ABOUT DERECHOS

Part of the NOAA-NWS-NCEP Storm Prediction Center web site
Prepared by Stephen F. Corfidi, Jeffry S. Evans, and Robert H. Johns (with the help of many others)

For feedback on "About Derechos," contact Stephen Corfidi

Last updated February 12, 2024; see What's New for recent additions and changes.

...CONTENTS...

...DERECHO FACTS PAGE...

Note: Some of the following information, in abbreviated, question-and-answer format,
is available in The Derecho FAQ (Frequently Asked Questions) page.

INTRODUCTION

Definition of a derecho
A derecho (pronounced similar to "deh-REY-cho" in English, or pronounced phonetically as "") is a widespread, long-lived wind storm. Derechos are associated with bands of rapidly moving showers or thunderstorms variously known as bow echoes, squall lines, or quasi-linear convective systems.

Although a derecho can produce destruction similar to that of a tornado, the damage typically occurs in one direction along a relatively straight path. As a result, the term "straight-line wind damage" is sometimes used to describe derecho damage. By definition, if the swath of wind damage extends at least 400 miles (about 650 kilometers), is at least 60 miles (about 100 km) wide, includes wind gusts of at least 58 mph (93 km/h) along most of its length, and also includes several, well-separated 75 mph (121 km/h) or greater gusts, the event may be classified as a derecho. The derecho-producing convective system also should contain certain radar-observed storm features such as bow echoes and rear-inflow jets (discussed below in Derecho-producing storms and Derecho development). This requirement was added in a revised definition of "derecho" introduced in 2016 that was intended to make the definition more physically-based --- i.e., focused on the meteorological processes believed responsible for the production of organized, damaging surface winds.

Origin of the term "derecho"
The word "derecho" was coined in 1888 by Dr. Gustavus Hinrichs, a professor of physics at the University of Iowa. Hinrichs used the term in a paper published by the American Meteorological Journal to distinguish thunderstorm-induced straight-line winds from the damaging, rotary winds of tornadoes. "Derecho" is a Spanish word meaning "right," "direct," or "straight ahead." (Click here to hear a pronunciation of the word "derecho"). In contrast, the word "tornado" is thought to have been derived from the Spanish word "tornar," which means "to turn." Because "derecho" is of Spanish origin, the plural form is spelled "derechos;" i.e., the letter "e" is not added after the letter "o."

A defining excerpt from Hinrich's paper may be seen in this figure that shows a derecho crossing Iowa on July 31, 1877. "Derecho" was adopted to a limited extent by the meteorological community during the 1880s. However, the word disappeared from use for nearly a century until it was resurrected by NOAA National Severe Storms Forecast Center (predecessor of the Storm Prediction Center) meteorologists Robert Johns and William Hirt in a 1987 paper that described derechos over the central and eastern United States.

Ray Wolf, of the Davenport, Iowa National Weather Service Forecast Office, created a web page about Gustavus Hinrichs. The page provides information on Hinrichs' background, and on his introduction of the term "derecho" in the late 1800s. Wolf's page also briefly discusses how the term "derecho" has come into more common use in recent years.

STRENGTH AND VARIATION OF DERECHO WINDS

Modified from illustration by Dennis Cain

DERECHO DEVELOPMENT

Derecho development necessarily is tied to the formation of bow echoes. A bow echo usually arises from a cluster of thunderstorms, but also may evolve from a single strong storm. Bow echoes most frequently occur when tropospheric winds are relatively strong and unidirectional (i.e., they vary little in direction with height). As the rain-cooled downdraft of a thunderstorm reaches the earth's surface, it spreads horizontally, most rapidly in the direction of the mean tropospheric flow. As the cool, dense air spreads outward, it forces the lighter, warm and moist air surrounding the storm up along the leading edge of the outflow, or gust front (see figure below, with mean flow assumed to be from left to right). The upward motion along the gust front typically is greatest along that part of the front that is moving most rapidly --- that is, progressing --- in the downwind direction (to the right in the figure). Gust fronts often are marked by low, ominous cloud formations known as arcus or shelf clouds. Photographs of such clouds on the forefront derecho-producing storms appear at the top of this page and here

Modified from illustration by Dennis Cain

The development of a thunderstorm's downdraft ordinarily marks the dissipation stage of that particular storm. But air forced up along a gust front can give birth to new thunderstorms. As new storms mature, the rain they produce reinforces the existing "pool" of rain-cooled air produced by earlier storms, allowing the gust front to maintain its strength. As this cold pool increases in size and elongates in the direction of the mean wind, it may induce an inflow of air known as the rear-inflow jet (dashed brown arrow in figure below) on the trailing side of the thunderstorm complex. This causes the updraft to tilt toward the rear side of the storm (i.e., to the left in the figure). Tilting of the updraft allows the thunderstorm to further expand, increasing the aerial coverage of the rain. This, in turn, adds to the pool of cold air accumulating beneath the storm and strengthens the gust front, causing it to bow outward in the downwind direction. The resulting acceleration in forward motion of the gust front subsequently forces more warm, moist air upward, creating still more storms, and the process repeats.

Modified from illustration by Dennis Cain

The rain produced by the newer storms reinforces the cold pool, strengthening the inflow of air from the back side of the developing storm complex and encouraging the downward transport of higher momentum air from aloft. These processes can enable the system to attain a nearly steady-state condition. At this point, the convective system typically exhibits a pronounced bow shape on radar (see figure below) and satellite, with an area of moderate to occasionally heavy rain located near the center of the cold pool, well behind the arc of intense rain immediately behind the gust front. As long as the thermodynamic and kinematic environments support the continued development of new thunderstorm cells in the downwind direction along the advancing gust front --- a process known as downwind or "forward" propagation --- the convective complex will persist, along with the potential for downbursts and microbursts.

Modified from illustration by Dennis Cain

An animated, visible data satellite view of a derecho-producing convective system appears below. This particular event (May 31, 1994) occurred in a relatively dry environment over Utah and western Wyoming. Because of this, there was minimal supplementary cloud cover that tends to obscure the active storm towers of systems occurring in more moist regimes. As a result, the case well-illustrates the process of forward propagation along a convective system gust front. As the arc-shaped gust front advances from southwest to northeast across Utah, the rapidly-growing, new updrafts appear as sharply-outlined whitish clumps (cumulus and cumulonimbus clouds). These stand in marked contrast to the more fibrous debris clouds (cirrus "anvils") that are left behind as the gust front continues northeastward, undercutting the older updrafts and severing their link to warm, buoyant air. The derecho-producing convective system is therefore seen to be composed of a series of discrete, downwind-developing, wind-producing thunderstorms. More on thunderstorm propagation and the impact of propagation on the predominant storm hazards posed follows in the next section.

DERECHOS AND FLASH FLOODS

On occasion, on the rear flank of a convective system responsible for a derecho, a nearly stationary band of thunderstorms may form. If such a band persists for an extended period, and/or if the line forms in a region of rocky or steeply-sloped terrain, flash flooding may follow. Situations of this type are not common, but when they do occur, the results may be devastating as the convective system's high winds are followed by the ravages of flooding. The "Ohio Fireworks" derecho-flash flood of July 4, 1969 is an example of what might be termed a concurrent derecho-flash flood convective system.  Eighteen people died, many others were injured, and widespread destruction occurred as derecho winds swept across Michigan, Ohio, Pennsylvania, and adjacent Lake Erie. A few hours later, more than two dozen people perished from subsequent flash flooding in Ohio. The Johnstown, Pennsylvania flood of July 20, 1977 was associated with a similar convective system that produced both flash flooding and high winds.

Paradoxically, the same unidirectional wind profile that is conducive to the downwind (or forward) propagation of convective systems --- that is, the repetitive, downwind development of new thunderstorms that can yield a derecho --- also is favorable for repetitive storm development in the upwind direction. When the direction of the environmental flow around a convective system varies little with height, the system's collective outflow of cool, surface air --- the cold pool --- over time necessarily elongates downstream, in the direction of the mean flow. The top part of the illustration below is a plan-view schematic of cold pool elongation for a case of unidirectional westerly flow, where the cold pool is shaded grey and "T" refers to "time." As noted in Derecho Development, the leading edge of the cold pool, or gust front (here depicted by weather map frontal symbols), is the main site of new thunderstorm formation. If thermodynamic conditions are favorable for repetitive storm development along the progressive or downwind-moving part of the boundary, that part of the elongating cold pool / gust front potentially could yield a derecho-producing convective system, as shown in the side-view schematics of the bottom panel in the figure below:

Illustration adapted from Corfidi et al. (2016)

As a cold pool in unidirectional environmental flow further elongates, the trailing, upwind flank of its gust front often becomes stationary or nearly so. This is shown in the right-most plan-view of the illustration above and in the plan-view inset of the figure below by the green-colored part of the outflow boundary, and by the alternating warm and cold frontal pips. Trailing gust front boundaries are especially likely to become stationary when the atmosphere is moist through a very deep layer, and the potential for strong, evaporatively cooled storm downdrafts is, therefore, comparatively limited. The persistent source of low-level uplift provided by the stalled gust front can then serve as the seat of repetitive thunderstorm development in the upstream direction. As individual storms grow and mature, they move parallel to the boundary, causing multiple episodes of heavy rain at locations along the line. Such convective evolution is known as echo training.  Prolonged echo-training in a moisture-rich environment nearly always results in excessive rainfall or flash flooding. Smaller scale or more intermittent episodes of echo training frequently occur on the rear flanks of derechos, and may cause localized flooding in the wake of a derecho's high winds. In addition, back-building lines of storms sometimes form atop the cold pool left in the wake of a derecho; the combined derecho-heavy rain producing storm structure is then known as a bow-arrow convective system. More information on the relationship between derechos and flash-flood producing convective systems is available here.

Illustration from Corfidi (2003)

DERECHOS AND HEAT WAVES

Some of the most intense summer derechos, especially those of the progressive type (see below), occur on the fringes of major heat waves. Examples include the July 1983 "I-94" derecho in the upper Mississippi Valley, the Mid-July 1995 derechos in New York and Canada, and the more recent Ohio Valley / Mid-Atlantic derecho of June 2012. The relationship is more than statistical. It turns out that the meteorological conditions favorable for large-scale heat waves often also are conducive to derechos. In the United States, this is especially true from the Upper Missisippi Valley and Upper Great Lakes into the Ohio Valley and Northeast.

The primary link between heat waves and derechos is the presence of an elevated mixed layer, or EML.  An EML is a layer of mid-tropospheric air that originates over the arid, elevated terrain. Because of their origin, EMLs exhibit sharp decreases in temperature with height. The large vertical temperature differentials (or "steep" lapse rates) in EMLs are analogous to those observed over black-topped roofs and parking lots on sunny days. Such thermal stratification encourages the formation of strong updrafts that can lead to the development of thunderstorms. In fact, the frequent presence of an EML on days othwerwise favorable for thunderstorm formation to a large extent accounts for the intensity of the storms commonly encountered over the Great Plains.

During a typical heat wave over the central and eastern United States, a large, stationary upper-level high pressure area usually is present over the south-central states. Persistent westerly winds on the poleward side of the high allow EMLs generated over the Rocky Mountains to extend eastward into the Ohio Valley and Northeast, well beyond their usual range over the Plains. Warm air aloft associated with the base of the EML acts as a "cap" or "lid" that prohibits thunderstorm development along much of the extent of the EML, southward into the heat wave-associated "high." But on the northern fringe of the EML, where low-level uplift frequently is focused along a stationary front marking the northern edge of the heat wave, updrafts that form in the strongly heated air near the ground may breach the cap, resulting in an explosive release of instability. If other conditions are favorable (e.g., low-level moisture is abundant along the front, winds are largely unidirectional, parallel to it, and increase with height), additional storms may erupt in concentrated fashion along the boundary, yielding a band of downstream-developing storms and, on occasion, a full-blown derecho.

The illustration below illustrates the idealized, large-scale meteorological environment favorable for progressive derechos on the northern fringe of a quasi-stationary high pressure cell associated with a major heat wave over central and eastern United States. The location of the EML in such situations is approximated by the brown-tinted band behind the word "Progressive," with the mean mid-tropospheric flow depicted by the arrow marked "Airflow," and the high pressure center by an "H." The region most susceptible to serial-type derechos in such a pattern also is shown.

Illustration by Dennis Cain

TYPES OF DERECHOS

Two primary forms of derechos may be distinguished. This classification largely is based on the overall organization and behavior of the associated derecho-producing convective system and reflects, in part, the dominant physical processes responsible for the thunderstorms and damaging surface winds.

Serial derechos
The type of derecho most often encountered during the spring and fall is called a serial derecho.  These 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 of derecho typically is associated with a strong, migratory low pressure system with strong low- to mid-tropospheric flow. An example of a serial derecho with a very extensive squall line and embedded smaller scale bow echoes is the one that 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.

Progressive derechos
The second type of derecho is called a progressive derecho.  These are associated with a relatively short line of thunderstorms (typically from 40 miles to 250 miles in length) that may at times take the shape of a single bow echo, particularly in the early stages of development. In some cases, the width of a progressive derecho and its associated bow echo system remain relatively narrow even though they may travel for hundreds of miles. 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 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 (see Fig. 2 in that event's discussion page). Progressive derechos may travel for many hundreds of miles along paths that are relatively narrow compared to those of serial derechos. They sometimes are associated with an area of weak low pressure at the surface. However, the low- to mid-tropospheric flow in the neighborhood of progressive systems typically is not strong, suggesting that, in contrast to serial-type events, the intense surface winds observed result more from storm-induced downdrafts than from the downward transfer of fast mid-tropospheric flow.

The following ilustration highlights the distinguishing synoptic-scale weather patterns associated with serial and progressive derechos:

Modified from illustration by Dennis Cain

Two other classes of derecho may be identified:

Hybrid derechos
Occasionally, derechos having characteristics of both serial and progressive events are observed. These events are known as hybrid derechos.  For example, the "Southern Great Lakes Derecho of May 30-31, 1998 was attendant to a strong, migrating low pressure system. However, the derecho path and the associated bow echo convective system had many characteristics of a progressive derecho.

Low-dewpoint derechos
Most derecho-producing thunderstorm systems originate in and/or move through areas of seasonably rich low-level moisture. For example, nearly all the cases included in Noteworthy Events occured with surface dewpoints at or above 70° Fahrenheit. In contrast, bands of widespread wind-producing storms sometimes form in environments of very limited moisture, for example, with dewpoints in the 40s or low 50s (°F). Such systems are known as low-dewpoint derechos.  Low-dewpoint derechos most often occur between late fall and early spring in association with strong low pressure systems, and are a form of hybrid derecho. On occasion they appear during the warmer half of the year, especially in drier regions. The May 31, 1994 Utah-Wyoming derecho shown in the animated satellite loop in Derecho Development (above) is an example of a low-dewpoint event. This system produced a 105 mph wind gust at Provo, Utah, where sixteen people were injured, and severely damaged the Saltair Pavilion on the Great Salt Lake. A wind gust of 140 mph was measured on Camelback Mountain in nearby Dugway Proving Ground. Surface dewpoints were in the mid-40s °F along the path of the storm. Low-dewpoint derechos are essentially organized bands of successive, dry downbursts or microbursts. Some low-dewpoint systems appear to be ordinary serial-type events that happen to form in drier-than-average environments, while others more closely resemble progressive derechos, producing serial, convectively-driven downdrafts. More information on derechos in environments of limited moisture is available here.

DERECHO FORECASTING

Formation
Reduced to simplest terms, a derecho-producing convective system consists of a line of downstream-replicating, ordinary thunderstorm cells. Forecasting derechos begins, therefore, with examination of the atmospheric environment to assess the likelihood for the sequential development of individual thunderstorms, and, in particular, thunderstorms that might produce strong surface winds.

Thunderstorms are reasonably well-forecast; the processes governing their formation have been fairly well-understood for the better part of a century. In contrast, derecho forecasting remains one of the most challenging tasks facing operational meteorologists today. This difficulty reflects two primary limitations: (1) the imprecise nature of the observational network (relative that necessary) to produce an accurate and detailed picture of the propensity for thunderstorms with strong surface winds, and (2) our incomplete understanding of the processes that lead to repetitive, linearly-organized storm development.

Many meteorological factors, some acting synergistically and some seemingly at odds, may come together to yield an environment conducive to derechos. The variation of these factors over space and time also is important in fostering or hindering development. Two nearly identical meteorological settings at a given location might yield vastly different outcomes --- that is, a derecho or no derecho --- depending upon how the atmosphere evolved to that point in time, and how conditions vary spatially around that location. Derecho formation also is dependent upon very small-scale processes involving condensation, melting, evaporation, and sublimation of water deep within rapidly changing clouds --- processes that are neither well-understood, nor readily observable.

In some ways, serial derechos are easier to forecast than progressive events because the primary forcing mechanisms responsible for their development (e.g., strong cold fronts or jet stream disturbances) are fairly well sampled by today's observing network. On the other hand, unlike many other significant weather events of any type, progressive derechos may arise in relatively benign large-scale environments, with little or no identifiable atmospheric forcing. Many progressive derechos, in fact, arise in a manner that might best be described as a form of "atmospheric boot-strapping," wherein seemingly random interactions between individual thunderstorm cells --- or a loosely organized group of cells --- sometimes lead to a much larger, more strongly-organized system. One factor that appears important in realizing such development is the release of very strong latent heating in a relatively confined region. This may occur through the simultaneous development of many vigorous individual thunderstorm cells near a weak front or other local lifting mechanism. Such latent heating involves the lofting of abundant rain, snow, and hail particles that can lead to the rapid formation of strong storm downdrafts and an expanding cold pool. The "Super Derecho" of May 8, 2009 is an example of a storm that exhibited this type of development.

Recent research has to some extent furthered our understanding of mechanisms behind repetitive, downstream thunderstorm development, and studies have shed light on the origins of strong winds in convective storms (e.g., the BAMEX program of 2003; see References). In addition, numerical forecast models exist that have, on occasion, correctly depicted derecho development. But significant advances in forecast skill likely will require deployment of a nationwide network of atmospheric sensing devices with greatly enhanced spatial and temporal resolution relative to that in place today --- in addition to further storm-scale research.

Movement
Once a derecho has formed, the forecast task reduces to determining where the parent convective system will move, and how long will it last. As noted in Derecho Development, a derecho will persist as long as the environment on the downwind side of its elongating cold pool is favorable for the formation of new storms. But both the location and rate of new storm development in the downwind direction typically vary over space and time, with factors such as the distribution of low-level moisture, atmospheric instability, and "capping" (warm layers aloft that hinder storm formation) --- amongst others --- typically involved. Such variables complicate the forecast process. At the same time, the background wind field around a derecho rarely remains static. Because the overall motion of a derecho-producing convective system is a combination of (1) advection (the movement of individual storm cells by the environmental wind outside the storms), and (2) propagation (the development of new storm cells relative to older ones), spatial and/or temporal changes in the environmental wind add further complexity to the forecasting of derecho tracks.

Various parameters and techniques have been developed in recent years to help forecast derecho formation, movement, and longevity (see References). These tools will require modification as observational and modelling studies provide further understanding of derecho mechanics.

TORNADOES AND OTHER CIRCULATIONS IN DERECHO ENVIRONMENTS

Tornadoes
Derechos and tornadoes can occur with the same convective system. This is particularly so with serial derechos associated with strong, migratory low pressure systems. The tornadoes may occur with isolated supercells (rotating thunderstorms) ahead of the derecho producing squall line, or they may develop from storms within the squall line itself. An example of a serial derecho that produced both extremely damaging straight-line winds and significant tornadoes from supercells embedded in the derecho-producing squall line is that which affected Florida during the early stages of the so-called "Storm of the Century" of March 12-13, 1993. Although not as common, tornadoes sometimes occur with progressive derechos. When they do, the tornadoes typically form within the bow echo storm system itself, and only rarely are associated with isolated supercells ahead of the bow.

The occurrence of tornadoes with derecho-producing convective systems reflects the fact that both tornadoes and strong convective wind gusts share, to some extent, common origins in the background atmospheric environment. In short, the great degree of thermodynamic instability; i.e., buoyancy, that gives rise to strong updrafts and, ultimately, the thunderstorms that spawn tornadoes also promotes the formation of storm downdrafts. In addition, both tornado and derecho environments are characterized by the presence of substantial vertical wind shear; i.e., large changes in wind speed and/or direction with height. While derecho-producing convective systems tend to be most favored when the vertical wind profile is unidirectional, a unidirectional wind profile may still contain appreciable shear. At the same time, in even a modestly sheared environment, small-scale stretching and tilting motions often present along storm gust fronts in a squall line may yield low-level circulations that, on occasion, can "tighten up" into a tornado.

Mesovortices
While storm downdrafts likely account for the greater proportion of damaging nontornadic winds in a derecho-producing convective system, observational evidence and numerical simulations show that corridors of enhanced wind damage in derecho-producing squall lines often are associated with embedded circulations or "vortices." These features may occur in various sizes and generally are referred to as mesovortices.

Mesovortices can arise in many ways. For example, the bookend vortices mentioned in Derecho-producing storms are counter-rotating circulations that arise as the rear-inflow jet of a developing convective system impinges on the system's gust front (see Derecho development). This forces horizontally-rotating "tubes" of air along the gust front to tilt vertically in storm updrafts on either side of the jet. Once formed, the pair of resulting vortices can enhance the rear-inflow jet and thereby initiate or accelerate the bowing process, as shown here. Bookend circulations may reach dimensions of several tens of miles. Counter-rotating vortices also occasionally arise from the downward tilting of horizontal vorticity by storm downdrafts, as shown in the following schematic modified from this study by Wakimoto et al. The horizontal vortex tubes (here depicted by green arrows) arise from differencies in buoyancy strength (that is, the tendency for spontaneous upward motion) across a thunderstorm or squall line's gust front.

Modified from Wakimoto et al. (2006b)

Once developed, it is the superpositioning of vortex-induced winds with those of the convective system's larger-scale outflow that can lead to corridors of enhanced damage in a derecho. Typically, with convective systems that form in the mid-latitudes where the mean tropospheric flow is westerly, this superpositioning yields the strongest winds on the equatorward side (i.e., the southern side in the Northern Hemisphere) of the vortices, where the wind fields are additive. An analogous additive effect accounts for the fact that the highest wind speeds in a hurricane generally occur in the storm's right front quadrant (when facing in the direction of storm motion).

DERECHO CLIMATOLOGY

Where and when derechos are most frequent in the United States
Derechos in the United States are most common in the late spring and summer (May through August), with more than 75% occurring between April and August (see graph below). As might be expected, the seasonal variation of derechoes corresponds rather closely with the incidence of thunderstorms.

Illustration by Dennis Cain

Given that the prevailing meteorological conditions favorable for derecho development occur with fair regularity over large parts of the continental United States each year, it is not surprising that the United States is one of the most derecho-prone regions of the world. In this country, derechos most commonly occur along two axes, as shown on the map below. One track parallels the "Corn Belt" from the upper Mississippi Valley southeast into the Ohio Valley; the other extends from the southern Plains northeast into the mid-Mississippi Valley. During the cool season (September through April), derechos are relatively infrequent but are most likely to occur from east Texas into the southeastern states. Although derechos are rare west of the Great Plains, derechos occasionally do occur over interior portions of the western United States, especially during spring and early summer (e.g, see this case that affected parts of Utah and adjacent states on May 31, 1994). It should be noted that because of limitations in the data collection and compositing techniques used to create the derecho climatology map, the number of events depicted along edges of the domain, especially near the Canadian border, is somewhat less than the actual number observed. Additional climatological information on United States derechos is available here.

Illustration by Dennis Cain

Derechos outside North America
While relatively uncommon, derechos also occur outside North America. Comparatively few, however, have been formally documented, and all of those in recent years. In Europe, a serial derecho occurred on July 10, 2002 over eastern Germany and adjacent portions of neighboring countries. The Berlin area was hardest-hit, with eight people killed and 39 injured, mainly from falling trees. Derechos in Finland, Spain and France, and Bulgaria also have been described. The Finland case was the first to have been documented poleward of 60 degrees latitude. The main factor limiting derecho development in Europe and other cooler parts of the world is the infrequent nature of elevated mixed layers (EMLs). In regions where EMLs are more common, derechos are more likely to occur (e.g., Australia and parts of Africa). In Bangladesh and adjacent parts of India, an intense type of storm known as a "Nor'wester" occasionally brings high wind and very heavy rain, usually in spring. Based on event descriptions and the meteorological environments involved, it appears that at least some "nor'westers" are, in fact, progressive derechos.

DERECHOS AND CLIMATE CHANGE

A common question of late is how climate change or, more specifically, global warming might affect derecho frequency, strength, and distribution. Given that earth's atmosphere always has been in a state of change, and that the time scales of climate change are very long relative to human lifespans, questions of this type are more academic than practical.

A warmer planet at first glance would appear to be more conducive to the development of the intense thunderstorms that comprise derecho-producing convective systems. But thunderstorm updrafts require the presence of strong vertical temperature gradients; any warming that occurs at the surface likely also would occur aloft. Thus, the net change in instability --- that is, the net change in the potential for strong updraft development --- likely would be minimal. In addition, although a warmer environment implies greater atmospheric moisture content and conditional instability (instability related to the release of latent heat during condensation; this is the type of instability that fuels a hurricane), all other factors being equal, the increased moisture also would yield more widespread low-level clouds. Such cloudiness would negatively impact storm initiation and derecho development.

There is nothing to suggest that a warmer world necessarily would favor stronger derechos. This is not only because vertical temperature differences likely would remain unchanged but also because derecho development requires the favorable coexistence of many interacting environmental factors over various scales of time and space. In particular, the small-scale processes involved in the initiation of individual storms --- and their growth and organization into long-lived mesoscale convective systems --- are incompletely understood. For this reason these processes are only crudely represented in both short-range (day-to-day forecast) and long-range (climate) numerical models. For example, increased moisture theoretically would be available for cloud and storm development in a warmer world. Increased cloud water content, in turn, generally enhances downdraft strength (through "water loading"). But it does not necessarily follow that storms with water-enhanced downdrafts would be more favorable building-blocks of derechos; if the downdrafts are too strong or ill-timed, their presence could short-circuit derecho development.

What can be said with greater certainty about derechos and climate change is that the corridors of maximum derecho frequency likely would shift poleward with time. This is because the bands of fast upper-level winds that arise from the equator-to-pole temperature gradient --- the jet stream --- would contract poleward in a warmer world. Because derechos tend to form on the equatorward side of jet streams, especially those that mark the northern fringes of warm high-pressure ("fair weather") systems, the areas most favored for derecho development also would shift poleward. It is unclear, however, how jet stream changes might impact derechos from a wind shear perspective. While derechos are not as sensitive to wind shear (changes in wind speed and/or direction with height) as are, for example, tornadoes, some degree of vertical shear is necessary for long-lived derechos. Because the most favorable wind shear environments are tied to the location of the jet stream, it seems reasonable to conclude that, overall, potential changes in shear would not appreciably impact derecho incidence or severity.

VISUAL ASPECTS

Another frequent question regarding derechos is whether or not their approach may be identified by characteristic cloud formations. The short answer is "No." Although some derecho-producing convective systems are indeed preceded by well-defined shelf or arcus clouds along the progressive portions of their gust fronts (recall examples shown at the top of this page), these formations are not always present. This is because the local wind and thermodynamic conditions that give rise to shelf clouds typically change appreciably over space and time. While the environment might favor the development of such clouds at some point during the life of a given event, it is uncommon for these conditions to exist everywhere and at all times during its existence. In addition, environments that give rise to dramatic shelf clouds sometimes appear when the larger-scale weather pattern is not favorable for derecho development. As a result, striking shelf clouds on occasion occur in conjunction with relatively benign thunderstorms. For example, the following image shows a turbulent, low-hanging shelf cloud that accompanied a small but intense cluster of thunderstorms near Ventnor, New Jersey on July 24, 1979. The winds aloft were comparatively light and not favorable for organized, sustained storms. But moisture-rich air in the lower levels and convergence (uplift) along sea-breeze boundaries contributed to circulations that briefly gave rise to an impressive shelf cloud, in addition to very heavy rain.

Photograph by Stephen Corfidi

Arguably some of the most dramatic and longer-lasting derecho-induced shelf clouds appear over the Great Plains. This, in part, reflects that region's proximity to the western United States plateau and its associated elevated mixed layer air, or EML, previously discussed. A very strong temperature inversion or "cap" that marks the base of the deep EML commonly is present over the Plains states on days when derecho-producing storms arise there. It is this inversion that contributes to the development of marked shelf-type accessory clouds as cool thunderstorm outflow winds force warm, humid, low-level air to rise through the stable layer. On occasion, more than one inversion and/or moisture discontinuity may be present near the base of EML. These environments are the ones that sometimes give rise to particularly menacing, multi-tiered shelf formations such as the one shown below. This photograph was taken near Fort Supply, Oklahoma as the intense "Southward Burst" derecho of May 27, 2001 approached from the north. Additional images from this event are available here.

Photograph by Douglas Berry

Whether or not a notable shelf cloud develops, the exceedingly dark aspect presented by the leading edge of many derechos largely is due to the blocking of sunlight by the deep and often horizontally-extensive clouds that comprise the parent storm system. During spring and summer this blocking effect is accentuated around sunset for east or southeast-moving storms as the low sun angle at that time maximizes the path taken by sunlight through the clouds. The high "optical density" of rapidly-growing cumulus and cumulonimbus clouds is another reason approaching derechos appear dark. Greater concentrations of small water droplets are present in vigorous convective clouds, where rapid condensation is taking place, compared to those in weaker cumuliform clouds or stratus types. Small droplets back-scatter (reflect) incident sunlight very effectively, allowing very little indirect light to pass through. As a result, derecho-bearing squall lines tend to be especially dark and foreboding when they lie between the observer and the sun. This is the case with the early evening sequence shown below, photographed near Vernon, Texas in April 2000.

NOTEWORTHY EVENTS

Many significant derechos (i.e., those that have caused severe damage and/or casualties), have occurred over North America during the last few decades. Most of these affected central and eastern parts of the United States and Canada. Linked below are brief summaries of some of these events. The list is not all-inclusive, and additional cases will be added as time permits. Information includes a map of the area affected, a description of the storm's impact and meteorology, and, where available, links to additional information.

Holiday weekend events
The human impact of the following events was enhanced by their occurrence on summer holiday weekends, causing many to be caught out-of-doors during the sudden onset of high winds...
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
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
July 4-5, 1999............"The Boundary Waters-Canadian Derecho"....ND, MN, ON, QB, NH, VT, ME

REFERENCES

Here is a list of selected of research papers on derechos and the convective systems responsible for their development. In addition to presenting analyses of some well-documented events, these papers describe what is known about the formation and mechanics of derecho-producing convective systems.

In the mid-1990s, the Atmospheric Environment Service of Canada assembled a video that documents the passage of a progressive-type derecho through the Pakwash forest of northwest Ontario on July 18, 1991. The video includes camcorder footage of the storm affecting a forested area, post storm aerial views of the forest "blowdown," and interviews with people who experienced the event. While the video clips were made some time ago, they remain useful in providing insight into the impact a derecho can have in real-time, .

Contact Stephen Corfidi for "About Derechos" feedback

ACKNOWLEDGEMENTS

Many people assisted in preparing "About Derechos" by providing photos, images, stories, suggestions, and other information. Their assistance was very much appreciated.

From Environment Canada: Phil Chadwick, Rene Heroux, Mike Leduc, Serge Mainville, Brian Murphy, Peter Rodriquez, Sarah Scriver, Dave Sills, and Pierre Vaillancourt.

From the U.S. National Weather Service and National Severe Storms Laboratory: James Auten, John Cannon, Ariel Cohen, Mike Coniglio, Sarah Corfidi, Mark Darrow, Greg Dial, Chuck Doswell, Roger Edwards, Randy Graham, Jared Guyer, John Hart, Victor Homar, David Imy, Sarah Jamison, Ed Jessup, Rusty Kapela, Steve Keighton, Richard Koeneman, Norvan Larson, Jeff Last, Jay Liang, Patrick Marsh, Dan McCarthy, Corey Mead, Peter Parke, Steve Pennington, Tom Reaugh, Kevin Scharfenburg, Russ Schneider, Todd Shea, Rich Thompson, Frank Wachowski, Jeff Waldstreicher, Steve Weiss, and Mike Wyllie.

Others who assisted include: Curtis Alexander, Bert Barnes, Karen Bennett, Marlin Bree, Dave Crowley, Lee Grenci, Botchy Grimm, Dave Lewison, Chouga McGuire, Pete Pokrandt, Colin Price, Kristina Reichenbach, and Robert Schlesinger.

Special appreciation is extended to Dennis Cain for the base schematic illustrations that appear in "Derecho-producing storms," "Derecho development," and "Derecho climatology," and are used in the National Weather Service's Jet Stream Online School for Weather.