Virginia Deluge Was an “Off the Charts Above a 1000-year Rainfall,” Says National Weather Service

JR: The disinformers like to say the extreme weather we are seeing today is nothing unusual. They don’t live in Texas, where “No One on the Face of This Earth has Ever Fought Fires in These Extreme Conditions.” Or my hometown area around the Catskill Mountains, where Hurricane Irene was “the most devastating weather event ever to hit the region.” Or around Binghamton, NY, where “An Extreme Rainfall Event Unprecedented in Recorded History Has Hit.”

Rainfall rates observed in southern Fairfax county around 6:00 p.m. on September 8. Some places saw rates between 3 and 4” per hour.

Capital Weather Gang Chief Meteorologist Jason Samenow has the details on one more record-exploding extreme event in this repost.

On Thursday, September 8, Ft. Belvoir received an astounding 7.03” of rain in three hours. According to the National Weather Service (NWS), that amount of rain in that amount of time was an “off the charts above a 1000-year rainfall (based on precip frequency from Quantico).”

Chris Strong, warning coordination meteorologist for the NWS in Sterling, emailed media a note on the frequency of rain return from last week’s event, for locations in Maryland and Virginia. It’s extraordinarily impressive and reproduced in full below…

National Weather Service note:

For our DC and Baltimore media colleagues…Some interesting stats that our hydrologist Jason Elliott passed on to us that you may find useful in your broadcasts/bloggings. Based on precipitation frequency computations, for last week’s rainfall:

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).

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.

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.

Chris Strong
NWS Baltimore/Washington

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