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Posted at 10:00 AM ET, 06/ 2/2009

Not Your Typical May - Or Was It?

By Capital Weather Gang

* Warm Followed by Showers/Storms: Full Forecast | NatCast *

By Don Lipman, Guest Contributor


If you're an outdoor tennis enthusiast, as I am, there wasn't much to cheer about last month. We didn't set any monthly rainfall records and barely broke a daily record (on the 26th), but it wasn't for lack of trying.

Though you may think that we haven't had such a rainy May in a long time, think again. We need go back only one year -- to 2008 -- to find a wetter May. In fact, May 2008, with 10.66 inches of rain (one third more than last month), was our third wettest May since records began, at least at Reagan National Airport (DCA), which is Washington's official measuring station! Including the weather records prior to DCA dating back to 1871, May 2009 will go down as the 6th wettest May with 8.05 inches.

It was probably the number of rainy days (21) last month that made it particularly unpleasant. Average is only 11. It's like comparing Seattle's annual precipitation to that of D.C. Most people would say that Seattle has a much rainier climate--and it does, but only because there are many more days (38% more) of overcast, drizzly days than in D.C. Annual precipitation, however, is actually slightly less in Seattle than in D.C.

Keep reading for more on May's weird weather, or was it...?

Actually, soggy Mays are not unusual at all in Washington. Statistically, it's the wettest month of the year--again, at DCA. From 1970 to 2000, the period upon which the current averages, or normals, are based, May has averaged 3.82 inches of rain (though September is a close second, averaging 3.79", and is one day shorter). By comparison, the driest month is February, but on a technicality because it's so short.

The wettest month ever (at least back to 1870) in D.C. was September 1934, with 17.45 inches of rain, an amount so large that it undoubtedly resulted from one or more tropical systems. And just for good measure, the driest month ever in D.C. was October 1963, when only a trace of rain fell. Finally, the wettest year of all was 1889, which piled up 61.33 inches (over 55% above normal) and the driest was the dust bowl year of 1930, which could only muster 21.66 inches of precipitation (45% below normal).

In the thermal department, the highest recorded temperature was 86°, one of the rare instances in which April's high temperature (92°) exceeded that of May. Otherwise, it was a fairly typical month, averaging 65.5 degrees, just .1° below normal (at DCA) but on the warm side at Dulles (IAD), which averaged 65.0 degrees, or 2.7 degrees above normal. (Average temperatures are so much lower at Dulles primarily because of the lower nighttime temperatures.) There were no daily temperature records set.

Following are a few of the May weather highlights at the Reagan National (DCA) and Dulles Airport (IAD) measuring stations:

Highest temperature 86° on the 9th 86° on the 23rd
Lowest temperature 45° on the 19th 38° on the 19th
Highest low temperature 68° on the 24th/25th 66° on the 16th/24th
Lowest high temperature 56° on the 4th 56° on the 4th
Average temperature 65.5° 65.0°
Consecutive days with a trace or more of rain (a dreary period)
1st to the 9th 1st to the 9th
Wettest day
26th - 1.65" 26th - 3.59"
A trace or more of rain fell on... 21 days 20 days

I'll leave you with a couple more interesting bits of climate trivia, courtesy the National Weather Service:
  • At DCA, total rainfall for the first three months of 2009 was just 5 inches, the fourth driest such period on record. In contrast, the April-May two-month period totaled 12.27 inches, making it the 5th wettest such period.
  • At IAD, May 2009 tied (with 1988) for the wettest on record (where records date back to just 1963) with 10.26", and was the wettest month there since June 2006.

About the author: After a 30 year "Intelligence Community" career, Don's second life focuses on tennis and weather. When not on the courts, he writes weather articles, which have appeared in community newspapers/newsletters and gives weather talks at senior centers and on cruise ships. He's doing a weather "gig" on the Emerald Princess on August 24th in the Baltic. Interested in going along?

By Capital Weather Gang  | June 2, 2009; 10:00 AM ET
Categories:  Lipman, Local Climate  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: Summer Heat, Then Spring Showers
Next: PM Update: Storms Back in Forecast, Severe?


Thanks, Don. I think we go through multi year boom and bust periods when it comes to rain. In 08 and now 09, we may be in a bust. A couple of years ago, people were complaining about a severe drought. It all averages out. Or, in the words of Kramer, "You're even steven!"

Posted by: jojo2008 | June 2, 2009 10:35 AM | Report abuse

Edit: I meant to say that in 08-09 were in a boom.

Posted by: jojo2008 | June 2, 2009 10:36 AM | Report abuse

As I've been saying, last month SEEMS to have been more rainy than May, 2008.

I didn't have so much trouble getting to dances, since there didn't seem to be so much rain during critical evening hours. The long spells of raw dreary weather last month made it feel like an extension of winter. There has also been more windy weather this spring, but not always in rainstorms.

I think we had more severe weather during May, 2008.

Posted by: Bombo47jea | June 2, 2009 11:56 AM | Report abuse

how about Dulles, with over 2" more than National, it must have been higher than the 6th wettest May on record. May is clearly our rainy season from thunder storms and such, while September seems to be our second rainiest month, usually from the tropical storms. Correct?

Posted by: rocotten | June 2, 2009 1:03 PM | Report abuse

As previously posted elsewhere, both Dulles and Ballmer were closer to records.

On average, May is wettest by absolute total, but Sept. is slightly higher per diem (30 days vs. 31).

Posted by: CapitalClimate | June 2, 2009 1:57 PM | Report abuse


As the last bullet in the above article states: "At IAD, May 2009 tied (with 1988) for the wettest on record (where records date back to just 1963) with 10.26", and was the wettest month there since June 2006."

Posted by: CapitalWeatherGang | June 2, 2009 2:17 PM | Report abuse

Save your hate mail; having been subjected to 3 years of nit-picking at PM Update (one of the main reasons for spinning off CapitalClimate), the Climate Capitalist knows his nits.

In no particular order of importance:
- If the wettest month ever is counted since 1870, then the wettest year ever was 61.33" in 1889, not 2003.
- "Days with precipitation" is mixing apples and oranges (or as my fortune cookie said, cheng zi).
The 21 for this year includes 5 days with only a trace, so the correct number to compare with the average of 11 is actually 16. (BTW, the 2 days with 1"+ were only 1.1 above average, and the number of cloudy days, 13, was 1 less than average.)
- If February is driest by a technicality (number of days), then May is only technically wettest, since the current average (3.82", not 3.89") makes it slightly drier than September's 3.79" on a daily basis.
- A little checking shows Sept. 1934 had 3.78" on the 7th from a tropical storm passing by Hatteras, but there was no organized tropical activity in sight for the 6" on the 12th-14th. Tropical activity did add 2.2" on the 16th.

Posted by: CapitalClimate | June 2, 2009 3:48 PM | Report abuse


We addressed the errors you identified.

Posted by: Jason-CapitalWeatherGang | June 2, 2009 9:01 PM | Report abuse

"The wettest month ever (at least back to 1870) in D.C. was September 1934, with 17.45 inches of rain, an amount so large that it undoubtedly resulted from one or more tropical systems."

That doesn't make any sense. IAD (and my neighborhood in Silver Spring) received about between 10" and 15" of rain last month and we had zero tropical systems. DCA simply didn't experience the brunt of the storms we had in May like a good portion of DC proper, and the close-in northern and western suburbs.

Posted by: crzytwnman | June 5, 2009 4:20 PM | Report abuse

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