From the ABC 7 Weather team

Neat facts from D.C.'s recent rain and flooding episodes

September 12, 2011 - 02:43 PM
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Fairfax County got an epic rainfall that comes only once every 500 year, plus other sodden factoids from D.C.'s recent storms.

Total rainfall from 1 p.m. Monday to 11 a.m. Friday, as estimated from radar readings. Washington, D.C., is located at top middle. Red = 5 to 10 inches, purple - 10 to 15 inches, gray = more than 15 inches. (NWS)

Still air-drying your welcome mats and cleaning fish from your car's glove box? Then perhaps you'll be interested in these far-out rain stats from Jason Elliott, a hydrologist at the National Weather Service. Looks like a rain this large in Fairfax and certain other suburbs typically only arrives once every 500 to 1,000 years. (If that map above looks familiar, it's because it is an updated version of the radar-estimated rainfall amounts posted here last week.)

Maryland, Wednesday

* The Bowie IFLOWS gauge recorded 4.57 inches in 3 hours, which is about a 200-year rainfall (based on precip frequency from Glenn Dale).

* For Upper Marlboro and near Ellicott City, Wednesday's rains were a roughly one in 50-100 year event.

* For Westview (near I-70 and the Baltimore Beltway), Wednesday's rains were a roughly one in 10-25 year event.

Virginia, Thursday

* The Kingstowne IFLOWS gauge (near Franconia) in Fairfax County recorded 5.47 inches in 3 hours, which is approximately a 500-year rainfall for that timeframe (based on precip frequency from Vienna & Clarendon).

* The Reston IFLOWS gauge in Fairfax County recorded 6.57 inches in 6 hours, which is also approximately a 500-year rainfall (based on precip frequency from Dulles).

* The Fort Belvoir AWOS (KDAA) reported 7.03 inches in 3 hours, which is off the charts above a 1000-year rainfall (based on precip frequency from Quantico).

Washington, D.C.

For a wide swath in the heavy rain axis thru the DC and Baltimore metro areas, rainfall was at least a one in 10-25 year event.

These stats were sent out today by NWS spokesman Chris Strong, who clarifies that it's far from a sure thing that the D.C. region won't get another epic-sized rain load in the next 500 years or so. He says: "Of course, return period doesn't mean that we won't see that kind of rain in those locations for several decades (or centuries). A 1 in 100 year rain means that there is a 1% chance of seeing that amount of rain in any given year. A 0.1% chance is true for a 1 in 1000 year event."

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