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The intense storm heading to the mid-Atlantic might become a derecho. Here’s what that means.

Heavy rain, swift wind, and hail are poised to strike Washington, DC, and the mid-Atlantic region.

A shelf cloud forming over Reston, Virginia ahead of the storm on May 14, 2018.
A shelf cloud forming over Reston, Virginia ahead of the storm on May 14, 2018.
Dave Dildine/WTOP
Umair Irfan is a correspondent at Vox writing about climate change, Covid-19, and energy policy. Irfan is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.

Intense winds, heavy rainfall, and hail are expected to strike the lower mid-Atlantic states, including Washington, DC, and the central Appalachian Mountains, on Monday evening in what could become a derecho, according to the National Weather Service.

“The risk for damaging winds is expected to continue, and become potentially widespread, through this evening,” NWS reported in its latest alert.

The Washington Post is reporting that the storm is likely to hit DC between 6 and 8 pm, with a severe thunderstorm watch until 9 pm.

Derechos can be devastating storms. The word, Spanish for “straight ahead,” was coined in 1888 by meteorologist Gustavus Hinrichs to distinguish storms producing winds in a straight line that knock down trees and buildings in the same direction from storms that produce rotating winds that cause tornadoes. A derecho storm system is also distinctly wide and fast-moving, causing wind damage for more than 250 miles (larger than the diameter of most hurricanes), with gusts pushing faster than 58 mph along most of its length.

About 70 percent of derechos occur between May and August, and most originate in the central part of the United States when strong, straight winds meet cool downdrafts. Stephen Corfidi, a researcher at University of Oklahoma’s Cooperative Institute for Mesoscale Meteorological Studies, explained that derechos are hard to forecast because there are so many moving parts and they act quickly on small scales.

“They are a particularly difficult nut to crack,” he said.

The key ingredients are a layer of cool dry dense air on top of warm moist air near ground level spread out over a large area. Warm air wants to rise, cool air wants to sink, and this instability results in a massive storm. If the winds are all pushing in one direction, it creates a snowplow effect driving an expansive wall of convective storms, a.k.a. thunderstorms, rapidly through a region.

“To distill it down in simplest terms, it’s a line of regenerating thunderstorms and they regenerate in a very organized ways,” Corfidi said. “They’re very efficient in restoring the equilibrium over a large area.”

Derecho frequency in the United States
Derecho frequency in the United States.

These storms can be dangerous and deadly. A derecho struck the mid-Atlantic region in 2012, killing 13 people and knocking out power to more than a million in the Washington-Baltimore area just as a heat wave struck. Winds gusting up to 80 mph knocked over trees, ripped shingles off roofs, and toppled power poles.

A derecho moving through the mid-Atlantic on June 30, 2012.
A derecho moving through the mid-Atlantic on June 30, 2012.

The storm’s rapid formation and swift gallop across the country made it difficult to prepare and caught many people off guard.

And as the climate changes, it’s unlikely that derechos will change in severity, but they may change their range. “There is nothing to suggest that a warmer world necessarily would favor stronger derechos,” according to NOAA. “What can be said with greater certainty about derechos and climate change is that the corridors of maximum derecho frequency likely would shift poleward with time.”

The current storm hasn’t yet reached the threshold of a derecho in terms of size, but forecasters expect it to easily clear the benchmark for wind, with gusts up to 75 mph expected.

It’s best to charge your devices as soon as possible and avoid being outdoors when the storm hits.

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