In wake of one of the strongest derechos the United States has seen in years, lets go over what a derecho is and the damage it can produce. The above loop is from August 10, 2020 when a derecho moved from Iowa over 700 miles into Ohio and lasted 14 hours. By definition, a derecho is a storm that produces wind damage that extends 250 miles, features 58 mph wind gusts, and many spread out wind gusts higher than 74 mph.
The word “derecho” was coined in 1888 by Dr. Gustavus Hinrichs, a professor of physics at the University of Iowa. Hinrichs used the term in a paper published by the American Meteorological Journal to distinguish thunderstorm-induced straight-line winds from the damaging, rotary winds of tornadoes. “Derecho” is a Spanish word meaning “right,” “direct,” or “straight ahead.” (Click here to hear a pronunciation of the word “derecho”). – SPC
Derecho is a long line of convective storms that produce widespread damaging wind gusts. These are likely straight-line winds that cause the damage and can be as bad, if not worse than tornado damage. As a derecho moves along, there are are often smaller clusters of microbursts or downbursts that produce the worst damage within the entire storm. There are almost always certain areas that are not severe depending on how strong the storm is. If there is enough instability, lift, and rotation along the front edge of the derecho, a brief spinup tornado can form. Stronger derechos can produce winds in excess of 100 mph and should often be treated with the same caution of a tornado.
A bow echo is commonly seen on radar associated with a derecho. This is an area of high dBz rapidly advancing, often ahead of the general flow, and creating damaging wind gusts along a gust front. These can again form in clusters along the front edge of the derecho. When observing radar, there will often be a squall line identified, also known more formally as a quasi-linear-convective-system (QLCS). The storm system that may have formed off a boundary like a cold front then starts to enter a very moist and unstable atmosphere. It is able to feed on itself with warm rising air adding to the storm as it advances. That cold air extending out from the storm can act as the ‘new front’ in which the storm uses to keep advancing forward.
These storms can form from a single-cell thunderstorm and evolve to hundreds of miles long. If you are in one place experiencing it, the wind will be a quick burst along the front edge followed by torrential rain and lightning. A shelf cloud is often one of the main signals that a burst of rain and wind is coming.
Power outages are often the biggest impact from derechos. Since they are a warm season event, this can leave many stuck in extreme heat without air conditioning for days at a time. In fact, derechos are associated with heat waves and can often occur during a heat wave. When temperatures are extremely high, there can be a deep layer if instability, especially between two and five miles up called the Elevated Mixed Layer (EML). This can help in the advancement of storms as they advance along their own gust front.
The United States is one of the most derecho prone regions in the entire world thanks to warm air from the Gulf of Mexico and cold air from Canada.
You can find much more information on derechos here. Below are a few tweets from the derecho in August 2020.