The third-largest city in the U.S. is being absolutely pummelled by a powerful storm that brought 161 km/h winds to other parts of the Midwest earlier on Monday.
Chicago is in the bullseye of a derecho, a powerful type of storm known for winds that pack a tornado’s punch (insert your own Windy City joke here). The National Weather Service warned that the storm will also drop “torrential rain” and called this an “EXTREMELY DANGEROUS SITUATION” (all caps theirs).
The storm system has left a trail of destruction across the Midwest. Winds hit 180 km/h in Iowa on Monday, tearing apart metal silos like a sheet of paper and leaving half a million residents without power as it cruised across the state and western Illinois. With the storm now barreling into a metro area of nearly 10 million, the toll of damage and downed power lines will surely rise. The city’s steel and glass skyline is also particularly vulnerable to the powerful winds that could shatter windows and create unsafe conditions for those in homes and offices and on the streets below.
The speed with which derechos can form and show up often only compound the damage they do. Capital Weather Gang noted that the short lead time for the forecast means that companies responsible for keeping the lights on rarely have time to prepare for the damage to power lines compared to, say, a hurricane. Though as we saw on the East Coast last week when Hurricane Isaias made landfall in the Carolinas and moved up the coast, even having the lead time of a good forecast can still end in destroyed electric infrastructure and leave customers waitings for days for things to get hooked back up.
Derechos are known as straightline wind storms. That means that while winds crank up to speeds associated with tornadoes and hurricanes, they don’t swirl. Instead, they generally push in a straight line (hence the name) from west to east in the U.S. It’s essentially a wall of wind and water, occasionally preceded by a shelf or roll cloud. When a derecho is approaching, it basically looks like the alien ships from Independence Day, which is somewhat fitting given the damage they can cause.
Similar to storms that produce tornadoes, though, derechos require a highly unstable atmosphere to form and do damage. The key ingredients are warm, moist air that gets sucked up into the storm and cooler air on the backside of the storm and below it. It’s this clash of hotter and cooler air that fuels the storms’ winds as they roar across the country. With much of the U.S. sweltering under intense heat — a hallmark of climate change — the warm air part of the equation was no problem. Similar monster storms have smacked the East Coast in recent years, including one that this Earther weather watcher got to enjoy the sight of from the rooftop of our old New York office in 2018 as it approached. (Pro tip: Heed weather forecasters’ advice and go inside before the first raindrops fall.)
After passing through Chicago, Monday’s derecho will continue to wreak havoc into the early evening hours. Severe thunderstorm warnings and watches extend eastward over Indiana and Michigan.