Introduction
Derechos are widespread, long-lived
windstorms associated with a band of rapidly
moving showers or thunderstorms. Coined by
Dr. Gustavus Hinrichs in 1888, "derechos", a
Spanish word which means "direct" or
"straight ahead".
Although a derecho's strength can produce
destruction similar to tornadoes, the damage
pattern produced by these events will occur
along relatively straight lines. Thus the
term, straight-line wind damage.
Derechos are produced by a family of
downbursts clusters. Downburst clusters
have overall lengths of 50 to 60 miles (80 to
100 kilometers).
A downburst cluster itself is made up of
several downbursts. A downburst is an area
of strong, often damaging wind produced by
a convective downdraft with the overall size
of the downburst varying from 4 to 6 miles (8
to 10 kilometers).
Within the downbursts are microbursts; smaller pockets of more intense wind.
While not shown in the illustration at right, within the microbursts are even smaller pockets of extreme wind
called burst swaths. Burst swaths can range from 50 to 150 yards (45 to 140 meters) long. The damage pattern from
burst swaths can often resemble a path of a tornado.
Due to this nature of the derecho, damage produced by these wind storms is highly variable along its path. Damage
surveys following derecho events have shown that within large areas of overall damage are much smaller pockets of
intense damage.
It is not uncommon for one house to be nearly destroyed while adjacent houses have relatively minor damage.
Derechos are produced by long-lived thunderstorm complexes that produce bow echoes.
Bow Echoes
Derechos typically are associated with bands of showers or thunderstorms that assume a curved or bowed shape.
The term "bow echo" is based on how bands of rain showers or thunderstorms "bow out" when strong winds
associated with the storms reach the surface and spread horizontally. A derecho comes from a long lived bow echo
or a series of bow echoes.
Bow echoes usually arise from a cluster of storms, but also may begin from just a single supercell thunderstorm. As
the rain cooled downdraft reaches the earth's surface, it spreads horizontally. This downdraft marks the dissipation
stage of that particular thunderstorm cell.
The cooler, denser air hugs the surface as it spreads, forcing lighter warm-moist air up into the atmosphere. The
boundary between the rain-cooled air and warm-moist air is called the gust front.
Air forced up by the gust front begins the next new thunderstorm cell formation. As this new cell matures, the rain it
produces reinforces the "pool" of rain-cooled air allowing the gust front to maintain its strength.
As the cold pool increases in size, it induces an inflow of air (orange arrow at right) on the trailing side of the
thunderstorm complex. This causes the updraft to tilt toward the trailing edge.
Tilting the updraft allows the cumulonimbus cloud to expand further, increasing the aerial coverage of rain which, in
turn, further adds to the cold pool of air under the thunderstorm and thereby strengthens the gust front causing it to
bow out.
The bowing out of the gust front forces more warm moist air up, creating new thunderstorm cells and the process
repeats.
The additional rain reinforces the cold pool and strengthens the rear inflow of air, with the thunderstorm complex
reaching a semi steady state condition. At this point there is a pronounced bow in the storm(s) as seen by Doppler
radar and an area of moderate to occasionally heavy rain near the center of the cold pool well behind the gust front.
As long as there are new thunderstorm cells forming on the gust front as it advances, replacing the older dissipating
cells, the cold pool and rear inflow air will continue. Also, all along the leading edge of the bow echo, thunderstorms
may be producing downbursts and microbursts.
About one half (46%) of bow echoes begin as unorganized thunderstorms; 30% form from squall lines, and about
one-quarter (24%) from supercell thunderstorms.
If the bow echo (or series of bow echoes) progresses more than 250 miles (400 kilometers) with widespread wind
gusts 58 mph (93 km/h) or greater, then the bow echo is classified as a derecho.
Derechos can be categorized into three main types; serial, progressive, and hybrid. These categories are largely
based on the overall organization and behavior of the thunderstorms producing the derecho.
Serial Derecho ▲
Progressive Derecho ▼
The type of derecho most often
encountered during the spring and
fall is called a serial
derecho (right). Serial derechos
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 typically is associated
with a strong upper level trough with a strong surface low pressure system. An example of a serial derecho 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 (right). These are associated with a relatively short line
of thunderstorms; typically 40-250 miles (65-400 kilometers) in length. Mainly a summertime occurrence, these
derechos usually are associated with a stationary front where the upper level air flow is parallel to the stationary front.
Progressive derechos may travel for many hundreds of miles along a path that is relatively narrow compared to those
of serial derechos. 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 (400 kilometers) 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. 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.
Where and When do
Derechos Occur
Percent occurrences of derechos by month.
All the weather we experience at ground level is
governed by the weather pattern in the atmosphere,
and derechos are no different. The type of derecho
that occurs is a good indicator of the weather pattern
and also is partly dependent on the time of the year.
The occurrence of derechos is divided into two
seasons; the "warm" season which is May, June, July
and August. 70% of all derechos occur during these
four months. The remaining eight month comprise the
"cool" season.
Progressive derechos are generally only warm
season events and are found moving over the north side of upper atmosphere high pressure ridges (image below). In
meteorology, we say they move "over the top" of the ridge.
As the Northern Hemisphere moves from spring to summer, the jet stream moves toward the North Pole along with
the storm track. Upper level pressure patterns also move north, with high pressure moving from the tropics to the
southern U.S.
In this position, the ridge of high pressure is over the northern plains and upper mid-west region of the country and it
is this location that experiences the majority of derechos each year.
Serial derechos, while not rare in the warm season, are primarily cool season events. They occur between the
upstream trough and downstream ridge (image left). Approximately 40% of all derechos are of the serial type.
The image (left) shows in the air pressure pattern at an approximate altitude of 18,000 feet (5,500 meters). The wind
follows the thin black lines from west to east (clockwise around the "high").
Where serial derechos occur depends upon the actual position of the pressure patterns. The atmosphere is in
constant motion, and the location of these pressure systems constantly move, albeit rather slowly.
For example, in the cool season, If the high pressure ridge is located off the east coast and the trough of low pressure
is located over Central Plains then a serial derecho could possibly occur over the Southeast U.S.
This is precisely what occurred on April 4-5, 2011 when a
serial derecho produced over 1,200 high wind reports
(blue dots on image right). Wind reports of 60+ mph (100+
km/h) with gusts to 90 mph (145 km/h) felled thousands of
trees and caused widespread power outages in addition
to 68 tornadoes.
Derechos in the United States 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 derechos 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.
Also, derechos tend to occur in families or groups. This means that when the first one occurs, there is a good chance
that more derechos will occur within the following few days, although not necessarily in the same location.
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.
Notable Derechos
Following are a few of the more historic derechos over the past 2 decades.
May 8, 2009 Super Derecho
This was one of the most intense and unusual derechos ever observed. The wind storm produced significant
and often continuous damage over a broad swath from the high plains of western Kansas to the foothills of
the Appalachians in eastern Kentucky.
Car crushed by a fallen tree in Memphis, TN during the Mid-South Derecho of 2003. The Commercial Appeal staff photo
by Alan Spearman, used with permission.
Multiple wind gusts in excess of 70 mph (113 km/h) and a few gusts over 90 mph (145 km/h) were
measured along its path. In addition, flash flooding was widespread on the northern edge of the system,
especially in Missouri.
But what made the event most unique was the appearance of an unusually strong, long-lived, larger-scale
circulation known as a mesoscale convective vortex. This feature was accompanied by a band of intense
surface winds and tornadoes that occurred independent of the severe weather directly associated with the
large-scale bow.
The Mid-South Derecho of 2003
During the early morning of Tuesday, July 22, 2003, a derecho formed over north central Arkansas and
moved rapidly east-southeast, reaching northern Alabama by mid-morning.
Although the storm weakened over northern Alabama, it re-developed over northwest Georgia and moved
across northern and central Georgia into South Carolina before ending by late afternoon. Many thousands of
trees were damaged or blown down. Two people were killed and 11 others were injured, mostly due to trees
falling on homes or vehicles.
Between 6 and 7 AM CDT Tuesday, the derecho passed through the Memphis, Tennessee metropolitan
area, producing some of the most intense winds during its existence. Numerous homes and buildings were
damaged, and at least 20 were destroyed
The "People Chaser" Derecho
On Sunday afternoon, May 27th, 2001, isolated supercell thunderstorms, some producing brief tornadoes,
formed over western Kansas. As the outflow of cold air from the supercell storms increased, a gust front
developed over southwest Kansas and began moving rapidly southeastward.
The thunderstorms associated with the gust front eventually evolved into a bow echo squall line. The
southeast-moving bow echo storm system produced a derecho with significant wind damage over a large
part of the southern Plains.
A homeowner in Memphis, TN working to extract a large, uprooted oak tree felled by the Mid-South Derecho of 2003.
The Commercial Appeal staff photo by Alan Spearman, used with permission.
The maximum wind gust in Kansas reached 100 mph (161 km/h) near Garden City. In Oklahoma, a
maximum wind gust of 94 mph (151 km/h) was measured at Quail Springs Mall in the northwest part of the
Oklahoma City metropolitan area; a gust to 93 mph (150 km/h) was recorded in Ardmore, Oklahoma.
The maximum gust in Texas was 78 mph (126 km/h) in suburban Fort Worth. In many areas affected by the
system, the severe gusts lasted 10 to 20 minutes before subsiding.
The "Right Turn" Derecho
During the late afternoon of Wednesday, July 12, 1995, thunderstorms formed over southeast Montana and
began producing winds that damaged homes and barns and blew over a mobile home. As the storm system
moved east across North Dakota, vehicles were overturned and a grain bin was destroyed.
Measured winds reached 70 mph (113 km/h) at Bismarck, ND. As the system approached Fargo, ND during
the early morning of July 13th, it became a well-defined bow echo storm with measured winds of 91 mph
(146 km/h) at the Fargo airport. Damage was extreme across Minnesota, with over five million trees blown
down and many buildings damaged and some destroyed.
Six campers were injured from the falling trees during the pre-dawn hours. Trucks with plows were needed
to clear many of the roads, and some areas were without power for a week. Damage totaled well over $30
million in 1995 dollars.
The Ontario-Adirondacks Derecho
On the evening of Friday, July 14, 1995, thunderstorms producing severe weather were occurring over
Upper Michigan and adjacent portions of Ontario, Canada near Sault Saint Marie. By late evening the
storms had evolved into a bowing line just northwest of the Mackinac Bridge in Michigan. At 10:17 PM EDT,
the thunderstorm gust front hit the bridge and a gust to 90 mph (145 km/h) was measured.
View of the 'People Chaser'
derecho storm system near Fort
Supply, OK. Photo by Douglas
Berry, used with permission.
Sustained winds above 80 mph
(129 km/h) continued on the
bridge for 10 more minutes.
Thus began the intense
"Ontario-Adirondacks Derecho"
that would cause hundreds of
millions of dollars worth of
damage, several deaths, and
many injuries as it raced
southeast from the northern
Great Lakes to the Atlantic coast.
The "Storm of the Century" Derecho
On Friday, March 12, 1993, a strong low pressure system formed in the Gulf of Mexico. This low pressure
system continued to strengthen dramatically and moved northeast to the mid Atlantic coastal region by
Saturday evening, March 13th.
Besides producing a record snowstorm over the eastern United States, this low pressure system was
accompanied by an intense squall line with embedded bow echoes ahead of the system's cold front.
The squall line produced a serial derecho as it swept across the Florida peninsula, Cuba, and adjacent
waters.
"The West Virginia Derecho of 1991"
On the morning of Tuesday, April 9, 1991, a bow echo formed over eastern Arkansas and began to produce
wind damage. As the day progressed, a serial derecho developed as the original bow echo raced northeast
and expanded in scale, while an additional bow echo formed farther north. Most of these casualties resulted
from falling trees, flying debris, and mobile homes being overturned or destroyed.
The most destructive damage extended from western Tennessee through Kentucky and West Virginia into
western Maryland and southern Pennsylvania. Winds along this band exceeded 80 to 100 mph (129 to 161
km/h) at some points. Literally thousands of buildings and mobile homes were damaged or destroyed.
Keeping Yourself Safe
The National Weather Service does not issue "derecho warnings" as by their definition derechos are wind events
produce by severe thunderstorms. Therefore, the NWS will issue a Severe Thunderstorms Warning if a derecho
approaches your location.
Two children examining the root structure of an uprooted tree caused by the "Mid-South
Derecho of 2003". The Commercial Appeal staff photo by Mike Maple, used with permission.
Derechos cause more fatalities than EF0 and EF1 tornadoes combined in spite of the fact that EF0 and EF1
tornadoes comprise over 80% of all recorded tornadoes. This is primarily due to the typical broad path of high winds
and subsequent damage.
Nearly half of fatalities attributed to derechos occur either in vehicles or boats. In boats, the vessels overturned due to
the high winds and the occupants drowned. In vehicles, some fatalities occur when 18 wheelers are blown onto their
sides, trees are blown onto automobiles, and when vehicles are driven into trees.
Besides their damaging winds, what makes derechos particularly dangerous is their forward speed; the rate as which
the move. It is not uncommon for a derecho to move at 60 to 70 mph (97 to 113 km/h). At such speeds, one does not
have much time to ensure their safety.
This can pose problems for campers and hikers. Often one will see threatening clouds approach and begin to seek
adequate shelter. However with derechos, their very fast forward speed greatly reduces preparation time and can
often catch individuals in exposed locations.
Typically the National Weather Service will issue a severe weather warning for your county when the storm (or
storms) is located in an adjacent county (or parish) and is expected to move into your area. However, with the fast
forward speeds of derechos, the warning may be issued for your county while the storm is still relatively far away.
View of Linden Avenue west of Barksdale Street on Friday afternoon, July 25, 2003, in
Memphis, TN three days after the "Mid-South Derecho of 2003". The Commercial Appeal staff
photo by Mike Maple, used with permission.
Instead of thinking you have extra time to get prepared; it is to be your wake-up call the potential for an extremely
rapid deterioration of your weather. It is your call for action. Be weather-wise, not otherwise.
Safety rules for derechos are the same for severe thunderstorms in general. As always, Rule #1 is at the beginning
of your day, CHECK YOUR WEATHER FORECAST.
While we will never be able to pinpoint when and where severe weather will develop, we can identify broader areas or
regions with the potential for the development of severe weather.
You may need to check several times daily, to see if you are, or will be, under a risk of severe weather.
In addition of obtaining your weather forecast from your local National Weather Service Forecast Office, check the
NWS Storm Prediction Center's (SPC) severe weather outlooks. SPC can provide several hours of lead time for
severe weather the threat of severe weather.
Also, if outdoors away from a computer, there are numerous weather apps that push NWS warnings to your
smartphone. In addition, there are apps that display NWS Doppler radar in real time. For your safety, be proactive in
monitoring the weather.