BACKGROUND: Many states across the U.S. experience derechos (pronounced "deh-RAY-cho"), widespread and long-lived windstorms associated with a band of rapidly moving showers or thunderstorms. Normally such conditions develop during the summer season in specific areas, such as the "Corn Belt" running from the Mississippi Valley into the Ohio Valley, or into the southern Plains. However, isolated derecho events have been known to occur in the interior regions of the western U.S. during the spring.
STRONG WINDS: To quality as a derecho, wind gusts must reach speeds greater than 57 MPH, although wind speeds are not constant. The strongest winds cluster together in "downbursts," and can sometimes top 100 MPH. Such powerful winds can cause a great deal of damage, felling trees and power lines, causing high waves on lakes or other bodies of water, overturning mobile homes, and collapsing barns or other outbuildings.
HOW THEY FORM: Derechos arise from bands of showers or thunderstorms that are often "curved" in shape, earning such storms the nickname "bow echoes." A derecho can be associated with a single bow echo or multiple bow echoes. These bow echoes can vary in scale, even dying out and redeveloping during the course of a derecho's formation. Derecho winds can be further enhanced by embedded supercells with the storm system that produced the derecho. Because of this, derechos often occur in the same storm systems that produce tornadoes. However, tornadoes are rotating storms, whereas derechos feature strong winds along a straight line.
TYPES OF DERECHOS: There are three basic types of derechos. A "serial" derecho is produced by multiple bow echoes embedded in a very long squall line (about 100 miles) and it sweeps across a very large area. This type usually develops from a strong, migrating low-pressure system. A "progressive" derecho arises from a fairly short line of thunderstorms (ranging from 40 miles to 250 miles in length), which often take the shape of single bow echoes. Progressive derechos tend to be linked to weak low pressure systems. Certain traits of "serial" and "progressive" derechos can sometimes combine into "hybrid" derechos.
The American Meteorological Society contributed to the information contained in the TV portion of this report.