Derecho

Primary reference(s)

NOAA, 2019. What is a Derecho? National Oceanic and Atmospheric Administration (NOAA). Accessed 26 November 2019.

Additional scientific description

A derecho (pronounced similar to ‘deh-REY-cho’) is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms. Although a derecho can produce destruction similar to the strength of tornadoes, the damage is typically directed in one direction along a relatively straight swath. As a result, the term ‘straight-line wind damage’ is sometimes used to describe derecho damage. By definition, if the wind damage swath extends more than 240 miles (about 400 km) and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event may be classified as a derecho (NOAA, 2019).

A derecho is a widespread convectively induced straight-line windstorm. Specifically, the term is defined as any family of particularly damaging downburst clusters produced by a mesoscale convective system (AMS, 2012a). Such systems have sustained bow echoes with book-end vortices and/or rear-inflow jets and can generate considerable damage from straight-line winds. Damage must be incurred either continuously or intermittently over a swath of at least 650 km (~400 miles) and a width of approximately 100 km (~60 miles) or more. The term derecho derives from a Spanish word that can be interpreted as ‘straight ahead’ or ‘direct’ and was chosen to distinguish between wind damage caused by tornadoes (AMS, 2012b), which have rotating flow, from straight-line winds.

The National Oceanic and Atmospheric Administration Storm Prediction Center reports that the types of Derecho include: serial derechos, progressive derechos, hybrid derechos and low-point derechos (NOAA, 2018).

Metrics and numeric limits

The winds associated with derechos are not constant and may vary considerably along the derecho path, sometimes being below severe limits (57 mph [92 kmh] or less) and sometimes being very strong (from 75 mph [121 kph] to over 100 mph [161 kph]). This is because the swaths of stronger winds within the general path of a derecho are produced by what are called downbursts, and downbursts often occur in irregularly arranged clusters, along with embedded microbursts and burst swaths. Derechos might be said to be made up of families of downburst clusters that extend, by definition, continuously or nearly continuously for at least 250 miles (about 400 km) (NOAA, 2018).

Examples of drivers, outcomes and risk management

Derechos are formed as follows. When the wet air in a thunderstorm meets the drier air surrounding it, the water in the air evaporates. When water evaporates, it cools the air around it. Since the cool air is denser, it rapidly sinks to the ground and creates strong winds. The downburst can suck more dry air into the storm, making even stronger downbursts or clusters of downbursts. Derechos occur when the right conditions for downbursts occur over a wide area (NOAA, 2019). See also NOAA (2018) for more detailed information about derechos.

Because derechos are most common in the warm season, those involved in outdoor activities are especially at risk. Campers or hikers in forested areas are vulnerable to being injured or killed by falling trees, and those at sea risk injury or drowning from storm winds and high waves that can overturn boats. Another reason those outdoors are especially vulnerable to derechos is the rapid movement of the parent convective system. Typically, derecho-producing storm systems move at speeds of 50 mph or more, and a few have been recorded at 70 mph. For someone caught outside, such rapid movement means that darkening skies and other visual cues that serve to identify the impending danger (e.g., gust front shelf clouds) appear at very short notice (NOAA, 2018).