The Weather Service logged about 100 reports of damaging winds (mostly toppled trees and wires) and hail (up to about the size of half-dollars) from the two thunderstorm complexes.
In the first wave late Wednesday afternoon into early evening, the most intense storms swept through Manassas and Alexandria, and near Fort Washington in Prince George’s County.
During the second wave later in the evening, the most vigorous storms targeted parts of Loudoun County, northern Fairfax County, the northwest part of the District and southern Montgomery county, where scores of trees were toppled.
Hot and humid weather in the 90s, which tied records at Dulles (high of 92 degrees) and BWI Marshall (high of 94 degrees) airports, helped fueled the storms. The Weather Service issued a severe thunderstorm watch as activity began to flare up in the mountains during the early afternoon. Then it hoisted dozens of warnings as the storms erupted as they crossed into the piedmont and coastal plain through the evening.
The storms gained such strength as the atmosphere strongly destabilized from southerly winds and strong heating from the sun. An increase in wind speed with altitude, or wind shear, led to intense storm updrafts and downdrafts, and helped organize cells into longer-lived clusters.
The Weather Service received several reports of wind gusts between 50 and 65 mph, including a 53 mph gust at Reagan National Airport in the first wave and 61 mph in Fairfax from the second. An isolated gust of 83 mph was also clocked by a river buoy near Occoquan, Va. in the second wave.
Although quiet weather is forecast in the Washington region Thursday, there is a risk for more thunderstorms, which could be severe, Friday.
Inside the Manassas microburst
The day’s most intense storm slammed Manassas and Manassas Park late in the afternoon, where it brought down scores of trees and unleashed large hail from what’s known as microburst or downburst, or rush of violent winds from the storm cloud to the surface. One tree fell on a mobile home and destroyed it, according to the Weather Service. Let’s examine, in some depth, how this storm developed.
Considerable buoyant energy was concentrated in lower and middle levels of the atmosphere. As storm cells matured, updrafts became very intense, levitating enormous masses of water drops and ice particles. Within these suspended cores of precipitation, large hailstones developed. Charge induction and separation generated bursts of concentrated cloud-to-ground lightning.
These cores, however, quickly collapsed, sending a volley of large hailstones and torrential rain toward the surface. As trillions of these particles descended, they dragged the surrounding air downward. Evaporation of rainwater and partial melting of hail chilled the air, increasing its density relative to surrounding air, further accelerating the air downward.
When these precipitation-cooled pockets of air impacted the ground, strong, horizontal gusts of wind developed and fanned outward.
The image above shows the initial storm cell located over Linton Hall in Prince William County, at 4:19 p.m. The left panel displays precipitation intensity. The purple core is a color code we use to denote a mixture of heavy rain and hail. The right panel shows the corresponding Doppler wind flow. Green regions denote airflow toward the Dulles radar; red is flow away (the radar is out of range of the graphic, beyond the northern edge).
The white “X” marks the location of the hail core in the left panel, and the point of microburst impact in the right. The green next to red, with alignment along a radar radial, is the classic “diverging” airflow signature of a downburst.
Switching to the second graphic, the storm is shown a few minutes later, having shifted eastward into the Manassas region. The Linton Hall cell has dissipated, and a new intense core (“Hail Core Cell 2”) has formed just to its southeast. This is how multicells behave; as new cells grow, older and adjacent cells dissipate. These cells cycle within the same aggregate storm.
At 4:32 p.m., foliage across Manassas was getting shredded by quarter-size hail, hurled sideways in turbulent swirls of downdraft wind. Just a few minutes later, a second downburst initiates and is imaged by Doppler radar, as the multicell slides east-northeast.
This storm complex was not a supercell; Doppler radar never detected a rotating updraft (called a mesocyclone). But violence can still be unleashed, in the form of damaging straight line wind and large hail. Nine times out of 10, damaging thunderstorm winds in our region are straight-line, due to downbursts, not tornadoes.
Below, find some amazing visuals and footage of the storms as they traversed the region …