Derechos 101 - what they are, how they develop and move, how they cause tremendous wind damage.
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What is a derecho?
A derecho is a fast-moving arc of severe thunderstorms, producing a long corridor of widespread wind damage, typically hundreds of miles long, by a hundred or more miles wide.
Derechos can occur year-round, but the most violent ones develop during the summer months. Derechos have caused numerous fatalities and injuries, and billions in property damage – including downed limbs and trees, damaged homes and vehicles, and widespread power outages.
Derechos are very different than tornadoes. They are much longer, widespread and more persistent wind storms. The winds are called “straight line” meaning they blow predominantly from one direction, and the most intense gusts blow downward and outward in a fan-like pattern.
In contrast, a tornado is a highly localized storm characterized by a narrow, typically short swath; in tornadoes, winds are directed inward and upward along a fast moving, spiral trajectory.
Derechos feature one or more prominent bow echoes that consistent of a solid arc or arrow-shaped complex of thunderstorms, as shown below. The movement of the bow echo across the landscape over many hours is what generates the long corridor of wind damage. Bow echoes often move from 50 to 70 mph and their arrival is marked by a sudden, destructive “wall of wind”. Damaging wind gusts typically range from 55-75 mph but are occasionally stronger.
Figure 1 (Corfidi et al., 2016)
What type of damage do derechos cause?
Derechos, particularly those during summer, create a significant level of societal disruption. Ashley and Mote (2005) demonstrated that the damage loss can total hundreds of millions of dollars for a single, intense derecho. They also demonstrated that derechos cause more fatalities in the U.S. than the majority of tornadoes, and fatality levels are comparable to most landfalling hurricanes in the U.S. They also indicate that most deaths occur in vehicles and boats, while most injuries happen in vehicles and mobile homes.
Below is an example of the wind damage corridor shown for an intense derecho on June 29, 2012 during which approximately 1,200 wind damage reports were officially logged.
Figure 2 (Corfidi et al., 2016; blue square = wind damage report, yellow box = significant wind gust [>74 mph], green square = large hail report, red box = tornado report)
When and where do derechos occur?
Nearly two thirds of U.S. derechos are warm season, meaning the months of May through August. One of the most prominent corridors for derecho activity extends NNW-SSE from the upper Midwest through the Ohio Valley, as shown in the figure below (courtesy of Guastini and Bosart, 2016). Note that the frequency of derechos in the corridor abruptly drops east of the Appalachians. Ashley and Mote (2005) found that an average of 21 derechos occur per year in the U.S.
Figure 3 (Guastini and Bosart, 2016)
Are there different kinds of derechos?
There are two different modes – a cool season and warm season type of derecho. The cool season variety (called a serial derecho) is typical of the southern Mississippi Valley and Gulf Coast region, and consists of a line of thunderstorms with irregular bowing segments (called a line echo wave pattern, or LEWP). It occurs in the setting of a large low pressure system, in the “warm sector” of that storm, and embedded in strong jet stream (upper level) winds. The derecho forms along the cold front and moves northeastward, as shown below. The derecho tends to move parallel to the mid-level, strong winds (which blow from the southwest), and slower than those winds. Thunderstorm downdrafts in these derechos very efficiently mix down momentum of the higher winds aloft, creating powerful gusts that blast the surface.
Figure 4 (Modified after Johns and Hirt, 1987)
The prevailing summer mode (progressive derecho) occurs along a stationary front separating extremely hot, humid air to the south, from cooler, drier Canadian air to the north. A relatively weak (compared to winter) belt of jet stream winds parallels the frontal boundary, at mid- and high levels. A single, prominent, bow echo develops and moves along the corridor of hot, humid air. The derecho’s corridor can span up to 1000 miles. The hot and humid air mass is very unstable, continuously regenerating vigorous thunderstorms in the bow echo.
Progressive derechos tend to outrun the belt of faster winds aloft and move orthogonal to these winds, from W to E or NW to SE. These derechos generate their destructive winds internally, through the agency of intense, convective downdrafts and a low-level jet of air along the back edge of the bow echo. In summary, progressive derechos generate their own damaging wind gusts through vigorous convective cells, whereas serial derechos tap into the strong winds of the larger, wintertime jet stream.
Figure 5 (Modified after Johns and Hirt, 1987)
Is there anything controversial about the naming of derechos?
Yes. The term “derecho” was originally coined by Gustavius Hinrichs, a chemist and natural philosopher, in 1888, for a type of “line storm” that occasionally blasted across the Great Plains (see painting below). He chose the term “derecho” because winds blow “straight ahead”, as opposed to the rotary winds of a tornado. The term seems to have fallen out of vogue until two severe storms meteorologists revived it in 1987 (Johns and Hirt), after writing the seminal, modern paper on derecho climatology and dynamics. At the time, they proposed a strict definition for a derecho, which has the following elements:
1) Concentrated area of convective-induced wind damage and/or gusts > 57 mph;
2) Wind damage is continuous along a corridor > 250 miles;
3) Wind damage that must demonstrate chronological progression along the corridor;
4) Three or more reports of “significant wind gusts” exceeding 74 mph, each separated by at least 40 miles
Figure 6 (John S. Curry, 1939 The Line Storm)
Some research studies since 1987 have excluded the more rigorous “part 4” in the defining elements above.
In 2016, a group of severe storms meteorologists (Corfidi et al.) recognized the lack of standardization with this definition. They took the additional step of proposing a new set of identification criteria. The motivation: It’s important to communicate an unambiguous definition of the term “derecho” to the general public, emergency planners and insurance industry. And, a single, universal set of criteria is needed to better align all those academic studies.
Corfidi et al. proposed a new, structurally-based definition (using Doppler radars), with less emphasis on wind observations. They further suggested expanding the derecho’s minimum corridor length to 400 mi. The idea was to cull out a lot of the weaker events…meaning many derechos formerly classified as “serial” would be renamed as “squall line windstorms”. The term “derecho”, in these researchers’ opinion, should be reserved for only the longest-lived, most intense events (perhaps just 10-15% of all annual events in the U.S. as their study suggests).
Personally, I have been a proponent of a unified definition (one agreed upon by forecasters and academic research scientists), but I also believe we should institute a tiered rating system i.e. different intensity categories of derecho. There are some events that are low-end, and some that are highly impactful – indeed, historical - that people will recall for a generation. Tornadoes and hurricanes are no different; we rate tornadoes on a wind intensity scale from zero through six, and hurricanes from Cat 1 to Cat 5. Why should derechos – which are also highly destructive windstorms – not similarly rated? We don’t necessarily need five or six categories, but perhaps three could suffice (low end [marginal], moderate, and high end) based on some combination of reported wind gusts and path length.
By the way, who decides whether a bow echo-generated windstorm actually constitutes a derecho? The answer: Whatever agency with scientific credibility makes the call! This can be the NOAA Storm Prediction Center, a local National Weather Service office, The Weather Channel, The Washington Post’s Capital Weather Gang…etc. It may not be the best practice to refer to a bow echo as a derecho while onging, as one needs to inspect the wind damage reports that come in after the fact, review the radar structure, etc.
An example of the thought process that goes into making the derecho “call” is found in the following story I wrote for The Washington Post:
Here is an article I wrote for the Washington Post about naming derechos:
One thing is for sure: Derechos are not “inland hurricanes” as I have seen them described in the news. That is just a terrible misnomer, on so many levels.
How do derechos generate damaging winds?
Derechos, particular progressive types, are organized around pairs of vertical, counter-rotating vortices, which are usually several miles in diameter, and embedded within front of the thunderstorm line. In between this vortex pair (called bookend vortices) is a bowing section, where part of the thunderstorm line races ahead, faster than adjacent regions of the line. The figure below, from Wakimoto et al. (2014), illustrates the basic dynamical processes that give rise to the bowing thunderstorm structure of a derecho.
Figure 7 (Wakimoto et al., 2014)
The counter-rotating vortices pull low-level airflow into the back of the storm line, and accelerate it toward the front. This generates much of the momentum that causes a jet of air to blast forward (several thousand feet deep) and distort the leading storm line into a bow or arc shape. The air also accelerates downward, toward the ground, because it is chilled by evaporating rain…becoming denser than surrounding air. The cool, descending flow spreads along the ground as a cold pool with strong, steady winds. The cold pool also acts as a giant “bulldozer”, lifting hot, humid air in front of the line upward, continuously regenerating the arc of thunderstorms.
The leading edge of the cold pool is called the gust front and when it arrives, the sudden surge of roaring wind is sometimes called a “wall of wind”. The powerful current that enters the back of the storm, supplying momentum to the cold pool, is called the rear inflow jet.
The most damaging gusts of wind come from individual storm cells within the bow echo, generated by convective downdrafts. Whereas the cold pool features widespread, steady winds on the order of 30-40 mph, individual gusts lasting mere minutes, and covering parts of neighborhoods, are called downbursts. Winds in downbursts commonly range from 50-60 mph to 70-80 mph, and even higher (sometimes exceeding 90-100 mph). Adjacent downbursts may merge into downburst clusters and add to the high momentum level of a bow echo’s cold pool. This is shown in the figure below.
Figure 8 (Halverson and Rabenhorst, 2020, after Theodore Fujita)
Part of the damaging winds within a bow echo come from the forward speed of the bow itself. If the gust front is moving from west to east at 50 mph, and a downburst embedded within fans outward toward the east with 50 mph winds, the combined wind total felt by a stationary observer is 100 mph! In other words, the effect of winds generated by different mechanisms, and acting in the same direction, is additive.
Many of the features discussed above (gust front, cold pool, bookend vortices, bow echo, rear inflow jet) can be identified using Doppler radar. This provides some measure of lead time to predict the arrival of the “wall of wind”, as a progressive derecho may be tracked for several hours by Doppler radars along its track.
How do derechos form?
Progressive derechos require a very unstable atmosphere (one in which warm, humid air from the ground rises to great heights in the atmosphere), wind shear (increase in environmental winds with altitude), and a triggering mechanism. The figure below illustrates a common derecho setup spanning the upper Midwest to the Mid Atlantic from June-August.
Figure 9 (Halverson and Rabenhorst, 2020)
In this setup, the central and eastern U.S. are dominated by a “heat dome” or giant cell of high pressure, with sinking air and very hot and humid flow surging north at low levels. Within the dome, one or more regional heat waves may be in progress. Along the northern periphery of the heat dome lies cooler, drier Canadian air. A stationary front separating these two air masses extends along this boundary, generally with a west to east orientation. Above the stationary front lies the jet stream, blowing from the W or NW. The jet stream is the narrow zone of wind shear, where winds increase with altitude.
One or more impulses – small ,wavelike ripples – typically course through the jet stream current. These may trigger an organized cluster of thunderstorms. When the corridor ahead is primed by unstable air, the cluster may act as the “seedling” of a derecho’s bow echo. Or…it may not. At other times, no discernable wave can be identified as an intense cluster of thunderstorms erupts. For theses reason, progressive derechos are often poorly forecast. We basically don’t know until “the day of” when a derecho initiates. While we can usually identify the instability and shear corridor a priori, a derecho may or may not develop.
Typically the initial storm cluster starts as a short line of storm cells, then the cold pool organizes and becomes more widespread, triggering additional line segments to erupt. These segments may merge into a larger convective line. We call this process “upscale growth” and it is not well understood. Once a solid storm line forms and is able to continuously regenerate, bookend vortices develop after a few hours, and distort the leading edge of the line into a prominent bow…as the entire storm complex begins to accelerate. Over time, the bow echo may veer toward the south (to right of the original track) as new cells grow and develop or “propagate” into the unstable air lying to the south.
The unstable air is important because it energizes buoyant updrafts in thunderstorm clouds, and the large amount of water vapor in the air creates heavy rainfall. This, in turn, generates strong downdrafts through water loading and evaporation. Often the zone of unstable air is “capped” by an elevated, hot, dry air layer streaming off the southern Rockies and even Mexico (as shown in the figure above). This inversion layer(also called elevated mixed layer, or EML) tends to suppress (delay) early and widespread growth of ordinary thunderstorms, leading to an explosively unstable situation where a concentrated zone of storms manages to initiate and rapidly become severe.
Wind shear is important because it helps separate updraft and downdraft currents, so that they reinforce one another rather than destructively interfere. Wind shear also contains inherent spin or rotation, which is tapped by the developing vortex couplet in a bow echo.
As hypothesized by Wakimoto et al. (2016), much of the rotation is generated internally by the storm, within especially deep regions of the cool pool, as shown in Figure 7, above.
It’s interesting how the rotation in the cool pool actually begins in the horizontal, as long “vortex tubes”. These then get drawn upward as “loops” of spinning air by strong updrafts. Each loop is thought to split into a pair of vertical vortices, and that’s how the bookend vortices develop.
Over enough hours, the northern bookend vortex (which rotates counterclockwise) may become dominant, as enough time elapses for that circulation to be influenced by the Earth’s counterclockwise spin (this process is called the Coriolis Effect). Such a large, dominant circulation (called a mesoscale convective vortex or MCV) further intensifies the storm system and adds to its longevity. Should that vortex reach the surface, the MCV may generate its own swath of wind damage,. A dramatic example of an MCV in the “superderecho” of May 8-9, 2009 across the Midwest is shown in the figure below.
Figure 10 (ShelbyOhWx.com, July 16, 2019)
How do derechos differ from other types of severe thunderstorms?
In a bow echo/derecho complex, the primary airflow is a strong, rapidly advancing cold pool containing strong, sustained straight line winds and embedded downbursts. We say that this type of storm organization is “downdraft dominant”.
A supercell (long-lived, rotating thunderstorm) is a solitary storm cellcontaining a helical or spiral updraft, called a mesocyclone, usually several miles in diameter. The entire storm is organized around the mesocyclone and its dynamics. The mesocyclone in a supercell generates most strong to violent tornadoes in the U.S. and large hail. Multiple gust fronts and downdrafts, including downbursts, can also be found in parts of the supercell. The basic storm’s organization and vigor is sustained by the mesocyclone, and we say that such a storm is “updraft dominant”. Some supercells last many hours and track across multiple states. Overall, supercells cover less total area than the bow echo of a derecho, but the track length of damage (straight line wind, tornadoes, large hail) may at times be comparable.
Figure 11 (LA Times, 2013)
Do derechos sometime contain tornadoes?
Yes. As if downbursts and the “wall of wind” do not provide enough destruction! Some derechos, especially serial derechos, can produce multiple tornadoes. These tend to be small, weak and short lived…and difficult to identify on Doppler radar. By the time one is potentially identified, and a warning is issued, the tornado has often dissipated. One study (Trapp et al., 2005) found that about one fifth to one quarter of all U.S. tornadoes are in fact developed by line storms such as bow echoes and derechos. The figure below shows an interesting post-bow echo survey illustrating the complex mix of downbursts, straight line wind and tornadoes that can occur.
Figure 12 (Forbes and Wakimoto, 1983
More resources on derechos:
Here is an article I wrote for the Washington Post on basic derecho science:
The NOAA Storm Prediction Center (SPC) has a great site about derecho science and individual derecho events, found here: https://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm
Academic (Peer Reviewed) References
Ashley, W.S. and T.L.Mote, 2005, Derecho hazards in the United States, Bul. Amer. Met. Soc., 86, 1577-1592.
Corfidi, S.F., M. C. Coniglio, A.E. Cohen and C.M. Mead, 2016, A proposed revision to the definition of “Derecho”, Bul. Amer. Met. Soc., 97, 935-949.
Forbes, G.S. and R.M. Wakimoto, 1983, A concentrated outbreak of tornadoes, downbursts nd microbursts, and implications regarding vortex classification, Mon. Wea. Rev., 111, 220-235.
Guastani, C.T. and L.F. Bosart, 2016, Analysis of a progressive derecho climatology and associated formation environments, Mon. Wea. Rev., 144, 1363-1382.
Trapp, R.J., S.H. Tessendorf, E.S. Godfrey and H.E. Brooks, 2005, Tornadoes from squall lines and bow echoes. Part I: Climatological distribution, Wea. and Forec., 20, 23-34.
Wakimoto, R.M., P.Stauffer and W.-C. Lee, 2014, The vertical vorticity structure within a squall line observed during BAMEX: Banded vorticity features and the evolution of a bowing segment, Mon. Wea. Rev., 341-362.
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