Derechos “are primarily a summer phenomenon,” said Harold Brooks, a senior research scientist at the National Oceanic and Atmospheric Administration’s National Severe Storms Laboratory. “If you make it more summery, you might expect them to increase.”
“The thing that makes derechos painful is that while their winds are frequently not strong enough to destroy buildings, they take out power lines and trees over a large area,” Dr. Brooks added. “Tornado paths may have intense destruction in the middle of it. In a derecho, it covers a much broader area.”
The week’s ominous weather shocked even professional weather watchers. Zach Sharpe, who heads the Iowa Storm Chasing Network, said he had never experienced such bizarre weather in December.
Much of Wednesday was unseasonably warm — “It felt like spring; there were people wearing shorts,” he said — but once the storm front approached, a blast of cold air brought instantly freezing temperatures and 80-miles-per-hour winds. “It was eerie to be chasing tornadoes 10 days before Christmas,” Mr. Sharpe said. “We were out in our vehicles, listening to Jingle Bells, while tornado sirens were going off.”
Still, scientists said, this week’s storm was so unusual and had so many different forces behind it, including a strong jet stream moving across the central states, that it can be difficult to disentangle the impact of global warming compared to other factors like La Niña, an intermittent climate phenomenon in the Pacific Ocean that can influence winter storms.
This storm was “about as messy an event as you can get,” Dr. Brooks said. “Is this a weird one-time event? Or maybe it won’t repeat for a long time? Will the seasonality change a lot? I don’t think we know.”
Victor Gensini, a professor of meteorology at Northern Illinois University, pointed out that all extreme weather events are now taking place against the backdrop of an atmosphere that has been profoundly shaped by humans burning fossil fuels. “Assume that all extreme weather events are impacted by climate change,” he said.