WEATHER NEWS: Explaining violent storms and a ‘rain train’ that blasted D.C. area
A pair of violent thunderstorms hammered areas south of Washington Wednesday afternoon; the cities of Fredericksburg and Warrenton were particularly hard hit. While the immediate D.C. area was spared those initial blows, it was soaked by a relentless stream of storm cells Wednesday night. This “rain train” caused areas of flooding and bewildered stormwatchers by its unusual north-to-south motion.
The Fredericksburg storm first developed north of the District before erupting in Prince William County, where it first caused wind damage. The storm then bolted southward along Interstate 95 taking down trees and wires all along its path — which ended south of Richmond. Winds were intense enough to strip siding from homes and even shift a shed off its foundation, according to reports from the National Weather Service.
This was the super intense storm that hit Fredericksburg earlier with violent downburst winds. Fast forward to around 25 seconds. https://t.co/zzIzt9xbx9
The Warrenton storm was comparably intense. It first caused tree damage in southern Loudoun and northern Fauquier counties around Middleburg and the Plains. It downed “dozens of trees” near Marshall, according to the Weather Service. In west-central Fauquier County, including Warrenton, there were “hundreds of trees blow down, many into homes with at least three structure collapses,” the Weather Service wrote. The storm also toppled trees around Culpeper before the storm weakened.
Amid the numerous the storms that swept through the region, there were also several reports of homes struck by lightning. At least one firefighter was hurt responding to a lightning struck home in Loudoun County.
The storms lined up into north to south rows like trains along a track, unloading tremendous rainfall in some areas.
Some of the heaviest rain focused along and just west of Interstate 81, where streams overflowed and roads were closed.
Around Washington, a train of storm cells that stretched as far north as central Pennsylvania cycled through the area starting around 7:30 p.m. and continued well after midnight. Minor flooding was reported along Rock Creek at Sherrill and Beach drives, where several vehicles were stranded or disabled.
Generally one to two inches of rain fell in the Beltway region; across the broader region amounts varied from around 0.1 to over 3 inches. Here are some of the totals:
Woodbridge: 3.1 inches
Front Royal: 2.5 inches
Fairfax: 2.29 inches
Fort Belvoir: 2.18 inches
Reagan National Airport: 1.86 inches
Rockville: 1.78 inches
Winchester: 1.4 inches
Dulles: 1.00 inches
BWI Marshall: 0.66 inches
Explaining the storminess
The Fredericksburg and Warrenton storms were isolated and occurred roughly one hour and 40 miles apart. The systems were compact, bow-shaped thunderstorm complexes that traveled rapidly from north to south — an atypical motion for our region during summer.
The storms initiated and tracked along a north-south oriented frontal boundary and were being steered by a deep flow of wind from the north.
The first complex congealed near Dale City around 2:15 p.m. and tracked south along I-95 before dissipating south of Richmond. The radar animation of the system is shown below. In the animation, the left panel shows radar reflectivity (the intensity of rain) while the right panel reveals Doppler-derived wind speed.
The storm complex initiates as an undulating, solid arc of cells which then rapidly bows out and elongates toward the south. A powerful surge of downdraft wind was responsible for the transformation to a bowing shape. Those winds were detected by the Doppler radar in Sterling, Va.) and shown by the patch of orange colors along the leading edge of the bow.
The image below reveals the peak strength of these winds — nearly 88 mph, and in the range of 78 to 96 mph at other times. At this distance from the radar, the lowest scan beam was several thousand feet above the ground, so these values do not reflect wind speeds at the ground. But they do reveal the intense momentum at play in this downdraft — a large percentage of which undoubtedly made it to the surface to create damage.
The second complex developed south of Purcellville just after 4 p.m., expanding into a bowing complex as it tracked rapidly to the south. The track of the storm took the violent core of wind just west of downtown Warrenton as can be seen in the radar loop below.
442p: Dangerous storm hitting Warrenton and may hit Culpeper a little after 5p. This storm will bring down trees. Winds 60-80 mph. pic.twitter.com/0mss82gqJw
Peak Doppler-derived wind speeds were in the 90 to 95 mph range as shown in the following radar snapshot:
The violent winds were the result of straight-line flow called a downburst, which occurs when an exceptionally strong downdraft strikes the surface and the airflow surges outward along the ground, literally as a blast of wind. The strongest winds occur in the direction that the storm is moving.
The great number of trees and scope of the devastation seems “derecho-like,” but in fact neither of these storm complexes qualifies as a derecho; derechos are defined by a minimum path of continuous wind damage at least 250 miles long.
Training storm cells and heavy rain
Some locations across the region picked up several inches of rain from this event, others not nearly as much. Interestingly, the heaviest rains fell in long, parallel, narrow corridors oriented north to south.
945p: Lots of rain still to cycle through from north to south (yes—this is unusual). Be careful driving near streams and turn around if you encounter standing water. pic.twitter.com/lVgTmv0ZTZ
This type of situation can happen when the frontal boundary that serves to lift the air into thunderstorm updrafts — in this case, a nearly stationary front oriented north to south — aligns parallel to the steering flow established by deep layer of winds above the ground.
The deep layer winds were from a very unusual direction for late June — due north — because the jet stream, a river of fast-flowing air in the upper atmosphere, is in a highly contorted pattern. A heat dome is lodged over the Southeast and the jet stream, along which storms track, is riding over top it, taking a sharp dip over the Northeast.