A derecho ( // , from Spanish : derecho [deˈɾetʃo] , "straight") is a widespread, long-lived, straight-line wind storm that is associated with a fast-moving group of severe thunderstorms known as a mesoscale convective system.
Derechos can cause hurricane-force winds, tornadoes, heavy rains, and flash floods. In many cases, convection-induced winds take on a bow echo (backward "C") form of squall line, often forming beneath an area of diverging upper tropospheric winds, and in a region of both rich low-level moisture and warm-air advection. Derechos move rapidly in the direction of movement of their associated storms, similar to an outflow boundary (gust front), except that the wind remains sustained for a greater period of time (often increasing in strength after onset), and may exceed hurricane-force. A derecho-producing convective system may remain active for many hours and, occasionally, over multiple days.
A warm-weather phenomenon, derechos occur mostly in summer, especially during June, July, and August in the Northern Hemisphere, within areas of moderately strong instability and moderately strong vertical wind shear. However, derechos may occur at any time of the year, and can occur as frequently at night as during the day.
Various studies since the 1980s have shed light on the physical processes responsible for the production of widespread damaging winds by thunderstorms. In addition, it has become apparent that the most damaging derechos are associated with particular types of mesoscale convective systems that are self-perpetuating (meaning that the convective systems are not strongly dependent on the larger-scale meteorological processes such as those associated with blizzard-producing winter storms and strong cold fronts). In addition, the term "derecho" sometimes is misapplied to convectively-generated wind events that are not particularly well-organized or long-lasting. For these reasons, a more precise, physically-based definition of "derecho" has been introduced within the meteorological community.
Derecho comes from the Spanish word in adjective form for "straight" (or "direct"), in contrast with a tornado which is a "twisted" wind.The word was first used in the American Meteorological Journal in 1888 by Gustavus Detlef Hinrichs in a paper describing the phenomenon and based on a significant derecho event that crossed Iowa on 31 July 1877.
Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. The resultant mesoscale convective system (MCS) often forms at the point of the strongest divergence of the upper-level flow in the area of greatest low-level inflow and convergence.[ clarification needed ] The convection tends to move east or toward the equator, roughly parallel to low-level thickness lines and usually somewhat to the right of the mean tropospheric flow. When the convection is strongly linear or slightly curved, the MCS is called a squall line, with the strongest winds typically occurring just behind the leading edge of the significant wind shift and pressure rise.
Classic derechos occur with squall lines that contain bow- or spearhead-shaped features as seen by weather radar that are known as bow echoes or spearhead echoes. Squall lines typically "bow out" due to the formation of a mesoscale high pressure system which forms within the stratiform rain area behind the initial convective line. This high pressure area is formed due to strong descending air currents behind the squall line, and could come in the form of a downburst.The size of the bow may vary, and the storms associated with the bow may die and redevelop.
During the cool season within the Northern Hemisphere, derechos generally develop within a pattern of mid-tropospheric southwesterly winds, in an environment of low to moderate atmospheric instability (caused by relative warmth and moisture near ground level, with cooler air aloft, as measured by convective available potential energy), and high values of vertical wind shear (20 m/s [72 km/h; 39 kn; 45 mph]) within the lowest 5 km [16,000 feet] of the atmosphere).
Warm season derechos in the Northern Hemisphere most often form in west to northwesterly flow at mid-levels of the troposphere, with moderate to high levels of thermodynamic instability. As previously mentioned, derechos favor environments of low-level warm advection and significant low-level moisture.
A common definition is a thunderstorm complex that produces a damaging wind swath of at least 400 km (250 miles), featuring a concentrated area of convectively-induced wind gusts exceeding 30 m/s (90 km/h; 50 kn; 60 mph). According to the National Weather Service (NWS) criterion, a derecho is classified as a band of storms that have winds of at least 30 m/s (90 km/h; 50 kn; 60 mph) along the entire span of the storm front, maintained over a time span of at least six hours. Some studies add a requirement that no more than two or three hours separate any two successive wind reports. A more recent, more physically-based definition of "derecho" proposes that the term be reserved for use with convective systems that not only contain unique radar-observed features such as bow echoes and mesovortices, but also for events that produce damage swaths at least 100 km (60 miles) wide and 650 km (400 miles) long.
Four types of derechos are generally recognized:
Winds in a derecho can be enhanced by downburst clusters embedded inside the storm. These straight-line winds may exceed 45 m/s (161 km/h; 87 kn; 100 mph), reaching 58 m/s (210 km/h; 110 kn; 130 mph) in past events. 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.
With the average tornado in the United States and Canada rating in the low end of the F/EF1 classification at 38 to 45 m/s (137 to 161 km/h; 74 to 87 kn; 85 to 100 mph) peak winds and most or all of the rest of the world even lower, derechos tend to deliver the vast majority of extreme wind conditions over much of the territory in which they occur.[ citation needed ] Datasets compiled by the United States National Weather Service and other organizations show 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 from 38 to 54 m/s (137 to 193 km/h; 74 to 104 kn; 85 to 120 mph) 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 derechos surpassed in this respect — by landfalling hurricanes, which at their worst may have winds as severe as EF3 tornadoes.
Certain derecho situations are the most common instances of severe weather outbreaks which may become less favorable to tornado production as they become more violent;[ clarification needed ] the height of 30–31 May 1998 upper Middle West-Canada-New York State derecho and the latter stages of significant tornado and severe weather outbreaks in 2003 and 2004 are only three examples of this. Some upper-air measurements used for severe-weather forecasting may reflect this point of diminishing return for tornado formation, and the mentioned three situations were instances during which the rare Particularly Dangerous Situation severe thunderstorm variety of severe weather watches were issued from the Storm Prediction Center of the U.S. National Oceanic & Atmospheric Administration.[ citation needed ]
Some derechos develop a radar signature resembling that of a hurricane in the low levels. They may have a central eye free of precipitation, with a minimum central pressure and surrounding bands of strong convection, but are really associated with an MCS developing multiple squall lines, and are not tropical in nature. These storms have a warm core, like other mesoscale convective systems. One such derecho occurred across the Midwestern U.S. on 21 July 2003. An area of convection developed across eastern Iowa near a weak stationary/warm front and ultimately matured, taking on the shape of a wavy squall line across western Ohio and southern Indiana. The system re-intensified after leaving the Ohio Valley, starting to form a large hook, with occasional hook echoes appearing along its eastern side. A surface low pressure center formed and became more impressive later in the day.Another example is the May 2009 Southern Midwest derecho.
Derechos in North America form predominantly from April to August, peaking in frequency from May into July.During this time of year, derechos are mostly found in the Midwestern United States and the U.S. Interior Highlands most commonly from Oklahoma and across the Ohio Valley. During mid-summer when a hot and muggy air mass covers the north-central U.S., they will often develop farther north into Manitoba or Northwestern Ontario, sometimes well north of the Canada–US border.
North Dakota, Minnesota, and upper Michigan are also vulnerable to derecho storms when such conditions are in place. They often occur along stationary fronts on the northern periphery of where the most intense heat and humidity bubble exists. Late-year derechos are normally confined to Texas and the Deep South, although a late-summer derecho struck upper parts of the New York State area after midnight on 7 September 1998. Warm season derechos have greater instability than their cold season counterpart, while cool season derechos have greater shear than their warm season counterpart.
Although these storms most commonly occur in North America, derechos can occur elsewhere in the world, with a few areas relatively frequently. Outside North America, they sometimes are called by different names. For example, in Bangladesh and adjacent portions of India, a type of storm known as a "Nor'wester" may be a progressive derecho.One such event occurred on 10 July 2002 in Germany: a serial derecho killed eight people and injured 39 near Berlin. Derechos occur in southeastern South America (particularly Argentina and southern Brazil) and South Africa as well, and on rarer occasions, close to or north of the 60th parallel in northern Canada. Primarily a mid-latitudes phenomenon, derechos do occur in the Amazon Basin of Brazil. On 8 August 2010, a derecho struck Estonia and tore off the tower of Väike-Maarja Church. Derechos are occasionally observed in China.
Unlike other thunderstorms, which typically can be heard in the distance when approaching, a derecho seems to strike suddenly. Within minutes, extremely high winds can arise, strong enough to knock over highway signs and topple large trees. These winds are accompanied by spraying rain and frequent lightning from all directions. It is dangerous to drive under these conditions, especially at night, because of blowing debris and obstructed roadways. Downed wires and widespread power outages are likely but not always a factor. A derecho moves through quickly, but can do much damage in a short time.
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 may be overturned from the high winds. Across the United States, Michigan and New York have incurred a significant portion of the fatalities from derechos. Prior to Hurricane Katrina, the death tolls from derechos and hurricanes were comparable for the United States.
Derechos may also severely damage an urban area's electrical distribution system, especially if these services are routed above ground. The derecho that struck Chicago, Illinois on 11 July 2011 left more than 860,000 people without electricity.The June 2012 North American derecho took out electrical power to more than 3.7 million customers starting in the Midwestern United States, across the central Appalachians, into the Mid-Atlantic States during a heat wave.
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are sometimes called thundershowers. Thunderstorms occur in a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds, and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms produce little precipitation or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.
A squall is a sudden, sharp increase in wind speed lasting minutes, contrary to a wind gust lasting seconds. They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to the increase to the sustained winds over that time interval, as there may be higher gusts during a squall event. They usually occur in a region of strong sinking air or cooling in the mid-atmosphere. These force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.
A squall line or quasi-linear convective system (QLCS) is a line of thunderstorms forming 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. Strong straight-line winds can occur where the squall line is in the shape of a bow echo. Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present. Some bow echoes which develop within the summer season are known as derechos, and they move quite fast through large sections of territory. On the back edge of the rainband associated with mature squall lines, a wake low can be present, sometimes associated with a heat burst.
The Storm Prediction Center (SPC) is a government agency that is part of the National Centers for Environmental Prediction (NCEP), operating under the control of the National Weather Service (NWS), which in turn is part of the National Oceanic and Atmospheric Administration (NOAA) of the United States Department of Commerce (DoC).
A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and Mesoscale Convective Complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.
A bow echo is 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. They can also become derechos.
A tornado outbreak is the occurrence of multiple tornadoes spawned by the same synoptic scale weather system. The number of tornadoes required to qualify as an outbreak typically are at least six to ten.
Col. Robert C. Miller, USAF (1920–1998), was an American meteorologist, who pioneered severe convective storms forecasting and applied research, developing an empirical forecasting method, identifying many features associated with severe thunderstorms, a forecast checklist and manuals, and is known for the first official tornado forecast, and it verified, in 1948.
An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. Roll clouds and shelf clouds are the two main types of arcus. Arcus clouds most frequently form along the leading edge or "gust fronts" of thunderstorm outflow; some of the most dramatic arcus formations mark the gust fronts of derecho-producing convective systems. Roll clouds also may arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts.
The rear-inflow jet is a component of bow echoes in a mesoscale convective system that aids in creating a stronger cold pool and downdraft. The jet forms as a response to a convective circulation having upshear tilt and horizontal pressure gradients. The cold pool that comes from the outflow of a storm forms an area of high pressure at the surface. In response to the surface high and warmer temperatures aloft due to convection, a mid-level mesolow forms behind the leading edge of the storm.
Severe weather refers to any dangerous meteorological phenomena with the potential to cause damage, serious social disruption, or loss of human life. Types of severe weather phenomena vary, depending on the latitude, altitude, topography, and atmospheric conditions. High winds, hail, excessive precipitation, and wildfires are forms and effects of severe weather, as are thunderstorms, downbursts, tornadoes, waterspouts, tropical cyclones, and extratropical cyclones. Regional and seasonal severe weather phenomena include blizzards (snowstorms), ice storms, and duststorms.
Joseph G. Galway, was an American meteorologist pioneering in the fields of severe convective storm forecasting and research. He was one of the first forecasters for the Severe Local Storms Unit (SELS) and the National Severe Storms Forecast Center (NSSFC), and developed widely used synoptic predictors associated with severe thunderstorms and tornadoes, such as the jet streak and lifted index.
Ronald William Przybylinski was an American meteorologist with primary areas of interest in bow echoes, mesovortices, and quasi-linear convective system (QLCS) tornadoes.
Atmospheric convection is the result of a parcel-environment instability, or temperature difference layer in the atmosphere. Different lapse rates within dry and moist air masses lead to instability. Mixing of air during the day which expands the height of the planetary boundary layer leads to increased winds, cumulus cloud development, and decreased surface dew points. Moist convection leads to thunderstorm development, which is often responsible for severe weather throughout the world. Special threats from thunderstorms include hail, downbursts, and tornadoes.
Convective storm detection is the meteorological observation, and short-term prediction, of deep moist convection (DMC). DMC describes atmospheric conditions producing single or clusters of large vertical extension clouds ranging from cumulus congestus to cumulonimbus, the latter producing thunderstorms associated with lightning and thunder. Those two types of clouds can produce severe weather at the surface and aloft.
The 1948 Tinker Air Force Base tornadoes were two tornadoes which struck Tinker Air Force Base in Oklahoma City, Oklahoma, on March 20 and 25, 1948. Both are estimated to have been equivalent to F3 in intensity on the modern Fujita scale of tornado intensity, which was not devised until 1971. The March 20 tornado was the costliest tornado in Oklahoma history at the time. On March 25, meteorologists at the base noticed the extreme similarity between the weather conditions of that day and March 20, and later in the day issued a "tornado forecast", which was verified when a tornado struck the base that evening. This was the first official tornado forecast, as well as the first successful tornado forecast, in recorded history.
Mesovortices are small scale rotational features found in convective storms, such as those found in bow echos, supercell thunderstorms, and the eyewall of tropical cyclones. They range in size from tens of miles in diameter to a mile or less, and can be immensely intense.
A line echo wave pattern (LEWP) is a weather radar formation in which a single line of thunderstorms presenting multiple bow echoes forms south of a mesoscale low-pressure area with a rotating "head". LEWP often are associated with a multiple-bow serial derecho and often produce tornadoes, some of which can be strong. The existence of a LEWP on radar means that a serial derecho has developed or is likely to develop soon, much as a hook echo indicates the same for a tornado.
The Spanish Plume is a weather pattern in which a plume of warm air moves from the Iberian plateau or the Sahara to northwest Europe giving rise to severe thunderstorms. This meteorological pattern can lead to extreme high temperatures and intense rainfall during the summer months, with potential for flash flooding, damaging hail storms, and tornado formation. Some of these intense thunderstorms are formed from thermal lows. Thermal lows are also known as heat lows. Thermal lows can be semipermanent features around some parts of Europe, particularly in the summer season. These thermal lows can be developed or created around Spain, Portugal, France etc., during the summer season because of the intense heat. Thermal low pressure can be located around the world, particularly in the summer or in tropical regions.