Fueled by afternoon temperatures that soared into the mid-90s amid suffocating humidity levels, the storms emitted frequent lightning that cut power to 18,000 customers in Northern Virginia, Maryland and the District.
The National Weather Service received nearly 20 reports of flooding across the region and about 30 reports of damaging winds.
For much of Monday evening into early Tuesday, flash flood warnings blanketed a large area east of the Beltway through much of Prince George’s County into Southern Maryland. Overnight, heavy storms triggered additional flash flood warnings for the District and its immediate western suburbs.
Rainfall analysis from the National Weather Service shows about 1 to 3 inches fell inside the Beltway, but up to 4 to 8 inches fell just to the southeast through southern Prince George’s County through eastern Charles County and into northern St. Mary’s County. Reagan National Airport received 2.47 inches.
Rainfall rates were as high as about 6 inches per hour southeast of the Beltway as a train of storms lined up and repeatedly passed over some of the same areas.
In some areas, water entered homes, while roads were closed because of overflowing streams in Vienna, Arlington, Alexandria, Largo and Upper Marlboro. Alexandria’s Four Mile Run rose 6.5 feet in 40 minutes.
Six people were rescued in high water along Old Ritchie Road in Prince George’s County. Another person stranded in floodwaters required rescue in Falls Church.
Water cascaded into the Cleveland Park and Woodley Park Metro stations, causing flooding.
The initial round of storms that developed after 6 p.m. Monday produced the strongest winds that brought down trees in several areas inside the Beltway, including the District. Reagan National Airport clocked a gust to 60 mph, while a wind sensor near Joint Base Anacostia-Bolling recorded a 69 mph gust.
But the storms that made the most noise arrived after 1 a.m. Tuesday.
On Twitter, many Capital Weather Gang followers said they were suddenly awakened by the vivid, crackling lightning and booming claps of thunder.
“I have never heard thunder like this,” tweeted Virginia Whorley in Alexandria.
“Good god the thunder and lightning in DC right now. My car alarm has gone off twice,” tweeted Betsy Whalen.
“There was a crack of lightning so loud it jolted me out of bed,” tweeted Meredith Hindley.
As the storms were slow movers, the sound and light show carried on for hours in some areas.
“I think I got about 2 hours of good sleep,” tweeted @Dominiqueiman.
Monday night marked the second in a row with damaging storms. On Sunday evening, a tree collapsed on a garage, injuring 19 people in Pasadena, Md.
Additional storms could flare up Tuesday afternoon and evening but are not expected to be as numerous or, in general, as severe.
How did Monday night’s storms happen?
The storms Monday evening and overnight were an example of convection gone wild from the Beltway, eastward and southward. The radar animation below shows an unusually active and widespread coverage of intense cells just after 11 p.m., with several zones of “training” storms — meaning repeated passage of cells over the same locations.
The training of cells, in a very moist and unstable air mass, led to some phenomenal, short-term rainfall totals at several locations.
The interesting aspect of this widespread activity was that there was no large-scale or “synoptic” trigger, such as a cold front, low-pressure region or potent upper-atmospheric disturbance.
Instead, the late evening activity was the result of an arc of potent thunderstorms that initially concentrated over southeastern Pennsylvania and southern New Jersey during the late afternoon. Downdraft currents from those clusters coalesced into a massive “outflow boundary” — an arc-shaped, shallow pool of chilled air that behaved as a small cold front.
That cold front dropped southward and eastward into Delaware and northeastern and central Maryland through the evening, becoming stationary along the D.C.-Maryland line. As the boundary advanced, it scooped up warm and humid air — firing off nearly continuous volleys of intense thunderstorm cells and clusters.
The following graphic, put out by the Weather Prediction Center, shows the boundary and generating cells over southern New Jersey during the late afternoon. It’s a busy graphic, but we want to elaborate how this setup caused storm cells to train repeatedly.
The large blue trapezoid is the Severe Thunderstorm Watch issued by the National Weather Service Storm Prediction Center. Within the box, note the wavy blue line with attached blue triangles. That is the leading edge of the developing, large outflow — advancing westward and southward.
Ahead of this boundary, over central and northeastern Maryland, the air mass had become exceptionally unstable — due to a combination of strong solar heating, hot winds arriving from the southwest, and high levels of humidity in the lower atmosphere. The red number “3000” refers to the value of buoyant energy, in Joules/kg of air. Anything over 2,000 signifies a very unstable atmosphere.
The next figure shows an analysis of the deep moisture available, termed “precipitable water,” to fuel intense storms and heavy rain. You will note the bull’s eye of 1.7 to 1.8 inches over the Interstate 95 corridor stretching from the District to Philadelphia, and the massive cluster of thunderstorms over South Jersey late in the afternoon.
Through the evening, low-level winds were directed from the southwest, toward the outflow boundary. This focused the continuous uplift of moist, unstable air along the boundary, over Delaware and eastern Maryland.
Meanwhile, middle- and upper-level winds were blowing from the north, parallel to the boundary. This set up the continuous movement or training of cells from north to south, over the same geographical region.