in Washington that was this warm was also this wet (or a September this wet, this warm). In other words, the combination of warmth and dampness this past month was the most extreme in 137 years of records.
September weather extremes and records
More than half of the days last month reported measurable rain, and if you add the days with just trace amounts, then 70 percent of the month (21 of 30 days) saw dampness. The heat at the front of the month was impressive, too. The high temperature of 95 degrees Sept. 4 was higher than on any day in August this year.
During the month, numerous records were set for daily rainfall and high overnight temperatures, as summarized below:
- Sept. 4: Record-high temperature of 95 tied 2008.
- Sept. 4: Record-high low temperature of 79 topped 76 from 2012.
- Sept. 5: Record-high low temperature of 78 tied 2012.
- Sept. 6: Record-high low temperature of 78 topped 77 from 1983.
- Sept. 9: Record rainfall of 1.59 inches topped 1.48 inches set in 1880.
- Sept. 4: Record-high low temperature of 72 topped 71 from 2015.
- Sept. 6: Record-high low temperature of 72 tied 1996.
- Sept. 9: Record rainfall of 1.4 inches topped 1.34 inches from 1999.
- Sept. 9: Record rainfall of 2.23 inches topped 1.35 inches from 1950.
- Sept. 17: Record rainfall of 2.22 inches topped 2.13 inches from 2003.
September weather pattern
A zone of high pressure over Bermuda was the dominant weather feature during September. It helped pump very warm and humid air into the region, which resulted in some of the record heat and frequent downpours. The entire eastern United States was much warmer than normal:
January-through-September temperature and precipitation rankings
For the past few months, 2018 had been trailing 2003 as the wettest year since 2000. Even though Washington was hit by the rains from Hurricane Isabel in 2003, this year still managed to surpass that year to move into the wettest position by about an inch and a half:
While September was quite warm, the overall 2018 average still trails several years since 2000 among the warmest. But it is slowly moving up the rankings and is in the sixth-warmest position:
September forecast evaluation
A month ago, just ahead of September’s start, we made the following predictions:
Temperatures: We predict temperatures to run above normal for the fifth month in a row, averaging between 74 and 77 degrees, which is warmer than last year’s 72.5 (the 30-year average is 71). We could get very close to or even exceed 1980s all-time warmest September of 77.1 degrees.
Rainfall: We expect about four to five inches, which is wetter than the normal of 3.72 inches and last year’s measly 1.43 inches. This could be the wettest September in six years and the sixth wetter-than-normal month in a row.
Here is what happened:
Temperature: 75.5 degrees — right inside the predicted range.
Rain: 9.73 inches. It was a blowout. We got the idea of a wetter-than-normal month correct, but our outlook was not wet enough.
Our view of a warmer- and wetter-than-normal month was verified, but the rain intensity was off. We give our outlook a B-plus.