Washington has had 20 more inches of rain than normal since April and over five inches more than normal in September. (National Weather Service)

By Jason Samenow and Ian Livingston

We have all kinds of stats that confirm what is obvious to Washingtonians: This month is one of the cloudiest and wettest Septembers the District has ever seen. The month is doing more than its fair share to put 2018 in the running for Washington’s wettest year on record.

You’re forgiven if you’re fed up with it.

“I would pay so much money for it to never, ever rain again,” tweeted Carly Cloud.
“This is awful!” added Lisa Dunn on Twitter.

September by the numbers

Through Sunday, Washington had posted 8.25 inches of rain for the month, which is 5.44 inches above normal and ranks fifth-most on record (through Sept. 23).

Rain falling early this week could easily push that number over nine or even 10 inches.

Washington’s rainiest September on record occurred in 1934, when an astonishing 17.45 inches fell. That record is safe, and the second-place total of 12.36 inches from 1975 probably is, too. But a top-five finish above 8.84 inches (from 2011) seems like a good bet this year. 

The rain this month has been both persistent and, at times, heavy. We’ve registered at least a trace of rain on 17 of 24 days and measurable amounts on 12 days. At least an inch has fallen three times (Sept. 7, 9 and 17).

Even on days when it hasn’t rained, stuck weather systems have frequently cast a bleak shadow over
the region. Through Sept. 24, we’ve compiled 14 cloudy days, seven partly cloudy days and just three sunny days.

Cloudy skies have prevailed on 13 of the past 16 days, and, no, this is not normal.
The top panel on the chart below shows it has been overcast at noon 70 percent of the time this September, way above the long-term average (climatology) of 27 percent. The chart also shows that every month this year has had more overcast midday hours than normal.

Top: Percent of the time it has been overcast at noon each month this year, compared with the long-term average (or "climatology"). Bottom: The frequency of overcast skies at noon during each September year from 1950 to 2018, but some years are missing. (IEMbot)
Although some data is missing, the bottom panel in the chart above shows that the 70 percent frequency of overcast skies at noon this September is the highest of any year on record (dating to 1950).  

2018 by the numbers

September’s 8.25 inches of rain (and counting) has pushed the 2018 precipitation total in Washington to 48.35 inches, which is more than 19 inches above normal. It ranks as the third-greatest amount on record year to date, trailing only 1886 and 1889.

Since April, Washington has picked up 40.70 inches of rain, which is the most on record. During this stretch, we’ve observed at least an inch of rain on 15 days, also the most on record. The 23.17 inches of rain we’ve seen since July 1 is the second-most on record.

With more than three months left this calendar year, Washington already has sailed past its average annual total of 39.74 inches. If no more rain fell the rest of the year after Sunday, 2018 would still rank as 23rd-wettest on record.

The record yearly rainfall total of 61.33 inches, set in 1889, is perhaps not out of reach. Let’s say, for example, Washington ends September with 50 inches on the year. Then it would need to average 3.78 inches over the remaining three months or a total of 11.34 inches to break the record. The normal rainfall from October through December is 9.62 inches, so Washington would need to exceed the normal amount by just a little each month to stay on record pace.