In researching the hurricane question, I first decided to see what the experts had to say about the weather phenomenon that swept through Southern Illinois. So, I went to the WGN website (www.wgn.com) to see what Tom Skilling was reporting. Here is what I found:
Today's Ask Tom Why
Dear Tom,
While visiting Southern Illinois this weekend people were referring to last Friday's storm
as an "inland hurricane." Was it?
Gail Robertson, Christine Steffy, Mary Essling,Daniel Beach, Sean Leidigh
Dear Gail, Christine, Mary, Daniel and Sean,
The storm that ravaged the southern Midwest on May 8th was not a hurricane, but a
derecho, a fast-moving and long-lasting line of thunderstorms producing an almost
continuous path of straight-line wind damage that often extends for more than a 1,000
miles. This massive thunderstorm complex was streaking east at speeds approaching
70 m.p.h. and packing peak winds in excess of 100 m.p.h. After a damage survey, Bill
Davis, head of the National Weather Service in Springfield, Mo., commented that the
storm was like an "inland hurricane" because of its size and its hurricane force winds,
and that terminology spread like wildfire.
ABOUT DERECHOS
Part of the Storm Prediction Center, NCEP, NWS, NOAA Web Site
Prepared by Robert H. Johns and Jeffry S. Evans (with the help of many others)
Contact Us: About Derechos Feedback
Information will be updated from time to time, so please check the "What's New"
page to see the what has been added or changed.
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.
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. 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-Canada 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.. 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, 1998produced 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
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.
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 derachos. 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
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
No comments:
Post a Comment