Well with the word origin being Spanish I would think Kim
Perez has it correct.
The word "derecho" is of Spanish origin, and means straight ahead. The strict translation of this word leads to the general public's understanding of a derecho: any straight-line wind associated with a thunderstorm. While correct to a certain extent, a more technical definition of derecho is a widespread convectively produced wind event resulting from the outflow boundary of a mesoscale convective system (MCS, see below). Thus, derecho producing mesoscale convective systems are referred to as DMCSs. The winds associated with derechos blow straight ahead, or are straight-line. The term straight-line is meant to be a contrast to the rotating winds associated with tornadoes. Johns and Hirt (1987) distinguished between two types of derechos, progressive and serial, differing in the mechanism of formation.
Progressive derechos are common in the spring and summer, as progressive derechos depend very little on strong synoptic or dynamic forcings. Instead, these derechos feed on the abundant potential energy available in the lower levels of the atmosphere during the late spring and summer, and are dependent on instability (Doswell and Evans, 2000). In addition, it was found that almost all progressive derechos form on the cool side of a pseudo-stationary front. Evidence for the existence of a progressive derecho can be found on the radar imagery, as progressive derechos produce the "bow echo," a term introduced by Fujita in 1978 (Doswell and Evans, 2000) often associated with derechos. DMCSs forming under these weak forcing conditions generally produce few tornadoes, compared to the DMCSs formed by strong forcings. In an 11-year study of derecho events by Doswell and Evans (2000), it was found that progressive derecho storms produce about 4 tornadoes each, while serial derecho systems produce almost 11.5 tornadoes each.
Serial derechos, unlike progressive derechos, form under conditions of strong synoptic and dynamic forcings. Generally, serial derechos are associated with low-pressure systems, where the dynamic lifting caused by the low, and associated fronts, produces an environment conducive for the genesis of MCSs. Serial derechos do not require the amount of instability necessary for genesis of a progressive derecho, but the environment must be unstable enough to support convection (Bentley and Mote, 2000). While progressive derechos are confined to the warm season, serial derechos can happen at any time throughout the year, because of the consistent cycle of low-pressure systems in the mid-latitudes. In fact, cool season derechos, as discovered by Mote and Bentley (2000), are predominantly serial derechos, as the cool season atmosphere does not possess sufficient potential energy for progressive derechos, yet when combined with synoptic forcings, are unstable enough to sustain the convection.
Mesoscale Convective Systems
Mesoscale convective systems are clusters of thunderstorms not meeting the size or time criteria of mesoscale convective complexes (MCC). MCCs are identified by criteria verified on IR satellite images. To become a MCC, a system must have a cloud shield greater than 100,000 square kilometers, with cloud top temperatures less than -32 degrees C. The interior of the system must be greater than 50,000 square kilometers, with cloud top temperatures less than -52 degrees C, as determined from IR satellite images. These size conditions must be met for 6 hours in order to qualify as a MCC. Additionally, at the time of maximum extent, the cloud shield must have an eccentricity (minor axis/major axis) of greater than or equal to 0.7. So, a MCC is a MCS that has met these criteria. Therefore, a MCS is often smaller or shorter lived than a MCC. As defined by the National Weather Service, MCSs still must meet certain temporal and spatial requirements. In order for the National Weather Service to label a system as a MCS it must have a continuous area of cloud top temperatures below -52 degrees C, that is at least 250 kilometers in length, and lasts for no less than three hours.
Basically, MCSs are self-sustaining, large thunderstorm clusters. Once an MCS is formed, it can feed off of itself, and specifically its outflow boundary, to generate new individual storms within the MCS. This self-sustenance and regeneration process accounts for the long life span of a MCS. A typical thunderstorm lasts about one hours, and an isolated supercell can last two to three hours, but it is not uncommon for MCSs to exist for ten hours. In a study of cool season derechos by Bentley and Mote (2000), it was found that the average duration for cool season DMCSs was 8.8 hours, which is considerably longer than the life-span of an isolated thunderstorm.
Potential Danger
There are two dangers associated with derechos: 1) the duration of the damaging winds and 2) the spatial coverage of such winds. Damaging winds are classified as those winds exceeding 25-30 m/s, or 50-60 knots. In derecho situations it is not uncommon for winds of this magnitude to last for over 30 minutes, with occasional gusts of 60-70 m/s. Derechos also, by definition, are widespread events, possibly affecting areas as large as 2000 km2 (Doswell, 1994). The area affected by derechos rivals the coverage area of a tropical storm, a parallel that places the magnitude of DMCSs in proper perspective. The potential danger of a derecho is immense because of the duration and spatial coverage of such events.
