Meteorological Terminology: What Exactly is a Derecho?

What is a Derecho?

While the term is rarely used, a derecho is a meteorological term that simply means a significant, long-lived wind storm. The word itself comes from the Spanish adjective for straight or direct. In fact, it was only August 10th of last year when the Stateline felt the effects of a derecho as one glided in from the west.

 In the end, it became one of the costliest thunderstorm events in U.S history, totaling up to $7.5 billion in damage. The reason why meteorologists rarely use this term is because the event has to meet a certain criteria to be called a derecho. The most common being that it must produce “continuous or intermittent” damage along a path at least 60 miles wide and 240 miles long, with frequent wind gusts of at least 58 mph.

Wednesday's Wind Event:

The event from August 10th more than qualified. But taking a look at Wednesday's event, it definitely has the mileage to be considered a derecho. From northwest Wisconsin to extreme northeastern Indiana, this line of strong storms traveled a whopping 450 miles. 

The one thing that will have be considered is if this wind event was able to produce frequent wind gusts up to 58 mph during that long stretch. As this line tracked southwards into the Stateline, areas in southeast Wisconsin and portions of northern Illinois experienced wind gusts up to 60-70, even up to 80 mph. Total filtered wind reports as of Friday morning came to 155, with the National Weather Service confirming 5 tornadoes in Wisconsin. 

Normally with this type of storm system, you can have areas along the leading edge of the line develop an inflow region. This helps a kink or an appendage to form, allowing for an area of rotation to develop and quickly tighten up. 

This type of tornado, also known as a quasi-linear convective system (QLCS) tornado are brief and are usually given a rating from an EF-0 to as a highs an EF-2. Surveys conducted by the National Weather Service in Milwaukee have concluded that a total of 5 tornadoes have been confirmed. One being an EF-0 with the other four being rated an EF-1. Additional surveys are scheduled to be conducted today.