The size of the bow may vary, and the storms associated with the bow may die and redevelop.ĭuring 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). 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. 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. Ĭlassic 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. 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. 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. 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. Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. 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. Etymology ĭerecho comes from the Spanish adjective for 'straight' (or 'direct'), in contrast with a tornado which is a 'twisted' wind. For these reasons, a more precise, physically based definition of 'derecho' has been introduced within the meteorological community. In addition, the term 'derecho' sometimes is misapplied to convectively generated wind events that are not particularly well-organized or long-lasting. 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). Various studies since the 1980s have shed light on the physical processes responsible for the production of widespread damaging winds by thunderstorms. However, derechos may occur at any time of the year, and can occur as frequently at night as during the day. A derecho-producing convective system may remain active for many hours and, occasionally, over multiple days.Ī warm-weather phenomenon, derechos occur mostly 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. 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 exceed hurricane-force. 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. ĭerechos can cause hurricane and tornado-force winds, heavy rains, and flash floods. A shelf cloud along the leading edge of a derecho in MinnesotaĪ derecho ( / d ə ˈ r eɪ tʃ oʊ/, from Spanish: derecho, '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.

Imy urban dictionary