Rainfall rates observed in southern Fairfax county around 6:00 p.m. on Thursday, September 8. A portion of southern Fairfax county experienced rates between 3 and 4” per hour at that time. ( Radarscope Iphone App via Ian Livingston)

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

Causes of the excessive rainfall event?

Last week, I wrote a piece on how different streams of moisture set up to converge over the D.C. region (as well as locations to the north). Not to be left out of the discussion is the potential effect of climate change, which is very likely already enhancing heavy rain events. As Andrew Freedman wrote earlier this year:

The physical mechanism behind the link between warming global average land and ocean temperatures and more frequent heavy precipitation events is rather simple to understand - as air warms, it can hold more water vapor, which means that more water can then be squeezed out of the atmosphere as liquid or frozen precipitation.

While this event cannot be blamed on climate change, warming probably gives existing rain events some added intensity and will most likely increase the frequency of very heavy rain episodes into the future.

Related: How does the early September 2011 East Coast flood event rank in the Washington D.C. area?