O’FALLON, Mo. — The severe thunderstorms and tornadoes that swept across Nebraska, Iowa and Minnesota and into Wisconsin and Illinois last week were a phenomenon called a “derecho,” the National Weather Service reported. It’s pronounced dah-RAY-choh — a Spanish word for “direct” or “straight ahead.” A derecho is a line of intense, widespread and fast-moving windstorms and sometimes thunderstorms that move across great distances. These are rare in the north-central United States, especially in December. There were at least 45 tornadoes, 12 of them rated EF-2. Thunderstorms carried winds of up to 80 mph in a wide swath for 260 miles long. Five deaths were blamed on the storm.
To be precise
The garden variety derecho carries a precise meteorological designator: “progressive derecho.” It moves arrow-like in a straight line. The derocho Wednesday night, however, was a “serial derecho.” It bowed outward in a path that was both long and wide — hardly arrow-like. The last previous derocho in the region was in August 2020 with 100 mph winds that lasted several hours and caused damage from eastern Nebraska, across Iowa and into parts of Wisconsin and Illinois. The 2020 storm was a progressive, not serial derecho and in August—not December.
What caused this derocho?
Moist air from the Gulf of Mexico usually cools as it moves into the upper Midwest. This cooling minimizes the risk of severe thunderstorms and tornadoes. But this year the winter has been too mild to cool the Gulf air. In fact, ahead of the storm, temperature were in the 70s on places. It was like a “warm July evening,” said weather historian Chris Burt at the National Weather Service. The pent-up energy exploded into horrific winds and tornadoes.