The cloud cover quantity in September was quite impressive as only two days qualified as mostly sunny (30% or less cloud cover). Reagan Washington National Airport (DCA) recorded at least a trace of rain on 18 days, tying 1934 and 1957 for third most such days on record. September 1888, the leader, had 20 days whereas 1945, second on the list, had 19.

Top 10 wettest Septembers on record in Washington, D.C.

Certainly the biggest frustration was the unceasing cloud cover, but there is some silver lining to this story.

We never hit 90-degrees last month. Not once. Last year we reached or exceeded 90F ten times (1/3 of the month). But not hitting 90-degrees is not super-special in our recent memory. We failed to reach that hot level just two years ago in 2009 too. And the normal number of 90-degree days seen over the 1981-2010 period is merely three. Our coolest morning low was 49F on Sep 16th with our warmest daytime high being a tie between Sep 4 and 14th at 87F. The normal temperature drops from around 84F at the start of the month to 74F by the end.

You can imagine with that level of change, we should see more variability compared to our core summer months. And indeed the high temperature trace of Sep 2011 (shown below) is quite variable and very different compared to the much hotter September 2010.

September 2011 high temperatures versus September 2010 high temperatures in Washington, D.C.

The September pattern was definitely unusual to some extent as frequent upper level low pressure areas stalled in the Midwest. That gave us a September “range” of warm/humid to cool/wet patterns. It is interesting to see that the main September warmth was seen in the West and to our north across Southern Canada instead.

September 2011 featured persistent upper level low peressure just west of the Ohio Valley., generating heavy cloud cover over the eastern third of the U.S. (NOAA)

And October 2011?

A phenomenon of cooler-than-normal waters in the central Tropical Pacific, known as La Niña, is gradually building again. Historically, that pattern yields a warm October for the Eastern U.S. Precipitation is a tougher forecast, but the historical analogs in recent years tend to lean in the wet direction. And it seems we will continue to see precipitation opportunities to keep October active on the rain front with above normal precipitation (sorry). You can see the National Weather Service’s final October temperature and precipitation outlook here: October temperature and precipitation outlook

Further Information:

The National Weather Service publishes nice monthly assessments usually within a week of the close of each month (should be available shortly):

You can click on your closest airport location here:

Reagan National (DCA)

Dulles Airport (IAD)

Baltimore-Washington (BWI)

Historical Washington, DC data provided by NOAA. See also: Ian’s excellent rundown on September climatology

(Ian Livingston contributed to this post.)