(Editor’s note: This post was originally published June 29, 2017. We’re republishing it on June 29, 2018 on the six-year anniversary.)

It’s been six years since the infamous June 29, 2012, derecho cut a massive trail of destruction through the heart of Washington, becoming one of our region’s watershed weather events.

The savage storm, for which there was little advance notice, delivered a knockout blow to the region, cutting power to the more than a million residents in the middle of its most intense heat wave on record.

Six years later, we’ve learned much about this particularly violent class of storms. But questions still remain about how effectively we’ll be able to detect and prepare for the next one. And it’s just a matter of time before another strikes.

The 2012 storm, revisited

On the night of Washington’s hottest June day ever recorded, 104 degrees, an unexpected and invisible “wall of wind” slammed into the western suburbs of D.C. around 10 p.m. Known officially as the “Ohio Valley-Mid-Atlantic Derecho,” during the next hour, a curtain of wind pushing 60 to 80 mph swept across the Mid-Atlantic. Lightning was nearly continuous, and sheets of rain were driven horizontally against homes and vehicles.

Just 30 minutes after the sudden, violent windstorm arrived, many neighborhoods were left in the dark. Tree limbs sagged into power lines, whole trees blocked roads and cut through the roofs of homes. The devastation was everywhere; the cleanup would take days, even weeks, and the power grid had to be completely rebuilt in many locations.

Derecho history

Since the 1980s, the Washington region has experienced several derechos. But until this pivotal 2012 storm, the term “derecho” was not part of our region’s lexicon. In fact, summertime derechos have been ravaging portions of the Midwest, Ohio Valley and Mid-Atlantic for decades, but few have reached the level of exceptional longevity and violence produced by the June 29 storm — with a track nearly 900 miles long (originating near Chicago), 22 fatalities, over 1,000 wind damage reports, and 9 million power outages nationwide.

Our region’s “derecho season” begins in early June and extends through late August. Derechos, by definition, are widespread wind storms generated by an arc of powerful thunderstorms. Something about these storms causes them to congeal and grow “upscale,” that is, merging into a powerhouse, mega-thunderstorm complex that continuously regenerates for up to 24 hours. The level of economic loss, lives lost and societal destruction, in extreme cases, is comparable to that of a landfalling hurricane.

Derecho science

When people think “severe thunderstorm,” a type of rotating storm called a “supercell” most often comes to mind — especially if they live in the Great Plains or Mid-South. These isolated, long-lived storm cells are driven by a rotating updraft, called a mesocyclone, that vastly transcends the life and intensity of ordinary storm cells. Supercells are infamous for dropping monster hailstones and producing most of our strong to violent tornadoes.

Derechos are larger convective systems, composed not of a single cell, but a phalanx of cells that bows out, arrowhead-like, as if a fist of wind punches through the line (which, in fact, is the case). Instead of updrafts, these storm systems are dominated — propelled forward, in fact — by a powerful, spreading, surging downdraft. In a thunderstorm, the downdraft is the chilly blast one feels just before the onslaught of heavy rain. But in a derecho, the windy blasts from dozens of extreme storm downdrafts (called downbursts) coalesce into a single, massive wind surge that races forward at speeds of up to 60 to 70 mph.

The wall of wind appears razor-sharp on Doppler radar, and it strikes like an iron fist. One second it’s dead calm, the next second all hell breaks loose, amid the rapid ramp-up of wind and its deafening roar. It’s a sudden, swift blow that leaves behind a trail of devastation hundreds of miles long; and when this windy fist comes at night, the experience is terrifying.

Such was the visceral experience of the June 29, 2012, derecho.

Another powerful derecho is inevitable

Five years after the 2012 trauma, two questions come to mind: 1. When will this happen again; and 2. how can we better predict these things?

To answer the first question, our region’s meteorological history since about 1990 suggests a “return interval” of derechos, of all intensities, of about three years. Some of these fizzle on our doorstep, some come through with a sound and fury.

There is a spectrum of derechos. June 29, 2012, was an exceptional rarity, on the far end of the violence scale. But when we turn back the calendar to June 1954, we have documented evidence (in the scientific literature) of another mega-type derecho that originated near Chicago, which subsequently blew into the District, unleashed winds in the 60-to-70-mph range, and left behind widespread damage.

Perhaps, then, two to three violent derechos can be expected to strike our region in a century. Time will tell when the next one approaches the District from the northwest, from Chicagoland. It’s hard to say that we are “due” because derechos don’t follow an exact timeline. It could happen this summer, or not for another 50 years. In the parlance of meteorology, the best we can say is that the truly nasty events are “episodic.”

Predicting the next derecho

Now, about the question of predictability. The June 29, 2012, storm was sneaky. For many years, the National Weather Service Storm Prediction Center has provided advance notice of major convective outbreaks and events, out to three days (and now, most recently, out to eight). The day before this megastorm, the probability was low-end for any type of severe storm, with no mention of a derecho. The morning of June 29, there was still no heightened awareness for the D.C. region. This is not to fault the pros at the Storm Prediction Center, because we simply had no real indication that a massive convective storm was about to sprout up across the Ohio Valley.

In fact, the specter of a powerhouse derecho did not become apparent until mid to late afternoon on June 29. A key event occurred that afternoon over Indiana. The initial cluster of thunderstorms that blew up near Chicago, early in the morning, collided with another batch of thunderstorms blossoming over Indiana, in the early afternoon. That merger — the “upscale growth” I mentioned earlier — was a key event in the development of a much larger, bowing surge of storms.

These are essentially random mergers between adjacent, stormy regions; they are not well captured in even the highest-resolution weather prediction models. BUT once the big bowing complex of storms finally congealed over Ohio, the models took notice, and by late afternoon they predicted that the ominous convective line would continue eastward, crossing the Appalachians, where the heat-baked D.C. region lay in unexpected wait.

That day, the mercury pushed 104 degrees in the District. At 7 p.m., a weather balloon launch from the National Weather Service forecast office in Sterling, Va. (near Dulles), revealed that a nearly unprecedented amount of buoyant energy had accumulated in the atmosphere. East of the mountains, a powder keg of unstable, superheated air was literally packed to the hilt by evening. This was another key event that sealed the District’s fate. Many derechos approaching from the Ohio Valley corridor do not survive intact across the Appalachians. Their dense, cold outflows of wind surge down the eastern slopes, outrunning the parent storm, disrupting the balance of air flowing in, and air flowing out.

The eve of June 29 was very different from the norm. A vast reservoir of hot air and high humidity lay waiting on the eastern side of those mountain slopes. Disruption be damned, that line of storms would surely regenerate in the super energy-rich air. So much energy was in that air mass, the setting sun had little effect; thermal inertia maintained extreme instability into the late evening. This became the thinking of meteorologists area-wide, starting around 7 or 8 p.m. Severe thunderstorm watches were rushed out by the Storm Prediction Center, covering vast tracts of territory east of the Blue Ridge. Broadcast meteorologists took to the airwaves portending a wild night ahead.

But in the early evening, folks were retiring, hunkering down into the din of air-conditioned homes and apartments, grateful for nightfall and respite from scorching afternoon temperatures. There was little situational awareness of the rapidly approaching derecho, not when a scorcher of a day under Venus-white skies, with nary a cumulus cloud, had claimed the headlines. The thought of refreshing thunderstorms was as remote a thoughts of water on the Moon.

But then the hammer blow arrived, preceded (by those looking out a window) by a nearly continuous, strobe-like flickering of lightning. For many, the power cut out mysteriously in the silence of a sultry summer night (power lines a few miles down the road quickly succumbing to the onslaught of wind), and THEN about 30 seconds later, the winds crescendoed into their deafening roar.

What we now anticipate better is the combination of atmospheric ingredients that conspire to generate a corridor of “derecho potential” arcing out of the Midwest, cutting through the Ohio Valley/Great Lakes, and terminating in the Mid-Atlantic (or, at times, New England). Such a corridor can be identified a few days in advance. But whether a derecho will TRIGGER within this corridor, and grow upscale into a major league system, remains the tricky part.

We now have heightened “derecho-potential” days, and such language comes out in Storm Prediction’s “convective outlooks.” Beyond this, though, such meteorologist-speak has not reached the lives of ordinary folk — typically just “hazy, hot and humid, with a chance of thunderstorms — some of which could be strong to severe”.

Unfortunately, until we see that big, bow-shaped complex starting to take shape over Ohio, about six or seven hours from crossing the Appalachians, all bets are off on how the day will end.

And then, if the beast emerges, will it survive its Appalachian crossing? There is not much research on this, and the few papers that address the topic suggest that when the crossing occurs after midnight, survival and thus the severe wind threat to the District is greatly diminished.

When you think “severe thunderstorm” in our region, you are reminded of the most familiar outcome — gusty winds, heavy rains and maybe a spattering of small hail. More often than not, the power stays on. The trees remain intact. These are the most familiar hoofbeats, those made by horses. But beware of the bull, the untamed, beastly derecho cut loose from of Ohio, because that encounter is destined to come once again.

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