What’s a Derecho?

It is a high-kicking chorus line of thunder­storms, 250 miles wide and advancing at 60 mph. Its 100 mph downbursts pound the ground, sometimes for 1,400 miles. Iowa is as prone to destructive derechos as Florida is to hurricanes. Derechos now happen much more often. Welcome to the new climate.

Figure 1. “Is that a rift in space-time, Daddy, or just a cloud?” This squall line in Sydney had winds of more than 160 km/h (100 mph, likely from downbursts) that blew down trees and power lines in Spring 2014, leaving 30,000 homes without electricity. Thanks to Nick Moir (that’s his daughter).

Figure 2. Large macro-downburst in New Mexico. This one is not from a derecho but that’s a downburst, a big one. A derecho tends to have many smaller downbursts; some are ‘wet’ like this one, but others are ‘dry,’ just violent winds spreading in all directions at 90–120 mph when they hit the ground. Airplanes coming in for a landing have been hammered into the ground by a micro-downburst. Thanks to Scott Kuhn.

The three U.S. derechos in early June 2020 caused many people to look up the word for the first time. A derecho (“deh-REY-cho”) usually has hurricane-strength winds over a wide (often 250 mi/400 km) bow-shaped front that looks like a “seam in space-time.” But the winds do not spiral in the manner of hurricanes and tornadoes; the damage mostly occurs because of downbursts.

When a downburst hits the surface, it spreads in all directions at perhaps 100 mph (160 km/h). The front itself speeds eastward at about 60–70 mph, though sometimes going NE or SE to detour around a high-pressure system. Large hail is fairly common. In the 5–10 minutes it might take for the front to pass over you, it is extremely noisy. Nearby trees may be snapping and falling, but you probably won’t hear such individual events because so much else is going on at the same time.

Seeing one approach in your rear-view mirror is scary. From the time it appears on the western horizon, it’s only 5–10 minutes until it is overhead. Unlike a tornado, it is so wide that there is no escaping it by turning north or south. If eastbound, you might be able to outrun it at 80 mph — until reaching the next traffic jam in the middle of nowhere, whereupon you may be pounded by one of those downbursts. If you are westbound, it will arrive very much sooner. Find a sturdy bridge to hide beneath; leave one window cracked open and cover yourself with a coat or blanket in case of flying objects; keep your seatbelt fastened, but loosely enough so you can hunker down.

Figure 3. Track of the “4th of July derecho” in 1999. Note that red X above.

Given how infrequent derechos have been, we are lucky to have a first-hand account from a trained meteorologist. Sarah Jamison, a National Weather Service meteorolog­ist, was enjoying the 4th of July holiday weekend in 1999 at a campground in western Maine (in Figure 3, look for the red X at right) when North Dakota weather reached out and touched her after a speedy 15-hour trip through Canada. Here’s the debrief[i].

July 4th was a very hot and humid day, with an afternoon high near 90°F/32°C. As the night progressed, Sarah mentioned that “the air was very stagnant with no wind, making it very un­comfortable in the hot tent.”

Awake, she noticed lightning in the distance. Quickly, the gust front hit and the wind rose alarmingly. Once everyone had run to the SUV in the parking lot, they could see trees falling all around with every lightning flash. About two dozen trees were blown down, several of which were larger than two feet (60 cm) in diameter.

After about five minutes, the winds began to weaken; within a half-hour, the storm had passed. One of the large trees and two smaller ones had fallen on Sarah’s tent, completely crushing it. Fortunately, no one in the group was hurt.

The roar of the storm’s winds was so loud that none could recall having heard the trees snapping or crashing to the ground. As is often the case with derechos, the damage that hit Sarah’s camp­ground was associated with a narrow band of intense, very damaging downbursts. The derecho cont­inu­ed to cause damage across central and southern Maine. Finally, after traveling over 1,300 miles (2,100 km), the derecho ended just before reaching the Atlantic coast. It had begun 21 hours earlier in North Dakota.

To summarize, a derecho is fast-moving, gives little warning, and those pounding down­bursts are loud. A derecho’s path can be both wide and very long. The derecho is a rarely seen type of severe wind­storm, named ‘straight’ in Spanish by a Danish physicist in 1877 for its path through Iowa.

I have successfully avoided derechos so far, even though I grew up in Kansas City in the 1940s and 1950s. I was already tuned in to extreme weather, back in the mid-50s, as I volunteered to serve in mobile Civil Defense communications teams. That gave me an up-close view of raw tornado damage, one that has stuck with me ever since. But there were no derechos back then; I never heard the word.

Figure 4. West of the Rocky Mountains, derechos are infrequent-but that may be changing, as Figure 5 shows.

Yet, as Figure 4 shows, Kansas City is now near the area that has the most derechos, about four every three years.

Out here in Seattle, they are never a topic in the weather forecast discussions. That is because, with three known exceptions, derechos do not occur west of the Rocky Mountains. The third exception is the spectacular one (Figure 5) that recently started in SE Utah, crossing the Rockies en route to Canada. It broke all the rules — or, at least, expectations.

Figure 5. A derecho footprint of high-wind damage stretching northeast a thousand miles (1,600 km) from almost-Arizona to almost-Canada in 15 hours. That’s an average ground speed of 67 mph

At about nine o’clock on the morning of June 6, 2020, a derecho began down in the SE corner of Utah. That’s hot semi-desert up on the Colorado Plateau where the San Juan River turns west, soon to join the Colorado River. About midnight, that derecho finally weakened when reaching the Canadian border in North Dakota.

From Rare to Common?

In thirty years, the U.S. has had more than 65 derechos making the news since their rate increased in 1991, a ten-times increase in their annual numbers. Since 1991, Europe has had 14, South America 4, and Asia 1. Australia classifies severe windstorms differently, but they still have long squall lines (Figure 1) that we elsewhere would recognize as derechos.

The U.S. has developed the derecho form of climate disease earlier than has the rest of the world, so it is worth a look at the derecho progression in the U.S. to see what may be ahead for other places.

And the derecho is no longer rare — sometimes, another arrives the very next day. And maybe the following day as well, as happened to Minnesota (MN in Figure 10) in 1995. Worldwide, a crude count of derechos suggests a ten-fold increase in annual numbers after 2006 (Figure 6).

Figure 6. Plot of worldwide derechos per year. Caution: earlier years are likely underreported, lowering that 10X ratio. These numbers are from voluntary additions to the Wikipedia article.

Derechos are big monsters. To qualify as a derecho, a windstorm needs to produce a continuous swath of wind damage that is (at least in the U.S. definition) longer than 400 km (250 miles). In Figure 7, the hypothetical derecho in England shows the scale.

The “Inland Hurricane”

In the U.S., the derecho is sometimes called an “inland hurricane” because of its high winds. But the derecho covers ground five times faster than a hurricane and twice as fast as a tornado — more like a fast freight train at 60 mph (100 km/h). They travel eastbound, though sometimes one is forced to detour to the northeast or southeast.

It’s not just faster ground speed and longer reach: a derecho also cuts a far wider swath of damage than a hurricane — say, 250 miles (400 km) wide (that’s London to Scotland in Figure 7) rather than the 50 miles (80 km) for a hurricane swath. The biggest tornado is a mile wide (1.6 km).

Figure 7. Derecho width diagram.

Hot and humid weather is the usual setup for a derecho, and we now have more hot and humid weather to create them. Reported derecho numbers[ii] (no billion-dollar threshold this time) are up 10X worldwide since 1990, the big step occurring about 2006.

More people are battered by a derecho than a tornado or hurricane, and it is far more likely to catch people unprepared. That June 29, 2012 derecho in Figure 10 killed at least 22 people but, it being in the middle of a heatwave, one must consider how many more people were killed in subsequent days because of the loss of electrical power for air-conditioning; a succession of hot, sleepless nights creates the deadliest heat waves via heat stroke. Those long delays in restoring power kill many additional people; undergrounding power lines and backup power are obvious solutions.

There were 9 million power outages from that 2012 derecho, and many took ten days to repair. After the Memphis derecho of 2003 (locals called it ‘Hurricane Elvis’) with downburst winds exceeding 108 mph (174 km/h), it took two weeks to restore power, leaving many residents unprotected from dangerously hot and humid conditions.

Figure 8. Note that the DC area got hit by two derechos the same day, the first at 8 AM and the second about 4 PM. All times EDT (GMT-4hr).

Like tornados, derechos may cluster.

Derechos may have some tendency to cluster, given that one setup is hot and humid.

Figure 9. July 1995 derecho paths, back in the days when color graphics were drawn with colored crayons. The orange one went from Montana (MT) to Pennsylvania (PA). Original NOAA figure annotated by author.

Figure 10. A year earlier in June 2012, an Iowa derecho swept east to DC, aided by a widespread heat wave.

Figure 11. Snapshot of radar mosaic shows two derechos racing eastward on March 9, 2002. Note how narrow and straight the left one is; the usual bands of rain lack such coherence.

The other way derechos can cluster, called serial derechos (Figure 11), delivers a one-two punch, where a second derecho arrives a few hours after the first.

Thermodynamics told us, back before the modern era of climate modeling began in 1968, that we were going to have more violent weather, simply because warmer air contains more moisture. But thermodynamics could not tell us what forms the violence would take. Derechos would have been unlikely to make the list, had there been one, because that form of severe windstorm was so rare and little known.

Among the emerging extreme weather threats, we see intensified derechos coming on the scene, especially in the U.S. Besides happening more often, they now occur in more places. There are many ways in which derechos might intensify: longer paths, wider paths, greater depth so it takes longer to pass over, more one-two punches, faster forward movement, and so forth.

So, think of the derecho’s swath being cut by a monster hurricane five times wider than usual — and maybe another, several hours later — and you will have a vision of our future if we fail to back out of the danger zone via removing the excess CO2 in the air.

Think blitzkrieg warfare, not the slowly shifting battle lines of World War 1 when things changed more slowly.

William H. Calvin, Ph.D., is a professor emeritus at the University of Washington School of Medicine in Seattle and is now the president of the CO2Foundation.org. His seventeenth book, Extreme Weather and What to Do About It, features a critique of the current climate message; describes five extreme weather shifts that occurred a decade ago; gives a justification for what makes climate an emergency now; considers design criteria for how to take the 50% excess of CO2 out of circulation; and offers a proposal for how to get started with a “Manhattan Project 2.0.”

[i] Derecho debrief adapted from www.spc.noaa.gov/misc/AbtDerechos/casepages/jul4-51999page.htm

[ii] These worldwide derecho numbers have not been worked up in the diligent manner that NOAA did for the billion-dollar U.S. events; they are merely my tabulation of volunteer listings in Wikipedia EN. A good international citizen science project would be to augment it with microfilm searches for pre-1980s derechos (and similar 150-km-wide squall line events) in newspapers and weather bureau archives for each country, preparing a list for the experts to work over.