Doppler estimated rainfall on July 11, 2011 (NOAA)

Last night’s thunderstorms, a few of which were severe, produced two bands of meaningful rain. One band tracked from northern Loudoun county through Montgomery county then into northern Prince George’s and Anne Arundel counties. A second band tracked from Prince William county through Charles county. Both bands generally dropped about 0.25-0.5” of rain, with some isolated areas receiving 0.5-1”.

But in between these two bands, there was a noticeable gap in precipitation (where less than 0.1” fell from southern Loudoun county through central and southern Fairfax county into the southern part of the District and southern Prince George’s county), a recurring theme in 2011.

This storm event marked the 5th occurrence of a large dis­cern­ible split in precipitation inside the beltway this spring/summer. It begs the question: is this merely just a coincidence?

Weather radar comic. . (Source )

Take a look at the four other D.C. split scenarios from earlier this spring and summer...

July 8, 2011 doppler estimated rainfall. (NOAA)

Just last Friday, appreciable rain avoided central and eastern Fairfax county and western parts of the District while soaking surrounding areas, as shown in the image above.

Doppler estimated rainfall on June 28, 2011. (NOAA)

Two weeks before that, on June 28, two bands of heavy precipitation largely remained north and south of the beltway.

Doppler estimated rainfall June 5-12, 2011 (NOAA)

After rain played keep away from the metro region June 5-12, I posted the image above, and wrote: “most of the region inside the beltway grossly underachieved in the precipitation department with multi-day totals of less than 0.10”

Doppler estimate rainfall May 16-19, 2011 (NOAA)

And on May 20, the granddaddy of all splits - the D.C. dry tongue - showed itself off after the beltway region eluded deluge after deluge for three straight days.

Each split has a slightly different configuration and orientation so I remain unconvinced there’s some atmospheric phenomenon or grand conspiracy causing this.

90 Day precipitation totals from April 12 through July 12 (NOAA)

And when you look at the total precipitation over the last 90 days (above), encompassing these five split scenarios, we find precipitation over the District (and close-in suburbs) is just slightly less than its surroundings.

Having said that, Reagan National Airport’s consistent proximity to the “rain hole” makes me a little skeptical about a recent study which found aircraft increase rain and snow around major airports. But then I suppose it’s hard for precipitation to be enhanced by planes, if storms don’t interact with them in the first places (i.e. the storm complexes keep avoiding the airport)

”How much rain fell?” resources

After storms, we got a lot of questions about where you can find out how much rain fell. Here are a few suggestions:

NWS daily climate report for Reagan National

NWS daily climate report for Dulles

NWS daily climate report for BWI

Community Collaborative Rain, Hail, and Snow Network - (Rainfall reports from citizen observers, you can search Maryland, Virginia, and the District)

WeatherBug station network (type in your zipcode for station nearest you)

National Weather Service Mesonet (select precipitation overlays)

Zoomable doppler estimated rainfall imagery (NOAA)

Doppler estimated rainfall image archive (NOAA)