This post was originally published on June 11, 2015.
This story presents a succinct definition of the storm, a primer on derecho science, major events from past years, and a discussion of why derechos are so difficult to predict.
Not your average thunderstorm
Thunderstorms do not exist to scare us and wreak havoc on property and lives; they are nature’s vertical heat pumps, moving excessive heat energy upwards, away from the surface. The heavy rain, lightning and thunder, and occasional severe weather are all by-products of this heat transfer process, which is called convection.
In extremis, thunderstorms organize into large, fast-moving arcs that accelerate the heat transfer. These lines rapidly scoop up the layer of unstable air layer near the surface, processing an enormous amount of hot and humid air. A super-intense downdraft of chilled air is sometimes manufactured, but this is just nature stabilizing the atmosphere: The heat is moved upward, and cool air sinks to replace it.
When widespread blasts of chilled air spread along the ground, felling trees and destroying property over a long track, a derecho is born. Simply put, the derecho is an atmospheric engine supercharged, fuel-injected, super-efficient at processing summer’s heat energy.
Before the infamous storm of 2012, the term “derecho” was barely part of our region’s lexicon. The public is familiar with tornadoes, and hurricanes, and even nor’easters. But why not derechos? After all, derechos have been occasionally pummeling Washington for decades, likely hundreds of years. The first scientific paper on the phenomenon was written in the late 1800s, a time when tornadoes were barely understood.
I don’t have a definitive answer, but I suspect it has to do with the unusual intensity of the June 29 derecho, striking the heart of our nation’s capitol when it was quite vulnerable. The derecho’s 4.5 million power outages – many multi-day – came at the height of a heat wave (the temperature hit 104 F in Washington that day, the hottest ever recorded in June).
The storm was largely unexpected and hit swiftly during the night. It came with a roaring, frightening, hammer-blow “wall of wind”. There were several fatalities, including people crushed by falling trees.
The event made such a foreceful and stealthy impact that the term “derecho” finally stuck.
Derechos, it is now recognized, exact a considerable societal toll. According to research published in the prestigious Bulletin of the American Meteorological Society, derecho damage and fatalities are on par with some U.S. landfalling hurricanes and Great Plains tornado outbreaks.
Serious tropical storms and hurricanes are infrequent visitors to the Washington region (a decade elapsed between Isabel in 2003 and Sandy in 2012) and we rarely endure EF-3 and stronger tornadoes (the F3 and F4 College Park and La Plata tornadoes of 2001 and 2002 were the last strong-category twisters to hit the immediate Washington region). But, as I discuss below, our city is hit by a derecho once every three years, on average.
Not all derechos are as intense as the one on June 29, 2012. But the phenomenon may be our region’s most frequent , high impact type of severe storm during the summer.
What exactly is a derecho?
A derecho is a widespread, long-lived, straight-line wind storm generated by a fast-moving arc of thunderstorms, frequently in the shape of a bow. Meteorologists have strict criteria that should be met: (1) the damaging wind swath must be at least 240 miles long; (2) the wind gusts must be at least 58 mph along the entire track; and (3) the track should contain several, well-separated gusts of at least 75 mph.
It’s useful to contrast the general properties of a derecho with those of the more familiar supercell thunderstorm. Supercells are a discrete type of severe local storm, generally long-lived (3+ hours), and organized around a powerful rotating updraft 3-5 miles in diameter (called a mesocyclone). A supercell’s primary severe weather includes destructive hail and strong-violent tornadoes. Downbursts and flash floods sometimes occur.
A derecho is a severe line storm (usually arc-, spearhead- or bow-shaped), long-lived (6+ hours), and dominated by a surging downdraft or outflow. Straight line wind damage is created when families of downbursts coalesce and fan out over a large area. The spreading blast of wind distorts the convective line into an arc or spearhead shape, essentially bowing a portion of the line outward.
Occasionally, derechos generate weak tornadoes (F0-F1) by different mechanisms than in supercells. Interestingly, the line’s outward bowing is enhanced by a pair of counter-rotating vortices on the line ends, which draw air into the back of the system. Often the northern vortex becomes quite large, creating a comma-head of cloud, wind and rain tens of miles across.
To recap: A derecho is a long track, violent wind storm created by a bow-shaped complex of thunderstorms, racing across the landscape at 50-60 mph. The bow is many tens to several hundred miles long. Damaging winds are generated within the bow by multiple downbursts that range in scale from 3-5 miles.
Where and when do derechos occur?
Derechos occur year-round across the U.S. and peak during summer, from late May through mid-August. The summer maximum occurs because these storms are nature’s way of efficiently processing large amounts of heat energy near the surface. Factors that delay the release of instability until late in the day – leading to an explosive atmosphere – are especially favored.
The instability is often coupled with moderate-to-strong upper level winds that increase with altitude – creating wind shear, within the core of the summer jet stream. These conditions readily become established along the northern periphery of a large “heat dome” straddling the eastern U.S.
A persistent corridor of instability and wind shear often stretches from the Upper Midwest through the Great Lakes and into the Mid Atlantic. Once the corridor becomes activated, one or more derechos traverse the corridor from northwest to southeast.
The June 29, 2012 derecho striking Washington, D.C. originated over Iowa during the early morning. Additional bowing systems followed on its heels. It’s not uncommon for a family of derechos to track repeatedly across the same geographical region, separated by several hours; usually the lead derecho is the most intense.
The Great Lakes Corridor is the principal summertime derecho highway across the U.S. but a second tract stretches across the southern Plains. The southern track is active during both summer and winter.
What are some significant derechos that have impacted the Washington, D.C. region?
When derechos arrive in Washington, they are not all equally intense and destructive. Like tornadoes, derechos exist along a spectrum in terms of length/duration, peak wind gust, and dimensions. These variations are not part of the official definition, which specifies the minimum set of criteria.
Sometimes the Appalachian cordillera or a derecho’s late-night arrival (thereby processing a more stable air mass) take the “bite” out of these systems.
Aggregate statistics culled over 15 years (1986-2000) suggest that up to 10-15 derechos impacted D.C., but several of these may have been remnants of stronger systems over the Ohio Valley. Since 2000, my own analysis shows that intense derechos have struck the D.C. region, on average, once every 2-3 years.
Some of these cases are presented below. In each example, the accompanying graphic shows the damage swath (wind damage reports, upper left), and the storm’s plan-view radar appearance over the D.C. region (lower right).
August 26, 2003. This derecho moved rapidly southeast out of Indiana. It produced widespread, severe wind damage across the greater D.C.-area leading to multi-day power outages. Hurricane Isabel hammered the region’s power grid less than a month later.
June 4, 2008. A series of two derechos traversed the Mid Atlantic, creating a corridor of high wind damage stretching from Indiana to the Atlantic Ocean. Several small tornadoes were also generated close to D.C.
June 29, 2012. An unusually intense derecho struck during a heat wave, with a total path length of nearly 1000 miles stretching from Iowa to the Atlantic. The storm hit D.C. around 10 p.m., traveling 50-70 mph with gusts exceeding 70 mph. Along its entire track, the NWS issued 300 severe thunderstorm warnings in advance of the storm. Up to 4.5 million utility customers lost power and there were 29 fatalities. A detailed account of this derecho can be found in a Weatherwise story (July-August 2014), written by the author.
June 12-13, 2013. Two derechos again converged on D.C. The first, traced back to Iowa, arrived over our region during the early morning and was dissipating. A second more intense system developed later in the day over Kentucky. The northern end of the second derecho contained a long-track mesovortex spawning the “Intercounty Connector Tornado”.
How difficult is it to predict derechos?
The challenge of prediction goes beyond that for ordinary thunderstorms. A large-scale, derecho-primed corridor can often be identified a couple days a priori. But the certainty of derecho initiation within this corridor, along with timing and location, can be very difficult to forecast. Fine-scale (mesoscale) models now provide some guidance.
Sometimes, though, a derecho’s expansion and intensification stems from the merger of several smaller storm clusters; this process is termed “upscale growth”. Such was the case during the early phase of the June 30 derecho – what appeared to be a chance merger between two separate convective systems over Indiana sealed Washington, D.C.’s fate 12 hours later. Upscale growth may be more of a stochastic (occurring by chance) process, and thus not readily amenable to deterministic model prediction.
Another forecast challenge is whether a derecho approaching the Appalachians will survive its mountain crossing. A recent study identified some useful forecast guidance, related to the amount of convective (buoyant) energy and wind shear on the lee side of the mountains. In this study, organized convective lines (not all of them derechos) were more likely to cross intact before midnight; those storm systems crossing after midnight tended to weaken significantly.
I see a nasty line of storms approaching on the radar. How can I tell if this is a derecho?
Understandably, our region suffers from “derecho anxiety”. And with easy access to weather radar animations, it’s easy to track an approaching storm. Sometimes ordinary squall lines, and even some severe convective lines, are mistaken for a derecho.
A convective storm is more likely to be a derecho if it is bow-shaped and fast-moving. Most importantly, the same line must have a history of producing wind damage upstream i.e. multiple severe thunderstorm warnings that have been issued across West Virginia and Ohio.
The radar appearance alone, however, can be deceiving. This was the case during the June 29, 2012 derecho. The ordinary radar view (left panel, below) was somewhat ragged in appearance, lacking a solid line of “red returns” characteristic of the most intense thunderstorms. And the classic bow or arrowhead shape was only weakly apparent.
However, the Doppler wind velocity image (right panel) suggested an entirely different story: An abrupt, massive “wall of wind” – a leading edge of 50-60 mph wind gusts – pushed through D.C. (orange and pink colors) from the west. The increase in wind speed from calm to damaging was nearly instantaneous. This curious aspect of perhaps the most intense derecho to hit D.C. in at least 50-60 years (if not longer) is not well understood, underscoring the need for more research into these high impact, summertime storms.
Even more about derechos…