Hail and graupel coat Interstate 66 in Arlington on March 22. (Janet Rice Elman via Facebook)

It was Mother Nature’s version of March madness. On Friday afternoon, a fast-moving squall line unleashed high winds, thunder and lightning, rain, hail, graupel and even snow in spots across the Washington region.

In a few places, an icy torrent even coated roadways, transforming a tranquil spring afternoon into a wild winter scene in seconds.

“I just stepped out the door, and within 30 sec, the ground was covered!," tweeted @debramayberry in Arlington.

The sudden squall followed soaking, record-challenging rains the previous day. But the sunny, mild morning — with temperatures warming into the 50s — offered few clues that such a tempest was coming.

Forecasters knew that a line of gusty showers with a bit of hail or graupel was possible as a vigorous disturbance dropped through the region. But the intensity of this squall line exceeded expectations. Wind gusts over 50 mph were clocked in Dulles, Leesburg and Winchester. Reagan National Airport posted a 59 mph gust.

“One of the craziest 10 minutes of weather I’ve ever experienced. Lightning, wind, hail, .25 of snow, a 20 degree drop in temps and now rain,” tweeted @DCUTV.

The squall’s most jaw-dropping feature was its alphabet soup of icy precipitation: small hail, graupel and snow.

“Whoaaa,” tweeted @PaulDNovak, “Day after tomorrow-esque.”

So what exactly is graupel? Weathercasters everywhere were using this term on the airwaves and TV to describe the ice falling out of this storm.

Most people are familiar with hail. Graupel is essentially a soft, mushy version of very small hail, about the size of BBs. Graupel is a bit more opaque than hail, solid white vs. the semi-translucent appearance of hailstones.

Both types of ice particles (graupel and hail) form in subfreezing clouds by riming. Riming describes the “flash freeze” of tiny water droplets into a spherical shape. During the cooler months, graupel signifies cloud updrafts that are fairly intense by wintertime standards.

Shown below is a radar capture of the squall line as it began consolidating, intensifying and bearing down on the Washington region. By mid-March standards, this was quite ominous-looking.


Ominous looking (non) thunderstorm line approaching D.C. last Friday afternoon. RadarScope.com

What circumstances created this squall line?

First, an unstable air layer, confined below 15,000 feet. Cold air in the middle atmosphere was moving in behind the strong low-pressure system that created all the rain the day before. A few hours of afternoon sun heated the ground to the low 50s, enough to create what we call a steep “lapse rate” (decrease in temperature with altitude). The mild air at the ground, because of its buoyancy, ascended into the cooler air above, triggering convective clouds.

Second, the line of storms organized along a potent disturbance in the upper atmosphere, called a shortwave trough. It’s a ripple of intense energy around 18,000 feet in the atmosphere, riding the jet stream. This ripple was exceptionally strong, and it moved across Ohio into West Virginia, then tracked across the Washington region during the midafternoon.

The graphic below shows an analysis of this shortwave. This is a very “busy” analysis diagram (par for the course for us meteorologists), but you can tell that something very “busy” appears over the region of Ohio-West Virginia. The pink-shaded tones show the energy center, over Ohio, around noon. This energy packet was moving rapidly toward the east-southeast and would begin affecting the D.C. region from 3 to 4 p.m.


Approaching packet of energy and upward motion at jet stream level (blue contours) on Friday afternoon. NOAA.

The blue contours, out ahead of the energy packet, reveal where the air was being forced upward vigorously. It’s hard to see in this diagram, but convection was starting to erupt and congeal into an arc-shaped squall line over the Potomac Highlands as early as noon. That arc initiated directly beneath the narrow ribbon of rapidly ascending air.

Here’s what made this storm a forecasting challenge. By all classic measures, there was not much instability to generate convective storms. What little there was was confined to a very shallow layer near the ground.

But the approaching shortwave created a lot of dynamic uplift in the lower atmosphere, helping to compensate for the limited instability. And the small quantity of instability was also compensated for by very strong lapse rates — in other words, lots of upward acceleration right off the ground.

Meteorologists recognize that fairly intense storms can erupt even when one of the classic ingredients (deep, large amounts of instability) is lacking; in other words, weak and shallow instability will do the trick, only if there is an especially large amount of coincident uplift created by an approaching, dynamic system such as a shortwave.

In nature, there is more than one “pathway” to an intense line of storms. The trick, as a forecaster, is to detect the varying contributions of multiple processes.

These shallow, dynamically generated storms exemplify a class of cool-season, severe weather system that has received increasing attention in the scientific literature, and similar attention in forecasting offices, over the past decade. They thrive in weak instability, combined with especially vigorous winds of the jet stream (called wind shear).

The squall line even put on a show after it passed, as mammatus clouds — more common in the summer — filled the post-storm sky. These pouch-like cloud formations develop from cold, dense sinking air on the periphery of thunderstorms.

See some examples below.