Dozens of severe-hail reports were logged by the National Weather Service. Hail that is about one inch in diameter, or roughly the size of a quarter or larger, is considered severe with the potential for damage. Hail as big as golf balls — which are 1.68 inches in diameter and more likely to leave dents in vehicles — was reported near Brambleton in southeast Loudoun County. Another storm also tossed large hailstones to the north, in Montgomery County.

Significant hail days are not terribly common around Washington, but when they happen, it is often during the spring before peak thunderstorm season arrives. This is because of the combination of strengthening spring sunshine, which creates strong currents of rising air in the lower atmosphere, and a lingering winter chill at the mid-levels, plus stronger jet stream winds. These factors allow spheres of ice to rapidly grow within bubbling clouds.

One striking aspect about yesterday’s “hailers” is that they developed over the D.C. region less than 24 hours after a similar widespread outbreak of severe hail-producing thunderstorms across north-central Pennsylvania on Sunday afternoon. It was the same weather front, but a different atmospheric disturbance along it, that triggered Monday’s hail.

The conditions that produced severe hail

The strong contrast between warm, relatively humid air in the lower atmosphere and cold, dry air higher up was a key factor in promoting the explosive thunderstorm growth that produced the large hail, but it wasn’t the only aspect.

A strong belt of winds from the west associated with the jet stream — a river of racing winds aloft — created a change in both the direction and speed of winds with height, or wind shear, which is typically required for powerful thunderstorms to develop.

The storm cells became powerful enough to keep growing hailstones suspended in the air, allowing them to swell to large sizes before falling to the ground. The longest-lived of the Monday storms qualified as supercells, at least at times. Supercells are characterized by strong and stable upward currents of air that persist for up to hours at a time. They are the types of storms most likely to produce severe hail and other hazards such as large tornadoes.

The swath of hail from southern Loudoun County to southern Prince George’s County seen on the image above was created by a single long-lived storm cell that is shown in the radar image below.

It had a very long track. The cell initiated northwest of Capon Bridge, W.Va., and pulsed up and down in severity. It appeared to have briefly generated hail over Winchester, Va., but then really got going over east-central Loudoun County, continuing in pulses through Fairfax County and southern D.C. and into Prince George’s County.

WTOP’s Dave Dildine reported cars suddenly parking under highway overpasses along the storm track, seeking refuge from the volley of large, potentially damaging hailstones. (As a brief aside, this is not advised for many reasons, including increased risk of accidents).

The radar image below, created with computer software that depicts the three-dimensional structure of the storm, shows intense precipitation lofted into freezing air above as the hail-generating supercell moved across Arlington. It is essentially a CT scan revealing the vertical structure of the storm, which towered to 45,000 feet, using microwave energy. This is shown in the figure below, as the hail-generating supercell was working across Arlington.

In this case, the mushroom-shaped inner core punches high up into the atmosphere like a fist. The air in that core was probably rising at speeds of 40 to 50 mph or faster.

The whole cloud is leaning, or tilting, because of the influence of the shearing winds. The white region below 10,000 feet represents the dense curtain of hail falling out of the storm. Few images so strikingly depict the impact of shearing winds on the development of a severe storm cell.

Photos and videos shared by readers

Impressive imagery and harrowing videos of the hail poured in via social media, especially on Facebook. One person said they had “never seen one this intense!”

Below are a few of the photos and videos we received. In the first photo, streaks of hail can be seen falling from the boiling mass of clouds amid the shining afternoon sun.