A barn destroyed in last year’s derecho. (NWS photo)

Today marks one year since an extremely rare and very powerful December derecho swept across Iowa, killing one person and causing widespread destruction.

Meteorologist Mike Fowle, at the National Weather Service, says the massive, long-duration storm is cemented in state history and will, hopefully, never be matched. Forty-nine of Iowa’s 99 counties were declared disaster areas.

“December 15th did set the Iowa record for most tornadoes in a single day. We had 63 reported tornadoes and that broke the previous record of 35 which occurred during the evening of August 31st of 2014,” Fowle says. “It was a really extraordinary event.”

While tornadoes can happen any day of the year, it’s very unusual to have one in December, let alone 63 in a single day. That one storm accounted for more than half of all tornadoes (114) statewide that year. A derecho is characterized as a widespread, long-lived, straight-line wind storm associated with a fast-moving group of severe thunderstorms.

Rudd Library hit by derecho. (NWS photo)

The December 15th derecho was the second one to hit Iowa in two years, following another on August 10th of 2020, which placed the term “derecho” into the vocabulary of everyone in the state. The August 2020 storm packed extremely powerful winds, peaking at 140 miles an hour near Cedar Rapids.

“The one in December had just slight differences in the amount of instability and then the wind profile and that allowed for a lot more tornadoes to actually form,” Fowle says. “Just small changes or subtle changes in the environment can cause big changes in the outcome, whether it’s all straight-line winds or tornadoes.”

The power was knocked out to more than 140,000 homes in the December storm, and the one person who was killed was a truck driver whose semi was blown into a Benton County ditch. Before the derecho the previous August, most Iowans had never heard the term, but forecasters knew it well. “Across Iowa, derechoes are not extremely uncommon. We do average about one derecho a year that would impact a portion of Iowa,” Fowle says. “We’ll see more down the road, that’s a given here across Iowa. Let’s just hope they’re not in December and they’re not at the strength of the August event.”

Coincidentally, Iowa had a derecho earlier this year, in July, but it was nowhere near as destructive as the previous two.

Radio Iowa