June 1, 2006 Derechos are elongated, straight-lined windstorms that often have bands of rapidly moving thunderstorms associated with it. These little-known atmospheric phenomena often stretch for hundred of miles and cause multiple tornadoes. Meteorologists usually issue a severe thunderstorm watch when they see an approaching derecho.
NORMAN, Okla.--A derecho is a weather term meteorologists admit most people don't know about. But in this past year of unpredictable weather, it's worth learning about.
"It caught it up just right and then just rolled it -- just it rolled along."
What warning coordination meteorologist Dan McCarthy's talking about is a huge Doppler radar that was whipped around by a storm called a derecho. The staff at the NOAA National Severe Storms Laboratory got an up close and personal look at what damage a derecho can do.
"The derecho lasts for hundreds of miles as it moves across the ground, causing damaging winds of up to 80 to 100 miles per hour," McCarthy, of the NOAA Storm Prediction Center in Norman, Oklahoma, tells DBIS.
A derecho is a widespread and straight-lined windstorm that often has a band of rapidly moving thunderstorms associated with it. These conditions are ripe for generating multiple tornadoes that can cause severe damage. "Some derechos can down homes ... Some derechos can roll over mobile homes ... Many derechos take roofs and take fences down. So they can be just as destructive as tornadoes."
The term derecho was coined by Gustavus Hinrichs, a physics professor. It's a Spanish word that means direct or straight ahead, while tornado is thought to be from the Spanish word tornar, which means to turn.
Meteorologists usually issue a severe thunderstorm watch when they see an approaching derecho.
McCarthy says, "If we recognize the situation that can go for hundreds of miles, we will add the particularly dangerous situation wording to it."
If you get that warning, move your family away from windows into a safe place and wait until the storm is over.
Derechos are more common in spring and early summer and occur more frequently in the Corn Belt and along the Southern Plains, but they have been reported as far north as Michigan, as far south as Florida, as far west as Texas, and as far east as Maine.
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.