As I write this piece, a significant derecho event is making its way across the midwestern part of the United States. It is creating a path of destruction. However, there were several posts on my social media feeds today asking “What is a derecho?” or “How could this much destruction be caused by a derecho? Here is a quick “101” on derechos.
A mesoscale convective system on August 10th, 2020 producing derecho winds.
Before I get into what they are, let’s discuss the current meteorological circumstances involving this derecho. The Monday weather discussion of the National Weather Service-Chicago said, “All eyes are on the incoming impressive convective complex surging eastward towards the river early this afternoon....Given the number of measured severe (and significant severe) wind gusts ongoing, see no real reason that this won`t continue well into and through our CWA (area of responsibility) given the thermodynamic environment in place.” On the same day NOAA’s Storm Prediction Center issued this dire statement, “A long-lived bowing MCS (mesoscale convective system), that appears to have peaked in terms of intensity and amplitude of significant severe wind gusts, will likely produce scattered damaging winds and isolated significant severe gusts as it spreads into Lower Michigan and Indiana this evening.” Indeed, this all sounds particularly dangerous, and images of damage are already emerging from across the region. What does all of this meteorological jargon mean?
Where derechos are likely to occur
NOAA defines a derecho as, “(pronounced similar to "deh-REY-cho" in English, or pronounced phonetically as "") a widespread, long-lived wind storm.” They are typically associated with organized bands of rapidly moving thunderstorms or rainstorms like squall lines, bow echoes, and quasi-linear convective systems. All of these systems are classified as mesoscale convective systems (for example, the MCS mentioned above). NOAA has a very informative website that goes into the origin of derechos.
According to NOAA’s website, “Although a derecho can produce destruction similar to that of a tornado, the damage typically occurs in one direction along a relatively straight path.” Meteorologists like me will often refer to this hazard as "straight-line wind damage.” Like the oft-misused term “blizzard,” there are very specific criteria that define a derecho:
- wind damage swath is greater than than 250 miles (about 400 kilometers)
- wind gusts that are 58 mph (93 km/h) or greater along much of the length of the line
- pockets of 75 mph (121 km/h) or greater gusts
There are ongoing efforts to tweak the definition, but I think you get the point that these things can be pretty serious. At this point, I should also remind the reader that “derecho” has become similar to terms like “Polar Vortex” or “bombogenesis.” These terms have been around meteorology for decades, but many people think they are new. For a thorough discussion of derechos, I recommend this 2005 scholarly study published in the Bulletin of the American Meteorological Society by Walker Ashley, a professor at Northern Illinois University, and Thomas Mote, a professor and associate dean at the University of Georgia.
Oddly, I find that people immediately default to tornadoes when they see wind damage when it is often straight-line winds. I am not sure what the psychology behind attributing wind damage to tornadoes is, but it is important to understand that derechos and other strong gust winds can meet severe storm criteria often associated with tornadoes or large hail.
