Video and images of the derecho that struck the eastern U.S. on June 29, 2012: Here
A derecho is a violent storm system that can produce widespread wind damage across a large area and is associated with a band of rapidly moving band of showers and thunderstorms. The strong to violent winds are typically associated ahead of the main system as the outflow from the storms becomes more concentrated. Wind damage is typically directed into one direction and can create widespread damage along a relatively straight path. These windstorms typically form in the late spring and summer and typically affect states east of the Rocky Mountains. On June 11, 2012, an event that looked like a derecho formed and affected parts of Missouri, Tennessee, Kentucky, Arkansas, Mississippi, and Alabama. In this post, we will look at how a derecho is formed, where they occur, and use the storms of June 11, 2012 as an example of what a derecho can do to a large area.
A derecho has a very specific definition in meteorology, and must include the follow criteria to be classified as a derecho:
1) There must be a concentrated area of convectively induced wind damage/gusts greater than 50 knots, or 60 miles per hour.
2) The area must have a major axis length of 400 kilometers, or 248.5 miles.
3) Wind reports should show a continuous and nonrandom pattern of occurrence. For instance, a swath of storms should consistently produce wind reports as the system moves to the east or southeast.
4) In the storm reported areas, at least three reports, separated by 64 km or more, should include wind gusts greater than 64 knots, or 74 mph.
5) Derechos are typically continuous and can sustain itself for hours. With that said, no more than three hours can elapse between successive wind damage events.
I am sure you have heard the terms “squall line” or possibly “bow echo”. Derechos can consist of bow echos or a squall line that tends to bow. When this happens, winds become stronger ahead of the system and can affect a large area with widespread wind damage. These systems are also considered to be called a mesoscale convective systems, or MCS. Derechos typically bow out throughout the entire system. According to NOAA, a typical derecho consists of numerous burst swaths, microbursts, downbursts, and downburst clusters. These downbursts can consist of wind speeds around 60 mph with gusts greater than 100 mph.
Typically, a cluster of storms develop during the late spring and summer time period. These cluster of storms can eventually evolve into a single strong storm. Have you ever felt a cool wind prior to a thunderstorm approaching your area? If you have, you are feeling the outflow winds from the storm. In other words, a rain cooled downdraft occurs in the thunderstorm and hits the Earth’s surface and spreads horizontally and pushes outward. The cool, dense air spreads out and the warm airmass ahead of the system typically moves along the leading edge of the outflow as an updraft. The winds in the troposphere, or the layer of the atmosphere where our weather occurs, typically become relatively strong and unidirectional. This is the beginning stages of a derecho.
In some systems, the downdrafts in the storms can weaken the systems as cooler air pushes towards the surface and stabilizes the atmosphere. However, in this case, warm air ahead of the system can actually refuel and energize the thunderstorm complex. The downdraft winds can create a cool pool along the surface. As more storms develop, it can help strengthen and elongate the cold pool at the surface. As this happens, the cold pool induces a inflow of air known as the “rear-inflow jet” that helps the updraft (winds moving up) of the thunderstorm to expand and further intensify the cold pool. It is the cold pool and the winds ahead of the system that begin to strengthen and straight line winds become more of an issue.
As the system continues to organize, the squall line can bow out and eventually become a derecho. Of course, for it to be a derecho, you have to have the above mentioned definitions to occur before it can be classified as one. Some derechos can form small, quick spin up tornadoes that only last a minute or two. These tornadoes are generally weak and are below EF-2 strength. However, there have been tornadoes stronger than that in some instances. Regardless, derechos can produce widespread damage unlike tornadoes that produce isolated damage in certain spots. With this is mind, derechos are serious storms that involves taking shelter as falling trees and power lines are likely as the storms progress.
Many of the times, outflow from strong thunderstorms is typically when you experience the strongest winds from the storm, and it occurs before the thunderstorm hits you. Example: Indiana State Fair Stage collapse. It might not seem like the storm has arrived yet, but the outflow can extend miles away from the actual storm that can produce heavy rain, frequent lightning, and hail. Typically, organized squall lines, bow echos, and derechos can be seen in advance to the west. With that in mind, it can be fairly easy to issue warnings ahead of a system about strong to violent winds. However, forecasting the formation of these systems can be tricky and difficult. For instance, it is difficult to determine exactly where the storms will form a day or two in advance.
So, who typically sees derechos and how common are they? Most of the time, anyone living east of the Rocky Mountains can see derecho events, particularly in the spring and summer months. The common time period for derechos develop in the United States is from April into August, with May, June, and July as the peak months for development. The best area to see these systems typically occur across Oklahoma, Kansas, Missouri, and Arkansas. We can also experience derechos across the southeast, the Great Lakes region, and the Ohio River Valley. Other parts of the world can experience derechos, but they are fairly rare events. Areas such as parts of Europe and across India can experiences these wind storms.
Example of a derecho:
On June 11, 2012, one of the first derechos to hit the United States in 2012 formed and impacted parts of Missouri, Tennessee, Kentucky, Mississippi, and Alabama. The derecho formed in the afternoon hours on June 11 and pushed into the southeast later that evening. Thousands of people lost power as trees fell to the ground snapping power lines. In Alabama, over 128,000 people lost electricity while 28,000+ people lost power in Mississippi. The derecho produced ominous shelf clouds, and it even produced a few tornadoes.
Take a look at the storm reports on June 11, 2012. You will notice a large area being affected by wind reports (blue dots). This provides us an idea of how large of an area the bowing squall line encompassed and allows us to define it as a derecho. There were numerous reports of trees down countywide and plenty of wind reports of 60+ mph wind gusts over the areas the derecho pushed through.
There were reports of a few tornadoes in the region, but the Storm Prediction Center and the local National Weather Service office in Birmingham, Alabama never confirmed it. However, Dr. Tim Coleman believes a weak tornado formed near Anniston, Alabama in Winston County.
Bottom line: Derechos are powerful wind storms that typically occur during the “warm months” or during the spring and summer months. The derecho initially starts as a cluster of storms that forms a squall line. This line of storms can eventually show a bowing structure indicating strong winds on radar. Within the derecho, bowing segments are possible indicating stronger storms and more concentrated winds in those areas. On June 11, 2012, one of the first derechos of 2012 developed and pushed through the southeast and caused thousands of power outages. These storms are very powerful and create wind gusts over 100 mph. If a derecho is moving into your area, go inside and stay away from windows and find a room away from trees that could fall and crash into your house.