Flooding in the Washington D.C. area on July 8th was simply unbelievable. Traffic was snarled throughout the metropolitan Washington area and numerous water rescues were required. According the National Weather Service, Reagan National Airport recorded 3.3 inches of rainfall in one hour. The agency even declared a flash flood emergency for the region. Weather Channel Meteorologist Mike Seidel's Tweet provides further perspective on the flooding, "It only took one hour for Washington, DC to gain Top-10 daily July rainfall status: 3.30" fell, leading to widespread flash flooding across the metro." Meteorologist Ryan Maue added that "Based upon preliminary data, between 2 and 3 billion gallons of water fell on Washington D.C. 68 sq miles (1.5-2.0 inches) from the slow-moving deluge." With rain falling from the sky at that intensity, urban flooding was inevitable. However, I noticed 3 very familiar things about the flooding in Washington D.C. on Monday.
The atmosphere was "juiced" with excessive moisture. NOAA's Weather Predictions Center provides a good backdrop for the meteorological processes associated with the event:
A large, slow-moving convective cluster moved over the D.C. Metro Area this morning, with DCA measuring 3.3 inches of rain in an hour. This has an annual exceedance probability of less than 1 percent, and other mesonet sites have observed similar (3+ inch per hour) rain rates....A low level trough extended from near the mouth of Chesapeake Bay up into northern Virginia, and this coincided with an axis of maximum PW values.
PW stands for Precipitable Water. The American Meteorological Society Glossary of Meteorology defines PW as "the total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending between any two specified levels, commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section." My former doctoral students Amanda Schroeder, currently a hydrometeorologist with the National Weather Service - Ft. Worth, studied large urban flooding events just like the one in the DC area for her dissertation research. She analyzed urban flood events over the past 40 years and discovered that extremely high Precipitable Water (PW), often in the top 1% of values for the location, were typically present. Of 40 flood cases in her study, PW values always exceeded 150% of climatological mean values.
In a Tweet earlier on Monday morning, I speculated that the DC flooding event was likely to have top 1%-level PW values. Climatologist Jordan McLeod responded that my hypothesis was correct:
Your suspicion is correct. The 12Z sounding at Dulles measured 2.06" of PWAT, which is only 0.01" shy of the daily record for all 12Z soundings going back to 1948. Highly anomalous for sure (graphic below).
When you couple excessive moisture values with an unstable air mass and a convergence zone providing a source of "lift," you get a scenario like the DC floods.
Urbanized land amplifies flooding. Typically, we focus on rainfall rates and how fast the system was moving in the aftermath of a flood. However, one of the most important aspects of an urban flooding event is the presence of "impervious surfaces" like roadways, parking lots, and so forth. Brian Bledsoe is the director of the Institute for Resilient Infrastructure Systems at the University of Georgia. He is also a professor of civil and environmental engineering. Bledsoe opined in the Washington Post:
Most of us intuitively grasp that intense rainfall interacts with increases in impervious surfaces such as roads, sidewalks, parking lots and rooftops to amplify the volume and speed of storm runoff. We are less inclined to think about where the water goes from there — how flood mitigation measures like ponds and detention basins can become less effective over time or fail if not properly maintained, or how the ability of streams and rivers to carry runoff changes due to sediment movement and other natural processes. It’s not just the rain that can change. It’s the rain, the urban footprint and the drainage systems all changing together.
Scientific studies also continue to suggest that the intensity of rainfall is changing and that our engineered stormwater removal systems may be overwhelmed by current (and future) generation of rainstorms.
Bad decisions. The final thing that was familiar is also the most frustrating. As I browsed social media, numerous pictures and videos were shared of stranded cars in or driving through flooded roadways. To be fair, some of the rainfall rates were so intense that some motorists likely were stranded. However, there were several examples of people making deliberate decisions to drive through flooded roadways. I even saw a loaded school bus plow right through a flooded roadway. As bad as that decision was, I was equally concerned that the bridge could have washed out (video here). It's as if the "Turn Around, Don't Drown" slogan exists only as a cute saying. Senior Weather Channel hurricane expert Rick Knabb tweeted:
At least 43 have died in U.S. floods while driving in 2019. Today, more examples of why we should consider investing in automated and remotely operated barricades for flooded roads. Too many people, including a few school bus drivers, will never abide by Turn Around Don’t Drown.
Whether this solution is ultimately the right one, I agree with Rick that more has to be done.
