A decade ago, “derecho” — the term used to describe a fast-moving, extensive, enduring and violent complex of thunderstorms — was launched into the national spotlight when destructive storms swept from the Midwest to the Mid-Atlantic, cutting power to millions and claiming 22 lives.
Since that infamous storm on June 29, 2012, a number of extreme heat-driven derechos have followed in its footsteps, breaking an array of records while leaving wide swaths of damage from Colorado to Canada.
In just the past six and a half months, two of the most destructive derechos on record have occurred. Both events blasted sections of the Midwest; the first on Dec. 15, 2021, and the second on May 12 of this year. Both events occurred on days with record-setting heat.
The record-breaking temperatures associated with these recent events is yet again prompting questions about whether rising temperatures from human-caused climate change is increasing derecho destructiveness and, more troubling, might portend even more extreme derechos in the future. As these storm complexes can produce wind damage comparable to hurricanes, cost billions of dollars and leave people without power for weeks, as the Iowa derecho did in 2020, the stakes are high.
Armed with 10 years of severe weather and climate change research since the 2012 derecho, scientists now say rising temperatures could well increase the fuel for these violent storms — making them stronger, more extensive and longer-lived — while shifting when and where they occur.
In a warming word, “given the right ingredients, I think it’s reasonable to make the argument that derechos could be more intense and could be larger or more widespread,” said Jeff Trapp, head of the atmospheric sciences department at the University of Illinois, who studies severe thunderstorms.
The derecho-extreme heat connection
Amid rising temperatures in recent years, derechos and other severe thunderstorm events have produced an increasing number of “significant” wind gusts — which the National Weather Service defines as 75 mph or higher.
All four of the top derechos, based on the number of reports of these significant gusts, have occurred since June 2020; all of the top nine derechos by this measure have happened over the past 10 years (including the June 2012 event). The top two — which each generated 64 significant reports — were the recent Dec. 15, 2021, and May 12 events.
Heat is the common denominator in many of these events:
- The Dec. 15, 2021, derecho, which unleashed destructive winds from Kansas to Wisconsin, was preceded by temperatures as much as 30 to 40 degrees above normal. Both Iowa and Wisconsin saw their highest December temperatures on record that day.
- The May 12 derecho, which hammered the zone from eastern Nebraska to southwest Minnesota, erupted on a day with record-high temperatures from Texas to Maine.
- When a destructive and deadly derecho slammed eastern Canada on May 21, record-setting heat swelled over much of eastern North America, with 90-degree heat reaching as far north as Vermont.
- While temperatures were only somewhat above average ahead of the Aug. 10, 2020, Iowa derecho, the costliest thunderstorm disaster in U.S. history, it was still very warm and humid as a heat dome swelled over much of the Midwest and South.
- The June 2012 derecho occurred on the hottest June day on record in Washington — the temperature hit a blistering 104 degrees. Nashville (109) posted its highest temperature ever observed, while Raleigh (105) equaled its all-time high.
It is not a coincidence that these derechos occurred on days with searing heat. Thunderstorms require instability, which acts as their fuel. It is often highest in environments that are exceptionally warm and moist. The amount of instability ahead of the 2012 derecho, for example, was extraordinary.
Extreme derechos often form on the edges of heat domes that broil large swaths of the country; the corridor along which they form is sometimes described as a “ring of fire.” In these areas, brisk middle-atmospheric flow associated with the jet stream readily overlaps extreme heat and humidity — a volatile combination that can fuel violent storms.
Climate change connections
Considering that scientists have found — through attribution studies — that climate change is making record-breaking heat many times more probable, wouldn’t it follow that this more intense heat is also making derechos stronger? It’s hard to say.
Connecting derecho intensity to climate change is complicated by two main issues. First, the sample size of extreme thunderstorm wind events is small and available data only dates back to 2004 — so it’s not possible to discern meaningful trends. Second, the recent increase in “significant” wind gusts noted in derechos may well be tied to an increase in weather monitoring.
“Increased mesonet [networks of weather stations] deployment and more home weather stations certainly contributes to the increase,” said Evan Bentley, a meteorologist at the Weather Service’s Storm Prediction Center, in an email.
Bentley said he expects that severe wind “records will likely continue to be broken” as observation networks further expand. As such, it may be challenging to tease out the contribution of climate change for these rare events.
But their seeming connection to record-breaking temperatures is an intriguing area of investigation, said John Allen, a professor of meteorology at Central Michigan University who researches how climate change may alter severe thunderstorms.
In 2018, Allen published a paper that projected more frequent severe thunderstorm days as rising temperatures increase instability. In an interview, he said that in a warming world, “there’s a greater breadth of favorability” for storm environments supporting derechos.
“We have more days with relatively large instability and that contribution could lead to bigger events,” Allen said.
But Allen said that the conditions required for derecho formation are “very specific” and that there are some factors that could “go against development” in a warming world. For example, jet stream winds that power derechos — which require strong temperature contrasts — could weaken with rising summer temperatures and retreat to the north.
Trapp explained that the sinking air associated with strengthening summer heat domes could more frequently quash storms. “Often with heat waves, we have suppressed conditions,” he said. “Air in relatively lower parts of the atmosphere is trapped. Those conditions are not conducive to thunderstorms that are widespread and that would create a lot of damage.”
But if increasingly hot conditions suppress storms in the summer, it could boost them in the spring and fall when the jet stream dips south, said Noah Diffenbaugh, a climate scientist at Stanford University who researches climate change and extreme weather events.
“If the odds of hot conditions are extending into the time of year that the atmospheric circulation is conducive to the storms, then global warming can increase the odds of those ingredients coming together [for a derecho],” Diffenbaugh said.
Trapp, Allen and Victor Gensini, a professor of meteorology at Northern Illinois University, agreed derechos may tend to increase in the spring and fall while perhaps shifting north some in the summer. The “parameter space favorable for derechos could be moving,” Gensini said.
Two derechos from the past year could be harbingers of what we’ll see more of in the future. The Dec. 15, 2021, derecho struck a region that had never before seen such a wintertime windstorm, while the derecho that struck Canada in May hit unusually far north and east.
Trapp, Allen, Gensini and Diffenbaugh all stressed that derechos are rare, complex events and that connections to climate change remain challenging to unravel. But Diffenbaugh said scientists have made considerable advances in linking human-caused climate change to extreme weather events since the 2012 derecho, which can pave the way for improved understanding.
“I think we’re in a much stronger position now than we were a decade ago to answer these questions,” he said.