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Explainer: What is a derecho? And was Wednesday's storm in Iowa one?

Specific criteria need to be met to label a storm system as a derecho. Here's what you should know about Wednesday's storm.

DES MOINES, Iowa — A rare December severe weather outbreak struck the central U.S. including Iowa on Wednesday.

At least 19 tornadoes and widespread severe straight-line wind gusts caused extensive damage and knocked out power to over 100,000 Iowa customers.

The damage wasn't as bad as the Aug. 10, 2020 derecho, but can this storm still be called a derecho?

What is a derecho?

In general terms, a derecho is a long-lived line of thunderstorms that produces intense wind gusts over a large area.

Specific criteria have to be reached in order to label a storm complex as a derecho:

  • The swath of wind damage must extend more than 250 miles, producing wind gusts of 58 mph or greater along most of its length
  • It must also include several, well-separated 75 mph or greater wind gusts
  • The damage path also needs to be greater than 50 miles wide

At first glance, it appears these criteria were met on Wednesday.

On Thursday, the National Weather Service in Des Moines said, "Preliminarily, it is being called a serial derecho. SPC (Storm Prediction Center) and local offices are still assessing the reports."

What is a serial derecho?

This type of derecho is typically associated with a strong low-pressure system and is most common in the spring and fall.

The winds surrounding strong low pressure systems like Wednesday's are typically strong before thunderstorms even form.

The line of thunderstorms only enhances that wind speed.

On Wednesday, non-thunderstorm winds peaked at just over 80 mph.

Thunderstorm wind gusts peaked at 100 mph in Council Bluffs and 88 mph in Audubon.

RELATED: Storm aftermath: National Weather Service confirms 13 total tornadoes in Iowa from Wednesday

RELATED: Families in Bayard, Jefferson left to rebuild following historic Iowa storms

The 2020 derecho was a progressive derecho.

Progressive derechos usually begin as one thunderstorm or small cluster of thunderstorms.

Over time, it will transform into a bowing line of thunderstorms.

Unlike serial derechos, progressive derechos strengthen when they move through areas of high instability, making them most common in the summer.

Unlike tornadoes, the National Weather Service does not officially confirm derechos.

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