Definition

A derecho 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. 

Although a derecho can cause destruction similar to the strength of tornados, the damage typically is directed in one direction along a relatively straight swathe. As a result, the term "straight-line wind damage" sometimes is used to describe derecho damage. By definition, if the wind damage swathe extends more than 240 miles (about 400 km) and includes 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.

Discussion

Derecho cause hurricane force winds, heavy rains, and flash floods. In many cases, convective-induced winds take on a bow echo (backward "C") form of a squall line, often beneath an area of diverging upper tropospheric winds, and in a region of both rich low-level moisture and warm advection. Derechos move rapidly in the direction of movement of their associated storms, similar to 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 or 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.

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" is sometimes misapplied to convectively generated wind events that are not particularly well organised or long lasting. 

Development

"How derechos develop" Source: wikicommons. Authors: Dennis Cain and Stephen Corfidi at the U.S. National Weather Service

Organised 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 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. 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

Flight safety risk 

Derechos can be hazardous to aviation due to embedded microbursts, downbursts, and downburst clusters. In addition, the powerful updrafts and high cloud tops can cause for dangerous conditions. Their sheer size also makes them very difficult to navigate around.