ABOUT DERECHOS

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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 and long lived windstorm that is associated with a band of rapidly moving showers or thunderstorms.

Click here to hear how the word "derecho" is pronounced.

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 by clicking here. 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".

DERECHO PRODUCING STORMS

What kind of storms cause derechos?
Derechos are associated with a band of showers or thunderstorms that are often "curved" in shape. These bowed out storms are called "bow echoes". A derecho can be associated with a single bow echo or multiple bow echoes. The bow echoes may vary in scale and may die out and redevelop during the course of derecho evolution. Further, derecho winds can be enhanced on a smaller scale by embedded supercells within the derecho producing storm system.

TYPES OF DERECHOS

Are there different kinds of derechos?
There are three types of derechos. One type is called a "serial" derecho and is 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 is typically associated with a strong, migrating, low pressure system. An example of serial derecho with a very extensive squall line and with embedded smaller scale bow echoes is the one that affected Florida, Cuba, and adjacent portions of the Gulf of Mexico, the Caribbean Sea, and the Atlantic Ocean on March 12-13, 1993.

The second type of derecho is called a "progressive" derecho and it is associated with a relatively short line of thunderstorms (typically from 40 miles to 250 miles in length) which may at times take the shape of a single bow echo, particularly in the early stages of development. In some cases, the width of the progressive derecho and its associated bow echo system remain relatively narrow even though they may travel for hundreds of miles. An example of this is the "Boundary Waters-Canadian Derecho" which occurred on July 4-5, 1999. In other cases, the progressive derecho and associated bow echo system may start out relatively small with a narrow path. However, with time the bow echo system and progressive derecho grow in scale, and they may exceed 250 miles in width towards the middle and latter portions of their path. Initially, the line of thunderstorms may start out as a single bow echo, but as it evolves, it becomes a short squall line, typically with more than one bow echo segment. An example of such development occurred with the "I-94 Derecho" on July 19, 1983 and the evolution can be observed on Fig. 2 of that case page. The progressive derecho is often associated with a weak low pressure system but it may travel for many hundreds of miles along a path that is relatively narrow compared with the path of a serial derecho.

The third type of derecho is "hybrid" and may have mixed characteristics of both a "progressive" and a "serial" nature. For example, the "Southern Great Lake 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.

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 the strong winds associated with a derecho relatively constant in speed during the period the derecho exists?
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 patches of stronger winds embedded within the general derecho path are called downbursts and they are often in clusters. A derecho is made up of a "family of downburst clusters" and by definition must be at least 240 miles in length. An example of a derecho where the varying wind speeds are quite evident is the one that occurred on July 4-5, 1980.

CASUALTY AND DAMAGE RISKS FROM DERECHOS

Who is most at risk from derecho events?
Since derechos are most common in the warm season, people involved in outdoor activities are most at risk. People camping or hiking in forested areas are very vulnerable to being killed or injured by falling trees. People in boats on lakes and other water sources risk drowning or injury from the strong winds and high waves overturning their boats.

A story about a camper's close brush with death in Maine during the July 4-5, 1999 derecho can be found by clicking here.

A story about a boater's scary encounter with the May 17, 1986 derecho on Lake Livingston in Texas can be found by clicking here.

A dramatic example of a boat being overturned by intense straight-line winds can be found on the Northern Indiana NWS web page by clicking here.

People in cars and trucks are also vulnerable to being hit by falling trees, falling power poles, etc. Further, people in high profile vehicles such as semi-trailer trucks, busses, and SUVs are at risk of their vehicles being blown over. At outside events like fairs and festivals, people risk being killed or injured by collapsing tents and other flying debris.

People that are not outside can also be at risk of death or injury in certain structures and situations during derecho events. Mobile homes, in particular, are at risk of being being overturned or destroyed. Barns and other outbuildings can be at risk of collapsing. People inside homes and businesses can be victims of large trees or branches falling through the roof, windows breaking and glass flying inside, or the building structure being significantly damaged (for example, the roof being peeled off).

A listing of what caused the 73 casualties during the July 4-5, 1980 derecho can be viewed by clicking here. This case is a typical example of people at risk being affected.

Another reason that derechos can be very risky to people involved in outdoor activities is their very rapid movement. Typically, derecho producing bow echo systems move at a speed of 50 mph or greater and a few have been clocked at 70 mph! For someone involved in outdoor activities the rapid movement means that the darkening of the sky and other visual clues that would alert them to the approaching danger occur extremely fast. Therefore, the visual clues of the approaching derecho gust front typically don't offer much time to take protective action.

What happens when a strong derecho hits a city?
As mentioned above, whether inside or outside a city, people are most at risk of being killed or injured when they are outside, in vehicles, or in mobile homes. Another factor that often affects large numbers of people after the passage of a derecho is the widespread loss of electrical power. And in large cities the power loss can affect hundreds of thousands of people. In some instances, portions of the city may be without electrical power for one to two weeks. Examples of large cities in which strong derechos resulted in long power outages in much of the metropolitan area include 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 mid Mississippi Valley into the southern Plains. 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 during the spring.

To see climatological maps for derecho events in the United States click here.

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 prior to June 2004. On July 10, 2002, a serial derecho occurred over eastern Germany and adjacent portions of other European countries. In Berlin and surrounding areas 8 people were killed and 39 were injured primarily from falling trees. In Bangladesh and adjacent portions of India a type of storm known as a "Nor'wester" occasionally occurs in the spring. From the description 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 storm system. This is particularly so with strong, migrating low pressure systems that produce "serial" derechos. The tornadoes may occur with isolated supercells (rotating thunderstorms) ahead of the derecho producing squall line, or they may be associated with the squall line itself. An example of a serial derecho, with both extremely damaging straight-line winds and significant tornadoes associated with supercells embedded within the derecho producing squall line, occurred in Florida during the "superstorm" of March 12-13, 1993.

HISTORIC DERECHO EVENTS

What are some of the more significant derecho events that have occurred in North America?
Records indicate that many significant derecho events...causing severe damage and casualties....have occurred in North America (typically in Canada and the United States) during the last few decades. Listed below are the dates and areas affected by some of these events. By clicking on the individual event dates you can see the areas affected and learn some of the details about what happened.

Independence Day Derecho Events

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
July 4-5, 1999.........."The Boundary Waters-Canadian Derecho"....ND, MN, ON, QB, NH, VT, ME

July 1995 Derecho Series

Series Overview.......Montana to New England
July 12-13, 1995......"The Right Turn Derecho"....MT, ND, MN, WI, MI, ON, OH, PA, WV
July 14-15, 1995......"The Ontario-Adirondacks Derecho"....MI, ON, NY, VT, NH, MA, CT, RI

Labor Day Derecho Events

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

Serial Derecho Events

April 9, 1991............"The West Virginia Derecho of 1991"....AR,TN, MS, AL, KY, IN, OH, WV, VA, MD, PA
March 12-13, 1993.."The Superstorm Subtropical Derecho"....FL, Cuba

Southern Plains Derecho Events

May 4-5, 1989.........."The Texas Derecho of 1989"....TX, OK, LA
May 27-28, 2001......"The People Chaser Derecho"....KS, OK, TX

Some Other Significant Derecho Events

June 7, 1982.............."The Kansas City Derecho of 1982"....KS, MO, IL
July 19, 1983............."The I-94 Derecho"....ND, MN, IA, WI, MI, IL, IN
May 17, 1986............"The Texas Boaters' Derecho".....TX
July 28-29, 1986......."The Supercell Transition Derecho"....IA, MO, IL
July 7-8, 1991............"The Southern Great Lakes Derecho of 1991"....SD, IA, MN, WI, MI, IN, OH, ON, NY, PA
May 30-31, 1998......."The Southern Great Lakes Derecho of 1998"....MN, IA, WI, MI, ON, NY
July 22, 2003............"The Mid-South Derecho of 2003"....AR, TN, MS, AL, GA, SC

DERECHO EVENTS IN 2004 AND 2005

What derecho events have occurred in the United States in recent years?
The Storm Prediction Center's severe weather records have been examined to determine those severe weather events that involve widespread damaging winds associated with convective storms. Information about these events, which include all the derechos that have occurred within the United States, has been gathered for the years 2004 and 2005. Click here to observe this information. (These records are preliminary and do not include the official National Weather Service report information listed in Storm Data. They also do not include reports from Alaska and Hawaii.)

PICTURES AND VIDEOS OF DERECHO EVENTS

Are there any pictures or videos of derechos?
There is a video tape that has been prepared by the Atmospheric Environment Service of Canada concerning the "progressive" derecho that affected the Pakwash forest in northwestern Ontario on July 18, 1991. This video tape includes camcorder footage of the storm affecting a forested area, post storm aerial views of the forest "blowdown", and interviews with people that experienced the storm.

On May 27-28, 2001, a "southward burst" derecho affected areas of the southern Great Plains from southwestern Kansas into central Texas. Many pictures of the gust front associated with this derecho as well as some videos of the damaging winds were taken by storm chasers on this day. A few of these pictures can be seen by clicking on the "historical derecho event" date at the beginning of this paragraph.

SCIENTIFIC PUBLICATIONS

I would like to know what scientific writings about bow echoes and derechos have been published. Is there a listing of such publications?
A list of scientific papers concerning bow echoes and derechos can be found by clicking here.

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Acknowledgments

Many people have assisted in preparing the "About Derechos" web pages by providing photos, images, stories, suggestions, and other information. The assistance from these people is very much appreciated. They include Environment Canada employees Phil Chadwick, Rene Heroux, Mike Leduc, Serge Mainville, Brian Murphy, Peter Rodriquez, Sarah Scriver, Dave Sills, and Pierre Vaillancourt. Also assisting were National Weather Service and National Severe Storms Laboratory employees (and former employees) including John Cannon, Mike Coniglio, Steve Corfidi, Chuck Doswell, Roger Edwards, Randy Graham, John Hart, Victor Homar, David Imy, Sarah Jamison, Ed Jessup, Rusty Kapela, Steve Keighton, Richard Koeneman, Norvan Larson, Jeff Last, Jay Liang, Dan McCarthy, Peter Parke, Steve Pennington, Tom Reaugh, Kevin Scharfenburg, Russ Schneider, Todd Shea, Rich Thompson, Frank Wachowski, Jeff Waldstreicher, Steve Weiss, and Mike Wyllie. Others assisting include Curtis Alexander, Marlin Bree, Dave Crowley, Dave Lewison, Pete Pokrandt, Colin Price, Kristina Reichenbach, and Robert Schlesinger.