ABOUT DERECHOS

Part of the NOAA-NWS-NCEP Storm Prediction Center web site
Prepared by Robert H. Johns, Jeffry S. Evans, and Stephen F. Corfidi (with the help of many others)

For feedback on "About Derechos," contact Stephen Corfidi

Last updated on April 7, 2011;
check "What's New" link below to see what has been added or changed.

Photo by Brittney Misialek, former WGN Weather Intern

Photo of the gust front "arcus" cloud on the leading edge of a derecho-producing storm system. The photo was taken on the evening of July 10, 2008 in Hampshire, Illinois as the derecho neared the Chicago metropolitan area. The derecho had formed around noon local time in southern Minnesota.

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....DERECHO FACTS PAGE....

DERECHO DEFINITION

What is a derecho?
A derecho (pronounced similar to "deh-REY-cho" in English, or pronounced phonetically as "") is a widespread, long-lived windstorm that is associated with a band of rapidly moving showers or thunderstorms.

Click here to hear a pronounciation of the word "derecho."

Because derecho is a Spanish word (see paragraph below), the plural term is "derechos." In this case there is no letter "e" after the letter "o."

What is the origin of the term "derecho"?
The word "derecho" was coined by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa, in a paper published in the American Meteorological Journal in 1888. A defining excerpt from this paper can be seen in this figure showing a derecho crossing Iowa on July 31, 1877. Dr. Hinrichs chose this terminology for thunderstorm induced straight-line winds as an analog to the word tornado. Derecho is a Spanish word which can be defined as "direct" or "straight ahead" while tornado is thought by some, including Dr. Hinrichs, to have been derived from the Spanish word "tornar" which means "to turn". A web page about Dr. Gustavus Hinrichs' background has been created by National Weather Service Science and Operations Officer Ray Wolf, and he provides more details about Dr. Hinrichs' development of the term "derecho" in the late 1800s. He also mentions how the term "derecho" became more commonly used in the late 1900s.

DERECHO-PRODUCING STORMS

What kind of storms cause derechos?
Derechos typically are associated with bands of showers or thunderstorms (collectively referred to as "convection") that assume a curved or bowed shape. The bow-shaped storms are called bow echoes. A derecho may be associated with a single bow echo or with multiple bows. Bow echoes themselves may consist of an individual storm, or may be comprised of a series of adjacent storms (i.e., a "squall line" or "quasi-linear convective system") that together take on a larger scale bow shape. Bow echoes may dissipate and subsequently redevelop during the course of given derecho. Derecho winds occasionally are enhanced when a rotating band of storms called a "bookend vortex" develops on the poleward side of the bow echo storm system. Derecho winds also may be enhanced on a smaller scale by the presence of embedded supercells in the derecho-producing convective system.

TYPES OF DERECHOS

Are there different kinds of derechos?
Three main types of derechos may be distinguished. This classification largely is based on the overall organization and behavior of the associated derecho-producing convective system and reflects, in part, the dominant physical processes responsible for the thunderstorms that produce the damaging winds.

The type of derecho most often encountered during the spring and fall is called a serial derecho. These are produced by multiple bow echoes embedded in an extensive squall line (typically many hundreds of miles long) that sweeps across a very large area, both wide and long. This type of derecho typically is associated with a strong, migratory low pressure system. An example of a serial derecho with a very extensive squall line and embedded smaller scale bow echoes is the one that affected Florida, Cuba, and adjacent parts of the Gulf of Mexico, the Caribbean Sea, and the Atlantic Ocean during the early stages of the "Storm of the Century" on March 12-13, 1993.

The second type of derecho is called a progressive derecho. These are associated with a relatively short line of thunderstorms (typically from 40 miles to 250 miles in length) that may at times take the shape of a single bow echo, particularly in the early stages of development. In some cases, the width of a progressive derecho and its associated bow echo system remain relatively narrow even though they may travel for hundreds of miles. An example of this type is the "Boundary Waters-Canadian Derecho" that occurred on July 4-5, 1999. In other cases, the progressive derecho and associated bow echo system begin relatively small, with a narrow path, but over time grow to exceed 250 miles in width. The line of thunderstorms of a progressive derecho often begins as a single bow echo that evolves into a short squall line, typically with more than one embedded bowing segment. Such development occurred with the "I-94 Derecho" over the north central United States on July 19, 1983 (see Fig. 2 in that event's discussion page). Progressive derechos may travel for many hundreds of miles along a path that is relatively narrow compared to those of serial derechos. Often they are associated with an area of weak low pressure at the surface.

The third type of derecho is known as a "hybrid" derecho; these have characteristics of both the "progressive" and "serial" types. For example, the "Southern Great Lakes Derecho of May 30-31, 1998 was associated with a strong migrating low pressure system. However, the derecho path and the associated bow echo system had many characteristics of a progressive derecho event.

In contrast to most derecho-producing thunderstorm systems which typically occur in association with very moist air, bands of widespread wind-producing storms sometimes occur in environments of very limited moisture. These systems are referred to as low dewpoint derechos. Such derechos most often occur between late fall and early spring in association with strong low pressure systems and are a form of serial derecho.

STRENGTH AND VARIATION OF DERECHO WINDS

How strong are derecho winds?
By definition winds in a derecho must meet the National Weather Service criterion for severe wind gusts (greater than 57 mph) at most points along the derecho path. In the stronger derecho events winds can exceed 100 mph. For example, as a derecho roared through northern Wisconsin on July 4, 1977, winds of 115 mph were measured. More recently, the derecho which swept across Wisconsin and Lower Michigan during the early morning hours of May 31, 1998 produced a measured wind gust of 128 mph in eastern Wisconsin, and estimated gusts up to 130 mph in Lower Michigan.

Are wind speeds relatively constant during the course of a derecho?
The winds associated with derechos are not constant and may vary considerably along the derecho path...sometimes being below severe limits (57 mph or less) and sometimes being very strong (from 75 mph to greater than 100 mph). The swaths of stronger winds embedded in the general path of a derecho are called downbursts, and these often occur in clusters. A derecho is made up of a "family" of downburst clusters and by definition must be at least 240 miles in length. The derecho of July 4-5, 1980 is a good example of an event that exhibited wide variation in observed wind speeds.

CASUALTY AND DAMAGE RISKS FROM DERECHOS

Who is most at risk from derecho events?
Because derechos are most common in the warm season, those involved in outdoor activities are most at risk. Campers or hikers in forested areas are vulnerable to being injured or killed by falling trees. People in boats risk injury or drowning from storm winds and high waves that can overturn boats.

Those in cars and trucks also are vulnerable to being hit by falling trees and utility poles. Further, high profile vehicles such as semi-trailer trucks, buses, and sport utility vehicles may be blown over. At outside events such as fairs and festivals, people may be killed or injured by collapsing tents and flying debris.

Even those indoors may be at risk for death or injury during derechos. Mobile homes, in particular, may be overturned or destroyed, while barns and similar buildings can collapse. People inside homes, businesses, and schools are sometimes victims of falling trees and branches that crash through walls and roofs; they also may be injured by flying glass from broken windows. Finally, structural damage to the building itself (for example, removal of a roof) could pose danger to those inside.

Another reason that those outdoors are especially vulnerable to derechos is the rapid movement of the parent convective system. Typically, derecho-producing storm systems move at speeds of 50 mph or greater, and a few have been clocked at 70 mph. For someone caught outside, such rapid movement means that darkening skies and other visual cues that serve to alert one to the impending danger (e.g., gust front clouds --- see photo at top of page) appear on very short notice. In summary, the advance notice given by a derecho often is not sufficient for one to take protective action.

A camper's close brush with death in Maine during the July 4-5, 1999 derecho is told here.

A boater's encounter with the May 17, 1986 derecho on Lake Livingston, Texas is described here.

The dramatic account of a boat being overturned by intense straight-line winds is given on the Northern Indiana National Weather Service's web page linked here.

The causes of injury or death for the 73 casualties of the July 4-5, 1980 derecho is given here. This event provides a good example of the various risks posed by derechos.

Do special hazards exist when a strong derecho hits a city?
Whether in an urban or rural area, those out-of-doors are at greatest risk of being killed or injured in a derecho. But a hazard of particular significance in urban areas is the vulnerability of electrical lines to high winds and falling trees. In addition to posing a direct hazard to anyone caught below the falling lines, derecho damage to overhead electric lines sometimes results in massive, long-lasting power outages. Hundreds of thousands of people may be affected; in the worst events, power may not be restored for many days. It is the dense concentration of overhead distribution feeders in urban areas, and their frequent proximity to large shade trees, that make cities especially vulnerable to electrical outages following wind storms. Examples of cities in which derechos have resulted in prolonged power outages that affected large portions of the metropolitan area include Baltimore, Maryland (June 29, 1980), Kansas City, Missouri (June 7, 1982), and Memphis, Tennessee (July 22, 2003).

DERECHO CLIMATOLOGY

Where and when are derechos most frequent in the United States? Derechos in the United States are most common in the late spring and summer (May through August) and typically occur along two axes. One axis extends along the "Corn Belt" from the upper Mississippi Valley into the Ohio Valley. The other warm season axis extends from the southern Plains into the mid Mississippi Valley. During the cool season (September through April), derechos are not as frequent but are most likely to occur from eastern Texas into the southeastern states. Although derechos are extremely rare in that portion of the United States west of the Great Plains, isolated derecho events have occurred in the interior portions of the western United States, especially during spring and early summer.

Here are climatological maps for derecho events in the United States.

Do derechos occur in other areas of the world besides North America? Derechos likely occur in other areas of the world where meteorological conditions are favorable for their development. However, only one such event has been formally documented in recent years. On July 10, 2002, a serial derecho occurred over eastern Germany and adjacent portions of neighboring European countries. In Berlin and surrounding areas, 8 people were killed and 39 were injured, mainly from falling trees. In Bangladesh and adjacent parts of India, a type of storm known as a "Nor'wester" occasionally occurs in the spring. From various descriptions and knowledge of the meteorological environments involved, it appears that some of these storms may be progressive derechos.

TORNADOES IN DERECHO ENVIRONMENTS

Can derechos and tornadoes occur with the same storm system? Derechos and tornadoes can occur with the same convective system. This is particularly so with serial derechos associated with strong, migratory low pressure systems. The tornadoes may occur with isolated supercells (rotating thunderstorms) ahead of the derecho producing squall line, or they may develop from storms within the squall line itself. An example of a serial derecho that produced both extremely damaging straight-line winds and significant tornadoes from supercells embedded in the derecho-producing squall line is that which affected Florida during the early stages of the "Storm of the Century" of March 12-13, 1993. Although not as common, tornadoes sometimes occur with progressive derechos. When they do, the tornadoes typically form within the bow echo storm system itself, and only rarely are associated with isolated supercells ahead of the bow.

NOTEWORTHY DERECHOS IN RECENT DECADES

What are some of the more significant derechos that have occurred in North America?
Many significant derechos (i.e., those that have caused severe damage and/or casualties), have occurred over North America during the last few decades. Most of these affected the United States and Canada. Listed below is a selection of some of the more noteworthy events in recent years; the list is not all-inclusive. Information provided in the links includes a map of the area affected, and a description of the storm's impact.

Holiday weekend events
The human impact of the following events was enhanced by their occurrence on summer holiday weekends, causing many to be caught out-of-doors during the sudden onset of high winds...
July 4, 1969................"The Ohio Fireworks Derecho"....MI, OH, PA, WV
July 4, 1977................"The Independence Day Derecho of 1977"....ND, MN, WI, MI, OH
July 4-5, 1980............"The 'More Trees Down' Derecho"....NE, IA, MO, IL, WI, IN, MI, OH, PA, WV, VA, MD
Sept. 7, 1998............."The Syracuse Derecho of Labor Day 1998"....NY, PA, VT, MA, NH
Sept. 7, 1998 ............"The New York City Derecho of Labor Day 1998"....MI, OH, WV, PA, NJ, NY, CT
July 4-5, 1999............"The Boundary Waters-Canadian Derecho"....ND, MN, ON, QB, NH, VT, ME