A derecho (/dəˈɹeɪ.t͡ʃoʊ/ , from Spanish: derecho [deˈɾetʃo], 'straight') is a widespread, long-lived, straight-line wind storm that is associated with a fast-moving complex of severe thunderstorms referred to as a mesoscale convective system.
Derechos cause hurricane-force winds, heavy rains, and flash floods. In many cases, convection-induced winds take on a bow echo (backward "C") form of squall line, often forming beneath an area of diverging upper tropospheric winds, and in a region of both rich low-level moisture and warm-air advection. Derechos move rapidly in the direction of movement of their associated storms, similar to an outflow boundary (gust front). The difference between derechos and conventional gust fronts is that derecho winds remain sustained for a longer period of time – often increasing in strength after onset – and may reach tornado- and hurricane-force. A derecho-producing convective system may remain active for many hours; occasionally even for several days.
A warm-weather phenomenon, derechos mostly occur in summer, especially during June, July, and August in the Northern Hemisphere, or March, April, and May in the Southern Hemisphere, within areas of moderately strong instability and moderately strong vertical wind shear. However, derechos can occur any time of the year. They are equally likely during day and night.
Studies since the 1980s have researched the physical processes responsible for the production of widespread damaging winds by thunderstorms. The most damaging derechos are associated with particular types of mesoscale convective systems that are self-perpetuating (meaning that the convective systems are not strongly dependent on the larger-scale meteorological processes such as those associated with blizzard-producing winter storms and strong cold fronts). The term "derecho" sometimes is misapplied to convectively generated wind events that are not particularly well-organized or long-lasting. For these reasons, a more precise, physically based definition of "derecho" has been introduced within the meteorological community.
Derecho comes from the Spanish adjective for "straight" (or "direct"), in contrast with a tornado which is a "twisted" wind. The word was first used in the 1888 American Meteorological Journal by Gustavus Detlef Hinrichs in a paper describing the phenomenon and based on a significant derecho event that crossed Iowa on 31 July 1877.
Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. The resultant mesoscale convective system (MCS) often forms at the point of the strongest divergence of the upper-level flow, and new storm cells are developed in the area with the greatest low-level inflow. The convection tends to move east or toward the equator, roughly parallel to low-level thickness lines and usually somewhat to the right of the mean tropospheric flow. When the convection is strongly linear or slightly curved, the MCS is called a squall line, with the strongest winds typically occurring just behind the leading edge of the significant wind shift and pressure rise.
Classic derechos occur with squall lines that contain bow- or spearhead-shaped features as seen by weather radar that are known as bow echoes or spearhead echoes. Squall lines typically "bow out" due to the formation of a mesoscale high-pressure system which forms within the stratiform rain area behind the initial convective line. This high-pressure area is formed due to strong descending air currents behind the squall line, and could come in the form of a downburst. The size of the bow may vary, and the storms associated with the bow may die and redevelop.
During the cool season within the Northern Hemisphere, derechos generally develop within a pattern of mid-tropospheric southwesterly winds, in an environment of low to moderate atmospheric instability (caused by relative warmth and moisture near ground level, with cooler air aloft, as measured by convective available potential energy), and high values of vertical wind shear (20 m/s or 40 knots within the lowest 5 km or 16,000 ft of the atmosphere).
Warm season derechos in the Northern Hemisphere most often form in west to northwesterly flow at mid-levels of the troposphere, with moderate to high levels of thermodynamic instability. As previously mentioned, derechos favor environments of low-level warm advection and significant low-level moisture.
In 2025, researchers from the University of Oklahoma, CIWRO, and Storm Prediction Center, published a study regarding the environment for derechos and non-derecho storms, and how the key factor was the bulk wind shear between 1 and 9 kilometres (0.62 and 5.59 mi) in the atmosphere.
A common definition is a thunderstorm complex that produces a damaging wind swath of at least 390–640 kilometres (240–400 mi), featuring a concentrated area of convectively-induced wind gusts exceeding 25 m/s (50 kn). According to the National Weather Service (NWS) criterion, a derecho is classified as a band of storms that have winds of at least 25 m/s (50 kn) along the entire span of the storm front, maintained over a time span of at least six hours. Some studies add a requirement that no more than two or three hours separate any two successive wind reports. A more recent, more physically based definition of "derecho" proposes that the term be reserved for use with convective systems that not only contain unique radar-observed features such as bow echoes and mesovortices, but also for events that produce damage swaths at least 100 km (60 miles) wide and 650 km (400 miles) long.
On 11 January 2022, the National Oceanic and Atmospheric Administration (NOAA) and Environment and Climate Change Canada (ECCC) formally revised the criteria for a storm to be classified as a derecho. A wind storm must meet the following criteria:
Wind damage swath extending for more than 400 miles (640 km)
Wind gusts of at least 58 miles per hour (26 m/s; 50 kn) along most of its length
Several, well-separated 75 miles per hour (34 m/s; 65 kn) or greater gusts
Prior to 11 January 2022, the definition for a derecho was:
Wind damage swath extending for more than 240 mi (390 km)
Wind gusts of at least 58 mph (93 km/h; 26 m/s; 50 kn) along most of its length