Thunderstorm Safety and Preparedness

Why the 2012 Mid-Atlantic Derecho Was a Benchmark Storm

Even though she didn't know it at the time, eight years ago, Erin Malawer lived through a benchmark weather event in Washington, D.C. — the Mid-Atlantic derecho of June, 2012. It hit right after a historic heatwave set record highs in cities from the midwest to the southeast. On Friday, June 29th, D.C. reached 104 degrees — the hottest June day that had ever been recorded in the city. Then, that evening around 10pm, a wall of wind hit the D.C. suburbs where Malawer lived with her husband and two boys.

She remembers it had finally gotten cool. "I remember sitting in bed watching television when we heard what sounded like a high-speed freight train," she says. "If I remember correctly, we weren't expecting a storm and we were confounded about the noise."

The sound was the derecho hitting. "The power went out almost immediately," she says, and it didn't come back on for five days.

The storm caused 4.25 million outages across eight states from the midwest to the southeast. One million of those outages were in the D.C. area alone. And all of this happened during one of the worst heatwaves in the region's history.

A derecho is a widespread windstorm that's often accompanied by a group of fast-moving, intense thunderstorms, called a squall line. The system is propelled by a strong succession of downdrafts — that cold blast of air often felt before a sudden downpour.

The winds that a derecho produces are called "straight-line winds" meaning that they push air in one direction, unlike tornadoes, which create swirling winds. Straight-line winds can blow over 100mph, which is on par with a low-grade tornado.

Derechos commonly emerge on the edge of a powerful heatwave, or rather, along the jet stream above it.

"You need a strong-enough jet stream disturbance riding over the top of that hot air to provide the instability and strong winds to produce the widespread wind damage of a derecho," says Jonathan Erdman, senior meteorologist at Weather.com. This jet stream is occasionally called "the ring of fire" due to its tendency to produce strong thunderstorms and derechos.

Typically, the more intense the heatwave, the more violent the derecho, as the Mid-Atlantic derecho of 2012 demonstrates.

"It produced a lot of lightning and thunder," recalls Malawer. "I wouldn't say I was scared — I think I was too stunned! I remember looking at my husband saying, 'What. Is. Happening?'"

Somehow, Malawer's two young boys slept right through it, but when they woke up, they were shocked by the destruction the storm had left in its wake.

"No air conditioning, no power, we passed trees that had crushed cars and parts of houses," she says, "It was like a reverse 'Wizard of Oz' for them."

FILE - In this June 30, 2012, file photo, an American Beech tree is down on Capitol Hill grounds in Washington across from the U.S. Supreme Court after a powerful storm swept across the Washington region. A gigantic line of powerful thunderstorms with tree-toppling winds is likely to threaten one in five Americans Wednesday is as it rumbles from Iowa to Maryland, meteorologists warn. The massive storms may even spawn an unusual weather event called a derecho, which is a massive storm of strong straight-line winds spanning at least 240 miles. (AP Photo/Manuel Balce Ceneta, File) — An American Beech tree is seen knocked down on Capitol Hill grounds in Washington across from the U.S. Supreme Court after a powerful storm swept across the Washington region.
(AP Photo/Manuel Balce Ceneta, File)

It was the same — if not worse for millions of others — who were in the derecho's path. The storm spanned over 900 miles from north to south and caused 23 fatalities. It resulted in $2.9 billion in damages — on the level with some of the most severe tropical storms that make landfall. It is still considered the most severe derecho to ever hit the Mid-Atlantic.

Malawer says the only storm she's ever experienced that compared to the 2012 derecho was a tornado that hit the D.C. in 2008.

A derecho typically hits the D.C. area every couple years. It's a less frequent meteorological event in the Mid-Atlantic because the region is protected by a wide-spanning mountain range: the Appalachians.

"Prior research suggests these lines of severe thunderstorms can, at times, have difficulty crossing the Appalachians," says Erdman. "In this 2012 case, however, the heat, humidity and jet stream disturbance were strong enough to overcome the mountains."

While derechos are notoriously difficult to predict with significant lead time because they often develop as a result of small atmospheric changes, the National Weather Service can detect the potential for major convective storm outbreaks at least eight days in advance.

It's hard to say when there will be another derecho on par with the 2012 derecho. Warmer summers are expected due to climate change, but NOAA notes this wouldn't just make warmer conditions on the ground. The air above would also get warmer, which means the atmosphere wouldn't necessarily be any more unstable and so wouldn't necessarily produce the conditions necessary for more severe thunderstorms and derechos. NOAA does say, however, that a warming world could cause the jet stream to move northward, which would suggest the derecho corridors could veer farther north.

Only time will tell when and where we'll see another severe derecho like the one in 2012, but advancements in predictive forecasts has helped improve lead times for these convective storms. So, the next time one looks imminent, people should have more time to prepare.