This is what a derecho looks like on radar. Pretty, huh?
Derecho at 3:14:44 PM EDT on June 4 2008. Base reflectivity on left, base wind velocity on right. Click to enlarge in a new window/tab.
This is what a derecho looks like on the ground. Not so pretty, huh?
Damage from the 7-11-11 derecho. Photo: NWS Des Moines, IA. Click to enlarge in new window/tab.
Before I go further into talking about derechos, let's discuss this kind of a thunderstorm. These storms form what are called "mesoscale convective complexes," or an MCS for short, which is a fancy way of saying "an organized area of thunderstorms." Given the right mixture of instability and wind shear, these storms can form into a line and grow in intensity.
A NWS graphic showing a mesoscale convective complex moving southeast on June 12, 2009.
We can make the definition of a derecho even shorter, by saying it's a destructive, long-track, long-lived bow echo. That brings us to our next question -- what's a bow echo? Sometimes, these mesoscale convective complexes can turn severe with damaging winds and some large hail. When they become severe, these lines of storms can start to bow due to something called a "rear inflow jet." The rear inflow jet can be caused by several factors, and it's an important factor in making the bow echo/derecho so strong. This area of stronger winds makes the middle of the line move faster and appear to bulge out, compared with the ends of the line. This creates a shape similar to an archer's bow, which is why it's called a bow echo.
Radar cross-section of the wind velocities in the 7-11-11 derecho in east-central Iowa, with the rear inflow jet denoted by arrows, pointing in the direction of the flow. Radar image and arrows from NWS Des Moines. Click to enlarge in new tab/window.
The rear inflow jet can be extremely strong (at times, exceeding 100 MPH) in the strongest bow echoes/derechos. As seen in the radar cross-section above, the rear inflow jet strengthens in this case as it nears the leading edge of the storms, and gets shoved down to the surface right at the edge of the system. When this jet hits the ground, it spreads out sort of like a ripple on a pond, causing severe straight-line wind damage.
We can see this happening during the 7-11 derecho in central Iowa. The next graphic is a base velocity radar image, which measures the speed and direction of an area of precipitation within a storm. They're great at detecting rotation in storms (leading to the possible development of a tornado), but they're also great for detecting severe winds.
In the below radar, the green and blue colors denote winds moving towards the radar site (which is located near Des Moines, IA), while the red/orange/brown colors denote winds moving away from the radar site. The darker the warm colors are, the stronger the winds are. In the following image, you can see winds at about 6,600 feet off the ground were blowing at 120 MPH. That's insanely strong. Unfortunately for residents in the path of this derecho, most of those winds translated to the ground, causing widespread damage from winds of 100 MPH+, or equivalent to that of a weak tornado. It did intense damage -- destroying homes, barns, grain silos, crops, phone poles, radio towers and even a television station's doppler weather radar.
Base velocity image from NWS Des Moines showing the rear inflow jet and strongest winds at the base of the bookend vortex near Garvin and Dysart IA. Arrows, lines and text on the radar image added by me. Click to enlarge in a new tab/window.
In the above image, I denoted where the rear inflow jet was located. The northern end of this derecho had something called a "bookend vortex." A bookend vortex (seen in the radar image below) is when the northern edge of the bow echo curls back and creates a counterclockwise vortex, which can be strong enough in some cases to produce tornadoes. In this case, however, it served to feed into the rear inflow jet and strengthen it. This is what happened in east-central Iowa, as denoted in the above wind velocity image as well as the cross-section (note that the rear inflow jet strengthened due to the bookend vortex just before diving down towards the ground). The strongest winds, and consequently the strongest damage, occurred where the rear inflow jet met the edge of the bookend vortex, near the towns of Garvin, Toledo and Dysart, IA.
Base reflectivity radar image from NWS Des Moines showing the bookend vortex. Arrows also by NWS Des Moines. Click to enlarge in a new tab/window.
The result of these intense winds were catastrophic. I'll let the following NWS storm survey pictures tell the story. There are more images submitted by the public at the NWS Des Moines website, which I can't embed due to copyright issues (however, all images below and above taken from the NWS fall under fair use/public domain laws).
7/12/11 -- An unfinished, metal framed industrial building completely collapsed from the derecho winds, Huxley IA. Photo: NWS Des Moines. Click to enlarge in a new tab/window.
7/12/11 -- House partially leveled due to the straight-line winds, Maxwell IA. Photo: NWS Des Moines. Click to enlarge in a new tab/window.
7/12/11 -- Grain silos punched in by the destructive winds, Marshalltown IA. Photo: NWS Des Moines. Click to enlarge in a new tab/window.
7/12/11 -- Ten power poles snapped in half along a rural farm road, Traer IA. Photo: NWS Des Moines IA. Click to enlarge in a new tab/window.
The two following images are from just east of the areas listed above, in the NWS Quad Cities IA territory. Garrison IA was the hardest hit in that area, with straight-line winds of 110-130 MPH ripping through and destroying nearly everything they could. To put that in perspective, it's like the town got hit with a high-end EF2 tornado, without the rotation.
What's left of the Garrison IA Fire Department after 110-130 MPH winds associated with the derecho moved through. Photo: NWS Quad Cities IA. Click to enlarge in a new tab/window.
The top half of the tiny Garrison IA public library destroyed due to 110-130 MPH derecho winds. Photo: NWS Quad Cities IA. Click to enlarge in a new tab/window.
The following image was taken at the NWS in Grand Rapids MI just before the derecho hit.
A large shelf cloud on the leading edge of the derecho approaching the NWS Grand Rapids MI office after daybreak on Monday July 11. The office's Doppler radar is seen on the right. Photo: NWS Grand Rapids MI.
The 7-11 derecho packed a good punch, but what about previous ones? There are a few derechos a year, with more forming in the more active years (such as this one). They usually form in the summer months, since this type of storm thrives on heat and humidity -- many a nasty heat wave has been broken by a derecho passage.
May 8, 2009 "Super Derecho"
Composite radar of the May 8 2009 derecho at hourly intervals, starting at 1000PM CDT May 7 in northeastern Colorado, and ending at 700PM CDT May 8 in western West Virginia. Click to enlarge in new window.
This was a derecho that started in Colorado and worked its way east to the Tennessee/North Carolina border, leaving large amounts of destruction and death across 10 states in its wake. The thunderstorms that would go on to produce the derecho formed in northeastern Colorado at 1000PM CDT on May 7 2009. The storms moved southeast into Kansas overnight, and began to intensify early in the morning as they moved across the state. It turned into a full-fledged derecho early in the morning while over south-central Kansas, and barreled towards southwestern Missouri. The storm caused unimaginable damage as it crossed through the Joplin area (which was recently devastated by an EF-5 tornado). It continued spawning tornadoes and causing more wind damage as it progressed eastward, eventually dissipating over the Appalachian Mountains early in the evening, 20 hours after the first storm initiation.
The derecho at its strongest near Joplin MO near 730AM CDT on May 8 2009. Radar image from SPC. The asterisk denotes the location of the TV transmission tower downed by nearly 100 MPH winds. This is "Figure 4" as referenced in the analysis below. Click to enlarge in new tab/window.
The SPC has a good page dedicated to this one derecho, with the following coming from part of their analysis:
Countless trees and power lines were downed, and 80-100 mph straight-line winds toppled the transmitting tower of KSN-TV in Joplin (Figure 4). The tower fell onto part of the studio building, forcing the station to remain off-air for several weeks. Roof and home damage was widespread. Numerous embedded tornadoes, including six rated EF-2 and one rated EF-3, added to the destruction in Missouri.
Even more significantly, while damaging winds continued to accompany the leading edge of the bow southward into Arkansas, a band of more intense winds developed in association with the previously-mentioned mesoscale convective vortex that evolved on the north end of the bow. This persistent vortex resembled the core of a tropical storm, complete with a small "eye," and produced sustained 70-90 mph winds as it swept east across southern Missouri between 8 AM and noon. The nearly continuous damage swath from such a meso-vortex was unprecedented. The winds were strongest on the south side of the circulation and were responsible for at least two deaths, numerous injuries, and widespread structural damage. Metal beams from a damaged factory building crushed cars near Sedgewickville in Bollinger County, and two buildings were destroyed in neighboring Perry County. At some locations, severe wind gusts lasted 45 minutes, and in forested areas, 80% of the trees were flattened.
May 30-31 Great Lakes Derecho
An SPC illustration of the progression of the derecho. Each + mark denotes wind damage caused by the storm. Red dots indicate tornado touchdowns.
This derecho formed just southwest of Minneapolis, Minnesota late in the evening on May 30th, and quickly strengthened as it made its way east across the Great Lakes region. This derecho was "noteworthy" (as the SPC lovingly calls it) in the amount of damage it produced, as well as the incredibly strong wind gusts that were recorded or estimated based on damage.
Estimated wind gusts (black) and measured wind gusts (red) as the derecho went through Wisconsin. Graphic from SPC.
Estimated wind gusts (orange) and measured wind gusts (red) as the derecho crossed through southern Michigan. The purple S denotes a seiche caused by the derecho's winds. Graphic from SPC. Click here to read a great article discussing seiches.
LinksThere are plenty more "noteworthy" derechos that have their own page over at the SPC which I encourage you to look up. You can find out more about derechos by visiting this page over at the SPC, and their noteworthy derechos links are located here.
For more information about the 7-11-11 derecho, numerous NWS offices have a page dedicated to the event. These links were active as of diary posting on July 16, 2011. Visit the following links for more about this event.
--NWS Des Moines IA
--NWS Quad Cities IA
--NWS Milwaukee WI
--NWS Chicago IL
--NWS Grand Rapids MI
--NWS Detroit MI
--NWS Northern Indiana
--NWS Wilmington OH
If you'd like to read some more scholarly/journal works on bow echoes and derechos, the SPC has compiled a catagorized list of their references for compiling the "About Derechos" page I linked above.