Derecho


A derecho 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 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, except that the wind remains sustained for a greater period of time, 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. 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.

Etymology

Derecho comes from the Spanish adjective for "straight", in contrast with a tornado which is a "twisted" wind. The word was first used in the American Meteorological Journal in 1888 by Gustavus Detlef Hinrichs in a paper describing the phenomenon and based on a significant derecho event that crossed Iowa on 31 July 1877.

Development

Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. The resultant mesoscale convective system 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, and high values of vertical wind shear.
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 in the atmosphere.

Classification and criteria

A common definition is a thunderstorm complex that produces a damaging wind swath of at least, featuring a concentrated area of convectively-induced wind gusts exceeding. According to the National Weather Service criterion, a derecho is classified as a band of storms that have winds of at least 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 wide and 650 km long.
On January 11, 2022, the National Oceanic and Atmospheric Administration and Environment and Climate Change Canada 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
  • Wind gusts of at least along most of its length
  • Several, well-separated or greater gusts
Prior to January 11, 2022, the definition for a derecho was:
  • Wind damage swath extending for more than
  • Wind gusts of at least along most of its length
On May 10, 2025, NOAA and ECCC formally revised the criteria for derechos in the United States and Canada to the following:
  • Widespread reports of wind gusts of at least, all occurring from the same mesoscale convective system
  • The MCS must be at least long
  • The MCS must last at least 3 hours
  • The forward speed of the MCS must be faster than the environmental mean wind speed
  • No more than one hour may occur between wind reports
  • Spatial gaps between wind reports may not exceed
  • The wind swath must be at least long
  • At least three wind reports of at least must be separated by
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.

Types

Four types of derechos are generally recognized:
  • Serial derecho – This type of derecho is usually associated with a very deep low.
  • * Single-bow – A very large bow echo around or upwards of long. This type of serial derecho is less common than the multi-bow kind. A few examples of a single-bow serial derecho are the derecho that occurred in association with the October 2010 North American storm complex, and the December 2021 Midwest derecho.
  • * Multi-bow – Multiple bow derechos are embedded in a large squall line typically around long. One example of a multi-bow serial derecho is a derecho that occurred during the 1993 Storm of the Century in Florida. Because of embedded supercells, tornadoes can spin out of these types of derechos. This is a much more common type of serial derecho than the single-bow kind. Multi-bow serial derechos can be associated with line echo wave patterns on weather radar.
  • Progressive derecho – A line of thunderstorms take the bow-shape and may travel for hundreds of miles along stationary fronts. Examples of this include "Hurricane Elvis" in 2003, the August 2020 Midwest derecho, the Boundary Waters-Canadian Derecho of 4–5 July 1999, and the May 2022 Canadian derecho. Tornado formation is less common in a progressive than serial type.
  • Hybrid derecho – A derecho with characteristics of both a serial and progressive derecho. Similar to serial derechos and progressive derechos, these types of derechos are associated with a deep low, but are relatively small in size. An example is the Late-May 1998 tornado outbreak and derecho that moved through the central Northern Plains and the Southern Great Lakes on 30–31 May 1998.
  • Low dewpoint derecho – A derecho that occurs in an environment of comparatively limited low-level moisture, with appreciable moisture confined to the mid-levels of the atmosphere. Such derechos most often occur between late fall and early spring in association with strong low-pressure systems. Low dew point derechos are essentially organized bands of successive, dry downbursts. The Utah-Wyoming derecho of 31 May 1994 was an event of this type. It produced a wind gust at Provo, Utah, where sixteen people were injured, and removed part of the roof of the Saltair Pavilion on the Great Salt Lake. Surface dew points along the path of the derecho were about.

    Characteristics

Winds in a derecho can be enhanced by downburst clusters embedded inside the storm. These straight-line winds may exceed, reaching in past events. Tornadoes sometimes form within derecho events, although such events are often difficult to confirm due to the additional damage caused by straight-line winds in the immediate area.
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.