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Posted at 10:15 AM ET, 09/09/2011

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


Radar estimated rainfall totals across the area from September 5 through 1 a.m. September 9. There are two fairly large areas where 15” or more were estimated (southern Fairfax county and southern Charles county). This is rough guidance and not meant as an actual total map. Totals on the upper left edge, for instance, are generally lower than actual. Image courtesy Weather Underground.
There are certain precipitation events that get etched into our memories because of their impact. Locally, most weather watchers can easily recall June 4, 2008, June 2006, Snowmageddon and that whole “nasty” winter. Further back there are ones like Agnes and the Knickerbocker storm.

This week will be one folks in the D.C./Baltimore area remember for a long time when it comes to flooding rain. The event has had it all: urban flooding, flash flooding and river flooding -- all over multiple days, impacting many communities. As rivers continue to crest both here and to the north it is too soon to quantify it fully, especially since even more (likely much less!) rain is probably still to fall. That said, we already know enough to come to certain conclusions...

Continue reading to see how the early September D.C. area flood fits into the historical record.


Waples Mill River Road in Fairfax on Thursday. By Capital Weather Gang photographer Kevin Ambrose.
We don’t have to think too far back for at least vaguely notable rain events in D.C. -- as measured at National Airport (DCA) -- itself. Of course there is Irene (with the greatest impact mainly south and east of the area), and the tropical downpours of late September last year . May 2008 had a five day stretch dropping 7.71” of rain at DCA. October 2007 featured four days of rain producing 6.18”. The very memorable June 2006 featured 11.37” of rain over the course of a week, with 9.41” coming in just two days. In this case, DCA was one of the high-mark spots.

And when it comes to climatology, everyone is quick to remind that no one actually lives at the airports. On days like Wednesday and Thursday, when epic rains fell both primarily to the east and west of places like DCA (which still got plenty of rain), this is a good idea to at least keep in mind. That said, the airports are a quality-controlled guide with the longest histories available, so let’s start there.

Below are rainfall statistics by day for the three regional airports: DCA*, Dulles (IAD), and Baltimore-Washington (BWI)**. (R) equals record rain.

AIRPORT: MONDAY, TUESDAY, WEDNESDAY, THURSDAY
DCA: 1.36”, 0.90”, 2.82”, 1.39” (6.47” event thru 9/8)
IAD: 1.99”, 1.07”, 1.11”(R), 2.27” (6.44” event thru 9/8)
BWI: 1.43”, 1.84”, 3.40”, 1.44”(R) (8.07” event thru 9/8)

The significance of these totals at these locations is not to be dismissed, though compared to other places across the area, and in the context of local rain history, it might seem not to be terribly excessive.

Record daily rainfall at DCA is 6.39” on Aug. 23, 1933 and only three days in nearly 141 years have seen rainfall of 6”+. The maximum Sept. daily rainfall is 5.16” on the 2nd in 1922. For BWI, the maximum daily and maximum Sept. daily rainfalls are 7.62” (8/23/1933) and 5.97” respectively. Despite a much shorter climate record only back to 1963, IAD’s maximum daily total of 10.67” is remarkably high. It came during June of 1972 (Agnes). IAD’s highest Sept. total is 5.94”.

Other than their record highest daily rain totals, neither BWI nor IAD has another 6”+ daily total, and BWI’s record is of equal length to D.C.’s. When combined with the fact that DCA has only three total days of 6” of rain or more, we can probably think of this as the daily rainfall “roof” with a bulls eye outlier result nearing 10” or so for the immediate area.


Rainfall reports for the period from 7 a.m. Wednesday through 7 a.m. Thursday showing heavy rainfall amounts to the east of D.C. prior to Thursday’s deluge to the west of D.C. Image courtesy CoCoRaHS.
On Thursday, another airport location -- not used by the NWS for climate records but still likely reliable -- Fort Belvoir, Va., recorded at least (last ob with rain total was 7:55 p.m.) an incredible 8.82” with as much as 7.03” coming during a three-hour stretch during the evening. It has received a stunning 13.52” since Monday. Though we don’t have historical data to compare this to at that location, the almost 9 inches easily tops both DCA’s and Baltimore daily records, and is very close to IAD’s record daily rain.

Going further “off the grid” (accuracy cannot be guaranteed but it “fits” both radar estimates and observed conditions) we see station totals up to 8”+ on Thursday alone in Falls Church, multiple spots in Reston over 7”, 7”+ in Lorton (9.6” according to one Cocorahs report), and 5-10”around Woodbridge (Cocorahs reports of 9.94” and 5.18”). Wednesday featured similarly (though a few inches less on the top end) high totals in a band to the east of D.C. and west of Baltimore. When weighed against known records of the airports, we can surmise that daily totals such as these are either near or at the top of local records across the area.


Rainfall reports for the period from 7 a.m. Thursday through 7 a.m. Friday showing heavy rainfall amounts to the west of D.C. following Wednesday’s deluge to the west of D.C. Image courtesy CoCoRaHS.
While flooding can come from other causes such as heavy rain in addition to snow melt, like in early 1996, our biggest rainfalls generally come from decaying hurricanes and tropical storms. In a broad scale, Agnes is perhaps one of the best comparisons outside the non-tropical June 2006 event, though the spatial extent of maximum rainfall is still somewhat unknown with the current event. Other systems like Connie (then the second punch from Diane), Fran, and Camille provided heavy rain nearby as well. Local big-river flooding may not end up as significant from this event as some others given that so much of the rain fell in the lower reaches of the watershed.

I purposefully spent the least time on event totals as the system responsible for the rain is still lingering enough to think it might drip a few more drops on some locations. Multi-day radar estimates and observed totals suggest that some spots are potentially into the 15”+ range to the southwest of D.C. (southern Fairfax county) and in parts of southern Maryland (southern Charles county) with numerous 8-10” totals in bands lining either side of the city. Data is still being collected to build on mapping and comparison accuracy.

In addition to examining the event on its own, it’s worth considering that many areas have gone from drought or near drought to more rain than needed in a short period of time. At BWI Aug.-Sept. has already become the wettest two-month stretch in history with plenty of September still to go. All in all, a pretty rare event (or set of events going back at least to Irene) for everyone in the D.C. area and an historic one for various segments of the region.

Here are some select other locations for the event through this morning from NOAA and our local National Weather Service office .

Virginia

Fort Belvoir: 13.52”
Newington: 13.48”
Franconia: 12.56”
Reston: 11.97”
Quantico: 9.01” and 9.39”
Oakton: 7.21”
Manassas: 6.16”

Maryland

Waldorf: 11.66”
Ellicott City: 11.36”
Crofton: 10.21”
Andrews Air Force Base: 9.2”
Gaithersburg: 6.97”
Downtown Baltimore: 6.9”
College Park: 6.72”
Annapolis: 5.32”

Another reference to pour through is CoCoRaHS.

...NOTES...

*The early part of DCA’s history was in downtown D.C. For simplicity the whole record is referred to as DCA as it is all considered one.

**The early part of BWI’s history was in downtown Baltimore. For simplicity the whole record is referred to as BWI as it is all considered one.

By  |  10:15 AM ET, 09/09/2011

Categories:  Floods, Local Climate, Latest

 
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