The first wave of rain drenched the area around Manassas on Wednesday morning, causing a sinkhole and several high-water rescues. Then in the evening, another wave unleashed a cloudburst around Baltimore, perilously close to Ellicott City, Md. Sirens sounded in the vulnerable historical district, which was engulfed by disastrous floods in 2016 and 2018, but the heaviest downpours remained just to its southeast. The rains expanded southwestward into parts of southern Montgomery County and Northern Virginia, where pockets of additional flooding occurred.

Finally, on Thursday morning, a large zone of heavy thunderstorms covering much of the region created more areas of flooding between western Prince William County and southern Prince George’s County. This rainfall dissipated midday.

But periodic bursts of heavy rain may flare up Friday, Saturday and Sunday. Because the ground is so wet, it won’t take much rain to lead to more flooding. Generally, 1 to 1.5 inches of rain in an hour is all it will take.

Since this soggy period started early Wednesday, widespread rainfall totals have reached at least an inch. However, numerous areas have seen 2 to 5 inches, often falling in an hour or two, which is where flooding issues have focused. Here are select totals:

  • Reagan National Airport: 0.76 inches.
  • Dulles Airport: 1.1 inches.
  • Manassas: 2 inches.
  • Oakton: 2.79 inches.
  • Bull Run: 3.25 inches.
  • BWI Marshall Airport: 3.9 inches (2.39 inches fell in one hour around 5 p.m. Wednesday).

Details on what to expect next

The entire D.C. metro region remains under a flash flood watch through Thursday night. This may be extended considering the threat of heavy rain — at least on an intermittent and scattered basis — through the weekend. The Weather Service predicts an additional 2 to 3 inches of rain through early next week.

We are stuck in a very humid air mass that is converging along a stationary front. The heavy rain Thursday morning was the result of an impulse in the jet stream flow passing just to our north, and it was responsible for triggering a widespread mass of heavy showers and storms across our entire region, which prompted several flash flood warnings.

These showers and storms were winding down midday Thursday, as the energy impulse moved off to the northeast.

Given the widespread cloud cover, which has kept solar heating to a minimum, we do not expect that the atmosphere will destabilize Thursday afternoon and evening as it did Wednesday. The coverage and intensity of showers and storms should be less.

Any cells that do trigger late Thursday, however, could still cause flash flooding problems, because the ground is so saturated.

Friday and into Saturday, the front is expected to make southward progress, as shown in the forecast map below. However, an upper-level disturbance will intensify to our west, probably triggering a wave of low pressure on the front. This system will continue to circulate sopping wet air aloft over our region, so the prospect of at least scattered heavy showers and storms will continue.

Saturday presents the highest chance of more excessive rainfall, but daily forecast details will need to be fine-tuned.

Reviewing Wednesday’s rain

A broad zone of heavy, persistent storms developed over central Maryland during the late afternoon on Wednesday, focusing over Baltimore and surrounding western suburbs. In response, numerous flash flood warnings were issued.

The general setup featured a stalled frontal boundary, amid a very unstable afternoon air mass — meaning the air was strongly heated, and loaded with humidity, close to the ground.

The convergence of winds along the boundary encouraged the air to broadly rise.

As the radar animation shows below, several scattered cells rapidly developed, then congealed into a much more expansive mass that essentially remained stationary for several hours. But other subtle dynamics were at play, governed by the movement and interaction of “outflow boundaries” — expanding pools of cooler air created by storm downdrafts.

The boundaries can be seen as fine, narrow arcs of weak radar signals moving about the image.

As the cooler, denser air advances, it lifts unstable air and can trigger new storm formation. Where outflow boundaries collide, the convergence and uplift of air in between can also initiate storms.

For instance, early in the radar sequence, you may note the collision of two boundaries along a west-east line extending from Wheaton to Laurel. Soon thereafter, a line of intense storms erupts along the location of the collision. That convective line remains stationary, dumping heavy rains.

Later in the sequence, an outflow boundary between Brookeville and Ellicott City races toward the northwest. As it undercuts and lifts unstable air, a new line of storms rapidly congeals along its trajectory … forming a winglike extension to the larger mass of storms farther east.

None of these exact boundaries, and boundary interactions, could be predicted in advance, even by our best (highest-resolution) forecast models. The small scale, and rapidly evolving nature, of these interactions is simply too fine for the models to resolve. This foils our attempts to predict the precise locations of flash floods, which typically unfold over regions of just a few square miles in extent.

A sobering example of this is shown above — a pinpoint region of extreme rainfall, which fell on the very edge of Ellicott City. While the warning sirens blared, Old Ellicott City picked up 1.5 to 2 inches in a short time, according to radar estimates. Several streams swelled but did not overflow their banks. Meanwhile, more than twice that amount (4.3 inches) rapidly accumulated just a few miles southeast of the city. This was indeed a very close call for the historical district.

Interestingly, the zone of heavy rain then pivoted west and southwestward, soaking sections of Northern Virginia. Flooding was reported in a few areas between Reston and Ashburn, where several inches of rain fell in the evening.

Owing to the chaotic behavior of the outflow boundaries, the torrential rain made a bizarre counterclockwise loop, circumventing the District, which experienced only a brief thundershower.