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A derecho (, from , 'straight') is a widespread, long-lived, straight-line wind storm that is associated with a fast-moving group of severe thunderstorms known as a mesoscale convective system.
Derechos cause Beaufort scale winds, heavy rains, and . 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 troposphere 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), except that the wind remains sustained for a greater period of time (often increasing in strength after onset), and may reach tornado- and hurricane-force winds. A derecho-producing convective system may remain active for many hours and, occasionally, over multiple 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 at any time of the year. They are equally likely during day and night times.
Various studies since the 1980s have shed light on the physical processes responsible for the production of widespread damaging winds by thunderstorms. In addition, it has become apparent that 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). In addition, 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.
Classic derechos occur with squall lines that contain bow- or spearhead-shaped features as seen by weather radar that are known as or spearhead echoes. Squall lines typically "bow out" due to the formation of a mesoscale high-pressure system which forms within the Stratus cloud 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.Parke, Peter S. and Norvan J. Larson (2005). Boundary Waters Windstorm. National Weather Service Forecast Office, Duluth, MN. Retrieved on 2008-07-30. 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 ( within the lowest 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 Wind shear between in the atmosphere.
On January 11, 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:
Prior to January 11, 2022, the definition for a derecho was:
On May 10, 2025, NOAA and ECCC formally revised the criteria for derechos in the United States and Canada to the following:
The 2025 revision was done to objectively define derecho events using an algorithm and eliminate ambiguity for the formal application of the term. The new definition was adopted in the United States and Canada based on an article from the Bulletin of the American Meteorological Society specifically for the Central and Eastern United States, the authors of which suggest in the article that such a definition may need to be adjusted for other regions to be consistent with local recordkeeping.
With the average tornado in the United States and Canada rating in the low end of the F/EF1 classification at peak winds and most or all of the rest of the world even lower, derechos tend to deliver the vast majority of extreme wind conditions over much of the territory in which they occur. Datasets compiled by the United States National Weather Service and other organizations show that a large swath of the north-central United States, and presumably at least the adjacent sections of Canada and much of the surface of the Great Lakes, can expect winds from over a significant area at least once in any 50-year period, including both convective events and extra-tropical cyclones and other events deriving power from baroclinic sources. Only in 40 to 65 percent or so of the United States resting on the coast of the Atlantic basin, and a fraction of the Everglades, are derechos surpassed in this respect — by landfalling hurricanes, which at their worst may have winds as severe as EF3 tornadoes.
Certain derecho situations are the most common instances of severe weather outbreaks which may become less favorable to tornado production as they become more violent; the height of 30–31 May 1998 upper Middle West-Canada-New York State derecho and the latter stages of significant tornado and severe weather outbreaks in 2003 and 2004 are only three examples of this. Some upper-air measurements used for severe-weather forecasting may reflect this point of diminishing return for tornado formation, and the mentioned three situations were instances during which the rare Particularly Dangerous Situation severe thunderstorm variety of severe weather watches were issued from the Storm Prediction Center of the U.S. National Oceanic & Atmospheric Administration.
One derecho developed a radar signature resembling that of a cyclone, with a central eye observable on surface observations with a minimum central pressure and surrounding bands of strong convection.David M. Roth. MCS with Eye - 21 July 2003. Retrieved on 2008-01-08.
North Dakota, Minnesota, and upper Michigan are also vulnerable to derecho storms when such conditions are in place. They often occur along on the northern periphery of where the most intense heat and humidity bubble exists. Late-year derechos are normally confined to Texas and the Deep South, although a late-summer derecho struck upper parts of the New York State area after midnight on 7 September 1998. Warm season derechos have greater instability than their cold season counterpart, while cool season derechos have greater shear than their warm season counterpart.
Although these storms most commonly occur in North America, derechos can occur elsewhere in the world, with a few areas relatively frequently. Outside North America, they sometimes are called by different names. For example, in Bangladesh and parts of East India, a type of storm known as "Kalbaisakhi" or "Nor'westers" may be a progressive derecho. One such event occurred on 10 July 2002 in Germany: a serial derecho killed eight people and injured 39 near Berlin. Derechos occur in southeastern South America (particularly Argentina and southern Brazil) and South Africa as well, and on rarer occasions, close to or north of the 60th parallel in northern Canada. Primarily a mid-latitudes phenomenon, derechos do occur in the Amazon Basin of Brazil. On 8 August 2010, a derecho struck Estonia and tore off the tower of Väike-Maarja Church. Derechos are occasionally observed in China.
Derechos may also severely damage an urban area's electrical distribution system, especially if these services are routed above ground. The derecho that struck Chicago, Illinois on 11 July 2011 left more than 860,000 people without electricity.Janssen, Kim, Mitch Dudek and Stefano Esposito, "Storm could break ComEd record with 860,000-plus losing power," Chicago Sun-Times, 11 July 2011. The June 2012 North American derecho took out electrical power to more than 3.7 million customers starting in the Midwestern United States, across the central Appalachians, into the Mid-Atlantic States during a heat wave.
The August 2020 Midwest Derecho delivered a maximum measured wind speed of , with damage-estimated speeds as high as in the Cedar Rapids, Iowa area. The storm was referred to as one of the largest "land-based hurricanes" in recorded history spawning 17 confirmed tornadoes across Wisconsin, Illinois, and Indiana. Ten million acres of crops were damaged or destroyed, accounting for roughly a third of the state of Iowa's agricultural area. Over a million homes across the Midwest were without basic services such as water and electricity. Iowa Governor Reynolds requested $4 billion in federal aid to assist in the recovery efforts. Winds were confirmed as having stirred up in Colorado and Nebraska, and then proceeded in force crossing 5 states including Iowa, Minnesota, Illinois, Indiana, and Ohio leaving destruction in excess of $7.5 billion in estimated damages.
The 21 May 2022 derecho in southern Ontario and western Quebec travelled lengthwise along the most heavily populated region in Canada, reaching peak wind speeds of 190 km/h. The derecho killed 10 people and caused $875 million property damage, the sixth largest "insured loss event"
Destruction of utility poles deprived some rural communities of telephone and electricity services for several weeks.
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