Downpours in Silver Spring on Saturday afternoon. ( Tim Brown via Flickr )

With its nonstop driving rain and temperatures in the 60s, Saturday’s storm could have easily been mistaken for a fall or spring nor’easter. But it was a once-in-a-generation July coastal storm that drew vast amounts of tropical moisture and unleashed historic amounts of rain in the D.C. region.

Rainfall totals July 21. (National Weather Service)

The storm unloaded four to six inches of rain across widespread areas with isolated totals of up to eight inches. The National Weather Service logged scores of reports of flash flooding, including more than a dozen high-water rescues because of vehicles stranded in floodwaters.

Flooding reports logged by the National Weather Service on Saturday, July 21. (NWS)

The rainfall totals of four inches, 5.02 inches and 4.79 inches in Washington, Dulles and Baltimore were all records for July 21. But they also ranked among the wettest days ever observed in the region for July or any time of year. Consider these statistics (gathered by Capital Weather Gang’s Ian Livingston):

  • It was Washington’s fifth wettest July day, and 24th wettest day for any time of year (records date back to 1871).
  • It was Dulles’s wettest July day and fifth most for any month of the year (records date back to 1963).
  • It was Baltimore’s second wettest July day and 10th wettest for any month (records date back to 1871).

The torrential period of rain was because of the near-perfect confluence of many factors, including processes related to the high-altitude jet stream, a potent coastal storm and extreme amounts of oceanic humidity.

The graphic below presents the jet stream flow in the upper atmosphere, during the early evening Saturday. The setup featured a very deep (intense) trough of low pressure, uncommonly intense for the midsummer months, extending from the Great Lakes to the Southeast coast. To the east of this trough, air streams from the south diverged or “spread apart,” drawing up an intense chimney of moist air from below.

Depiction of upper-level airflow, position of surface low (red L) and zone of rising air (red oval) on Saturday evening. (Modified from NOAA)

The dynamic “sweet spot” for rising air lay directly over the Mid-Atlantic, and enabled a cyclone to develop along the coastline — which also featured a gradient in temperature (cooler air inland, warmer air offshore). The thermal gradient helped to power this storm, which was more akin to a wintertime system. The coastal low was intensifying as it moved slowly up the coast, and was located close to Washington-Baltimore by midnight.

The slow speed of this moisture-laden storm system contributed to excessive rain totals area-wide. But several other factors combined in a synergistic manner. As shown in the graphic below, a powerful jet of low-level, moist air was drawn into the system from the south and east. The core of this current was nearly 65 mph at around 5,000 feet and amounted to a pipeline of moist Atlantic air feeding directly into the zone of intense rising motion.

Depiction of low-level airflow feeding into the storm (purple arrows), position of surface low (red L) and the intense rain band on Saturday evening. (Modified from NOAA)

On the west side of the storm, an intense rain band about 50 miles long by 30 miles wide set up shop across Washington, Northern Virginia and Central Maryland. This band was essentially stationary, parked for hours across our region.

Within the broader band, smaller, cellular elements of heavy rain unloaded from south to north, in the manner of “echo training.” This type of rain band, again, is much more typical of winter storms; the type that delivers prodigious quantities of snow. These bands become established in a zone where the airflow is both stretched and convergent; what meteorologists term a “deformation zone” in the flow. They are also notoriously difficult to predict more than a few hours in advance.

The rain band is shown in greater detail in the next figure. The orange and red shadings, based on radar reflectivity, imply rain rates on the order of more than an inch per hour.

Radar snapshot showing detail of extremely heavy rain contained in the stationary rain band over the District. (Radarscope)

Tremendous amounts of water vapor streamed into the storm off the Atlantic, aided by the low-level jet discussed above. In the next figure, this region is highlighted in green shades. Here, precipitable water values soared into the 2.3- to 2½-inch range, which is about the maximum ever observed in the Mid-Atlantic during summer. (Precipitable water is an integrated measure of total water vapor in a vertical column from surface to top of the troposphere). The slug of deep moisture fed directly off the ocean into the stationary rain band parked over the District.

Depiction of precipitable water (contours and green-shaded regions) mapped to radar, and also the surface low (red L) and its track, on Saturday evening. (Modified from NOAA)

Among the constant torrents of rain, did you notice how cool it became Saturday afternoon? The day’s high temperature (in the 70s) occurred early in the morning, then dropped into the mid-60s by midday.

Like the winter months, cool air damming was largely to blame. This refers to a shallow layer of cool air wedged between the storm and the mountains. The approaching coastal low drew down cooler air from the north, and the combination of heavy cloud cover and evaporation of rain provided significant additional cooling. These processes rapidly built a wedge of “midsummer chill” close to the ground. In fact, this wedge helped lift milder and moisture-laden air off the Atlantic, further boosting the efficiency of the rainmaking processes in this storm.