Derechos

Many people have never even heard of a derecho (pronounced day-ray-sho) before. A derecho causes an enormous amount of damage with straight-line winds that extend for several hundred kilometers along a squall line's path. The word derecho was coined by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa, in a paper he published in the American Meteorological Journal in 1888. Derecho is the spanish for "straight ahead" or "direct". On a side note, many believe the word tornado originated from the spanish verb tornar which means "to turn".

IN DEPTH: What is a Derecho?
I gave a one sentence definition of a derecho in the opening paragraph. Put into different words, it is a widespread and long-lived windstorm associated with a band of rapidly moving showers and thunderstorms. The winds are sometimes so strong that they will push the squall line outward. This forms a shape on RADAR that meteorologists call a 'bow echo'. Below are a couple of examples of a bow echo on RADAR that produced a derecho windstorm. Notice that the stronger precipitation (red) is occurring in a bow-like shape.


Derecho moving through the mid-west.
Photo Courtesy: http://i.imwx.com/web/multimedia/images/blog/MCV8May_regrad12z.jpg


Derecho moving through Texas.
Photo Courtesy: http://www.srh.noaa.gov/images/fwd/highwind07/web_radar_0057Z_metar_01Z_may307.PNG

Don't be deceived by these images; not all bow echos produce a derecho, so don't always assume if you see this on RADAR that the strong windstorm is heading your way. By definition a derecho must be at least 240 miles in length, and its winds must meet the National Weather Service (NWS) criterion for severe wind gusts, greater than 57 mph, at most points along its path. A stronger derecho event can have winds in excess of 100 mph. On May 31, 1998 a derecho plowed thru Wisconsin and lower Michigan with wind gusts measured up to 130 mph. Bow echoes can vary in scale and have even been known to die out and redevelop during the course of a derecho formation. It is important to realize winds are not constant across the stretch of a derecho and they can vary considerably. A derecho is composed of several patches of stronger winds embedded in a general derecho path, called downbursts. In addition to the winds, another dangerous aspect of a derecho is the speed at which it moves. A typical bow echo system can move at speeds of 50 mph or greater, giving those caught up in an outdoor activity very little time to take shelter in a safe place.

THE THREE KINDS OF DERECHOS
Serial: Serial derechos are produced by multiple bow echoes embedded in a very long squall line (approximately 100 miles). They sweep across a very large area and are developed from a strong, migrating low pressure system. Tornadoes are also common to occur within a serial derecho.

Progressive: Progressive derechos are produced by a fairly short line of thunderstorms (40 - 250 miles in length) and they often take the shape of single bow echo. Unlike serial derechos, progressive derechos normally spawn off a weak low pressure system. This type of derecho is narrower than the serial derecho during the early stages of its existence, but as it advances it continually grows until it becomes almost as large as a serial derecho. Tornadoes are possible with progressive derechos; however, they are not common.

Hybrid: Hybrid derechos come from a strong low pressure system like serial derechos, but its other characteristics resemble those more like the progressive derechos.

WHEN AND WHERE THEY OCCUR
The most common target of these windstorms is right here in North America, mainly the United States and Canada. Approximately 20 derechos will occur in the United States every year. Derechos can however form anywhere in the world where conditions are favorable: where moist air masses, heat, and a large fairly level terrain exist (a non-level terrain could attribute to breaking up the winds as they sweep across the land). Prior to 2004, the only other recorded derecho event around the world occurred over eastern Germany in 2002. A similar storm, called a "Nor'wester", exists in Bangladesh and adjacent portions of India. From the descriptions of the events that occur, they seem to resemble a progressive derecho; however, this is not yet confirmed.

The most common time for derechos to occur is late spring thru summer, from May to August. July is the most dangerous month due to the warm and muggy afternoon conditions that can easily trigger these violent thunderstorms. In the United States, they occur along two axes: one which runs along the "corn belt" from the Upper Mississippi Valley into the Ohio Valley, and another which starts in the mid-Mississipppi Valley and ends in the Southern Plains. During the cool season from September to April they are not as frequent, but mostly occur in the region from eastern Texas into the southeastern states. Any derecho event west of the Great Plains is extremely rare. In the United States, derechos have occurred as far west as Texas, as far east as Maine, as far north as Michigan, and as far south as Florida.

DAMAGE ASSESSMENT
So what is this windstorm capable of doing? Well, for one, it can knock over trees and power lines fairly easily. On July 15, 1995 a powerful derecho went through New York State during the early morning and blew down several trees in Adirondack State Park. The winds are also strong enough to take down fences and roofs, collapse barns, overturn mobile homes, and cause high waves on lakes. With all this, it is very common to see many killed and injured from this natural disaster. I'll end with a picture of a shelf cloud, a characteristic commonly associated with bow echos. You can definitely see the strong winds from this derecho pushing out ahead.


Photo Courtesy: http://www.adjusterpro.com/insurance-adjuster-blog/wp-content/uploads/2009/05/storm81.jpg



Sources:
Ahrens, C. Donald. "Thunderstorms and Tornadoes." Meteorology Today: an Introduction to Weather, Climate, and the Environment. Belmont, CA: Brooks/Cole, CengageLearning, 2009. 380. Print.
"Derecho." Learn Weather Phenomenons. Web. 26 Aug. 2010. http://weatherfreaks.net/derecho.
"Derecho -- Meteorologists Describe Little-Known Giant Windstorms." Science Daily: News & Articles in Science, Health, Environment & Technology. 1 June 2006. Web. 26 Aug. 2010. http://www.sciencedaily.com/videos/2006/0602-derecho.htm.
Johns, Robert H., Jeffry S. Evans, and Stephen F. Corfidi. "About Derechos." Storm Prediction Center. 22 July 2010. Web. 26 Aug. 2010. http://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm.
Whitt, Kelly. "Understanding Derecho Storms: The Frightening Power of Strong, Long-Lasting Straight Line Winds." Meteorology & Climatology. 7 Jan. 2008. Web. 26 Aug. 2010. http://meteorologyclimatology.suite101.com/article.cfm/understanding_derechos.