Derecho
Encyclopedia
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
s in the form of a squall line
Squall line
A squall line is a line of severe thunderstorms that can form along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. It contains heavy precipitation, hail, frequent lightning, strong straight-line winds, and possibly tornadoes and waterspouts....
usually taking the form of a bow echo
Bow echo
A bow echo is a term describing the characteristic radar return from a mesoscale convective system that is shaped like an archer’s bow. These systems can produce severe straight-line winds and occasionally tornadoes, causing major damage.-Research:...
. Derechos blow in the direction of movement of their associated storms, similar to a gust front
Outflow boundary
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump...
, except that the wind is sustained and generally increases in strength behind the "gust" front. A warm weather phenomenon, derechos occur mostly in summer, especially June and July in the Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
. They can occur at any time of the year and occur as frequently at night as in the daylight hours.
Criteria
The traditional criteria that distinguish a derecho from a severe thunderstorm are sustained winds of 58 mi/h (50 kt or 93 km/h) during the storm (as opposed to gusts), high or rapidly increasing forward speed, and geographic extent (typically 250 nautical miles [280 mi or 460 km] long). In addition, they have a distinctive appearance on radar (bow echo); several unique features, such as the rear inflow notch and bookend vortex, and usually manifest two or more downbursts.Although these storms most commonly occur in North America, derechos can occur elsewhere in the world, although infrequently. Outside North America they are sometimes called by different names. For example, in Bangladesh
Bangladesh
Bangladesh , officially the People's Republic of Bangladesh is a sovereign state located in South Asia. It is bordered by India on all sides except for a small border with Burma to the far southeast and by the Bay of Bengal to the south...
and adjacent portions of India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, a type of storm known as a "Nor'wester" can be a progressive derecho.
Etymology
Derecho comes from the SpanishSpanish language
Spanish , also known as Castilian , is a Romance language in the Ibero-Romance group that evolved from several languages and dialects in central-northern Iberia around the 9th century and gradually spread with the expansion of the Kingdom of Castile into central and southern Iberia during the...
word for "straight". The word was first used in the American Meteorological Journal in 1888 by Gustavus Detlef Hinrichs
Gustavus Detlef Hinrichs
Gustavus Detlef Hinrichs was a chemist who published his findings on periodic laws within the chemical elements well before Dmitri Mendeleev or Lothar Meyer.-Life:...
in a paper describing the phenomenon and based on a significant derecho event that crossed Iowa on 31 July 1877.
Characteristics
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
, and hence are also called a bow echo or spearhead radar echo. The size of the bow may vary, and the storms associated with the bow may die and redevelop. Winds in a derecho can be enhanced by downburst
Downburst
A downburst is created by an area of significantly rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds. Unlike winds in a tornado, winds in a downburst are directed outwards from the point where it hits land or water...
clusters embedded inside the storm. These straight-line winds can exceed 100 mi/h (160 km/h) (in some cases, sustained wind) in these clusters and straight-line wind gusts of up to 200 mi/h (320 km/h) are possible in the most extreme cases. Tornadoes sometimes form within derecho events, although such events are often difficult to confirm due to the additional damage caused by straight-line winds in the immediate area.
On the other hand, with the average tornado in the United States and Canada rating in the low end of the F/EF
Enhanced Fujita Scale
The Enhanced Fujita Scale rates the strength of tornadoes in the United States based on the damage they cause.Implemented in place of the Fujita scale introduced in 1971 by Ted Fujita, it began operational use on February 1, 2007. The scale has the same basic design as the original Fujita scale:...
1 classification at 85 to 100 mi/h peak winds and most or all of the rest of the world even lower, derechoes tend to deliver the vast majority of extreme wind over much of the territory in which they occur. Data compiled by the United States' National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
and other organizations shows that a large swath of the north-central United States and presumably at least the adjacent sections of Canada and much of the surface of the Great Lakes can expect winds above 85 mi/h to as high as 120 mi/h (135 to 190 km/h) over a significant area at least once in any 50-year period, including both convective events and extra-tropical cyclones and other events deriving power from baroclinic sources. Only in 40 to 65 percent or so of the United States resting on the coast of the Atlantic basin and a fraction of the Everglades are derechoes surpassed in this respect—by landfalling hurricanes, which in the worst case can have winds as severe as EF3 tornadoes.
Particularly dangerous situation
A particularly dangerous situation is a type of enhanced wording first used by the National Weather Service's Storm Prediction Center in Norman, Oklahoma on certain severe weather watches...
severe thunderstorm
Severe thunderstorm watch
[File:Annual_severe_thunderstorm_watch_frequency_in_the_United_States.svg|thumb|right|300px|Map of average annual severe thunderstorm watches in the United States between 1999 and 2008.Legend:...
variety of severe weather watches
Severe weather terminology (United States)
This article describes the United States National Weather Service severe weather terminology. The NWS defines precise meanings for nearly all its weather terms. This article describes NWS terminology and related NWS weather scales...
were issued from the Storm Prediction Center
Storm Prediction Center
The Storm Prediction Center , located in Norman, Oklahoma, is tasked with forecasting the risk of severe thunderstorms and tornadoes in the contiguous United States. The agency issues convective outlooks, mesoscale discussions, and watches as a part of this process...
of the US National Oceanic & Atmospheric Administration.
Downbursts in general are extremely common in derechoes and the rotor variety can also cause damage in a circular pattern.
Climatology
Derechos in North America form predominantly from May to August, peaking in frequency during the latter part of June into July. During this time of year, derechos are mostly found in the Ohio ValleyOhio River
The Ohio River is the largest tributary, by volume, of the Mississippi River. At the confluence, the Ohio is even bigger than the Mississippi and, thus, is hydrologically the main stream of the whole river system, including the Allegheny River further upstream...
, Upper Mississippi Valley
Upper Mississippi River
The Upper Mississippi River is the portion of the Mississippi River upstream of Cairo, Illinois, United States. From the headwaters at Lake Itasca, Minnesota, the river flows approximately 2000 kilometers to Cairo, where it is joined by the Ohio River to form the Lower Mississippi...
and the
Great Lakes
Great Lakes
The Great Lakes are a collection of freshwater lakes located in northeastern North America, on the Canada – United States border. Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario, they form the largest group of freshwater lakes on Earth by total surface, coming in second by volume...
region including southern Canada, mostly in Southwestern Ontario. During mid-summer if a hot and muggy airmass covers the north-central US they will often develop further north in Manitoba
Manitoba
Manitoba is a Canadian prairie province with an area of . The province has over 110,000 lakes and has a largely continental climate because of its flat topography. Agriculture, mostly concentrated in the fertile southern and western parts of the province, is vital to the province's economy; other...
or Northwestern Ontario
Northwestern Ontario
Northwestern Ontario is the region within the Canadian province of Ontario which lies north and west of Lake Superior, and west of Hudson Bay and James Bay. It includes most of subarctic Ontario. Its western boundary is the Canadian province of Manitoba, which disputed Ontario's claim to the...
, near or just north of the US-Canadian border. North Dakota
North Dakota
North Dakota is a state located in the Midwestern region of the United States of America, along the Canadian border. The state is bordered by Canada to the north, Minnesota to the east, South Dakota to the south and Montana to the west. North Dakota is the 19th-largest state by area in the U.S....
, Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...
and upper Michigan
Michigan
Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....
are also vulnerable to derecho storms when such conditions are in place. They often occur during periods of extreme heat along stationary front
Stationary front
A stationary front is a boundary between two different air masses, neither of which is strong enough to replace the other. On a weather map, this is shown by an inter-playing series of blue spikes pointing one direction and red domes pointing the other. They tend to remain essentially in the same...
s on the northern side of where the most intense heat and humidity is occurring. Late-year derechos are confined to Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
and the Deep South
Deep South
The Deep South is a descriptive category of the cultural and geographic subregions in the American South. Historically, it is differentiated from the "Upper South" as being the states which were most dependent on plantation type agriculture during the pre-Civil War period...
, although a late-summer derecho
New York State Labor Day Derechos
The New York State Labor Day Derechos were two derecho events that occurred on Labor Day, September 7, 1998. One derecho moved through northern and central New York state, and the other would start in southeastern Michigan and move through northeastern Ohio, Pennsylvania, New Jersey, and Long...
struck upper parts of the New York State area after midnight on 7 September 1998.
Derechos have been known to occur in other parts of the world. One such event occurred on 10 July 2002 in Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
. A serial derecho killed eight people and injured 39 near Berlin
Berlin
Berlin is the capital city of Germany and is one of the 16 states of Germany. With a population of 3.45 million people, Berlin is Germany's largest city. It is the second most populous city proper and the seventh most populous urban area in the European Union...
. They have occurred in Argentina and South Africa as well, and on rarer occasions close to or north of the 60th parallel in Northern Canada. Primarily a mid-latitudes
Mid-latitudes
The mid-latitudes are the areas on earth between the tropics and the polar regions, approximately 30° to 60° north or south of the equator. The mid-latitudes are an important region in meteorology, having weather patterns which are generally distinct from weather in the tropics and the polar...
phenomenon, derechos do occur in the Amazon Basin of Brazil.
Classification
- Serial derecho - Multiple bow echoes embedded in a massive squall line typically around 250 miles (400 km) long. This type of derecho is usually associated with a very deep lowLow pressure areaA low-pressure area, or "low", is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low-pressure systems form under areas of wind divergence which occur in upper levels of the troposphere. The formation process of a low-pressure area is known as...
. One example of the serial type is a derecho that occurred during the Storm of the Century (1993) in FloridaFloridaFlorida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
. Also because of embedded supercellSupercellA supercell is a thunderstorm that is characterized by the presence of a mesocyclone: a deep, continuously-rotating updraft. For this reason, these storms are sometimes referred to as rotating thunderstorms...
s, tornadoTornadoA tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es can easily spin out of these types of derechos. - Progressive derecho - A small line of thunderstorms take the bow-shape and can travel for hundreds of miles along stationary fronts. An example of this is the Boundary Waters-Canadian DerechoBoundary Waters-Canadian DerechoThe Boundary Waters – Canadian Derecho, , was an international derecho that occurred during the afternoon and evening hours of July 4 and the early morning hours of July 5, 1999. It traveled and lasted 22 hours.-North Dakota:In the pre-dawn hours of July 4, a band of thunderstorms developed...
of 4-5 July 1999. Tornado formation is less common in a Progressive than Serial type. - Hybrid derecho - Has characteristics of a serial and progressive derecho. These types of derechos are associated with a deep low like serial derechos, but are relatively small in size like progressive derechos. An example is the Late-May 1998 Tornado Outbreak and Derecho that moved through the central Northern Plains and the Southern Great Lakes on 30-31 May 1998.
According to the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
criterion, a derecho is classified as a band of storms that have winds of at least 50 knots (58 mi/h or 93 km/h) along the entire span of the derecho, which occurs over a time span of at least 6 hours.
Damage risk
Since derechos occur during warm months and often in places with cold winter climates, people who are most at risk are those involved in outdoor activities. Campers, hikers, and motorists are most at risk because of falling trees toppled over by straight line winds. Wide swaths of forest have been felled by such storms. People who live in mobile homes are also at risk; mobile homes that are not anchored to the ground can be overturned from the high winds. Derechos may also severely damage an urban area's electrical distribution system, especially if these services are routed above ground. An example would be the Chicagoland Derecho of 2011, in which over 860,000 customers were left without power.See also
- List of derecho events
- Convective storm detectionConvective storm detectionConvective storm detection is the meteorological observation of deep, moist convection and consists of detection, monitoring, and short-term prediction. This term includes the minority of storms which do not produce lightning and thunder. Convective storms can produce tornadoes as well as large...
- MesocycloneMesocycloneA mesocyclone is a vortex of air, approximately 2 to 10 miles in diameter , within a convective storm....
- Severe weatherSevere weatherSevere weather phenomena are weather conditions that are hazardous to human life and property.- Examples Include :Severe weather can occur under a variety of situations, but three characteristics are generally needed: a temperature or moisture boundary, moisture, and , instability in the...
- Extreme weatherExtreme weatherExtreme weather includes weather phenomena that are at the extremes of the historical distribution, especially severe or unseasonal weather. The most commonly used definition of extreme weather is based on an event's climatological distribution. Extreme weather occurs only 5% or less of the time...
External links
- Facts about derechos (Storm Prediction CenterStorm Prediction CenterThe Storm Prediction Center , located in Norman, Oklahoma, is tasked with forecasting the risk of severe thunderstorms and tornadoes in the contiguous United States. The agency issues convective outlooks, mesoscale discussions, and watches as a part of this process...
) - What is a derecho? (University of Nebraska at Lincoln)
- What is a derecho? (Meteorologist Jeff Haby's education page)
- Derecho Hazards in the United States (Walker Ashley)
- Gustavus Hinrichs, Discoverer of the DERECHO (National Weather Service)
- Origin of the term "Derecho" as a Severe Weather Event (Meteorologist Robert H. Johns)