Derechos arise from extratropical MCSs when squall-line thunderstorms create a sustained series of downbursts as they advance. These cold downbursts blast outward from the squall line striking, then hugging, the surface as the storm advances. Derechos most commonly form along nearly stationary fronts lying approximately west-east that separate very warm, moist, and unstable air on the south from a relatively cool, dry air to the north. The derecho typically moves eastward along the front, veering toward the warm air mass to the south (or right). Severe-storm researchers distinguished between two types of derechos, progressive and serial, which differ in their formation process and peak season of occurrence.
Progressive derechos generally emerge in the spring and summer spawned by the abundant solar energy that heats the surface and the lower atmosphere. These are the most common type, accounting for about three quarters of all derechos. Progressive derechos do not depend on strong synoptic or dynamic forcing of convection. Almost all progressive derechos form on the cool side of a semi-stationary front. They often manifest as bow echoes on weather radars, forming a bulge in the center of a squall line. Typically, conditions found ahead of the bow-echo arch include high winds, small hail, and the chance of weak tornadoes. When the bow echo bulge reaches 80-320 km (50-200 miles) wide, the derecho has been born.
Courtesy US National Weather Service, NOAA
Once formed they can race across the landscape at 65 km/h (40 mph) and are self-sustaining.
In contrast, serial derechos arise from strong synoptic-scale or dynamic forces most often associated with strong, propagating low-pressure systems. Here, lifting caused by the low pressure cell and associated fronts produces the environmental conditions conducive for MCS genesis rather than instability generated by strong solar heating. Serial derechos do not need the strong unstable conditions required to produce a progressive derecho, but the environment must be able to support convection. As a consequence, serial derechos can arise at any time of year.
Derechos are most commonly experienced from the central and northern Plains across the Midwest into the Ohio Valley, but can occur in northeastern and east-central States and the southerly regions of eastern Canada. They are rare south of 32 degrees North latitude (the Gulf Coast region). Roughly a dozen derechos are observed across the US each year, mostly in the late spring and summer months. Cool-season derechos are predominantly serial derechos while progressive derechos are generally confined to the warm seasons.
| Here's the follow-up discussion on this article: | View all related messages |
For a complete listing of article comments, questions, and other discussions related to Keith C. Heidorn's Meteorology topic, please visit the Discussions page.