In Bangladesh and adjacent parts of India, a type of storm known as a "Nor'wester" occasionally occurs in the spring. How does a derecho develop? National Weather Service Image via National Weather Service meteorologist Samuel Shea. The image (left) shows in the air pressure pattern at an … According to NOAA, a typical derecho consists of numerous burst swaths, that is, air that flows downward from a storm and affects a large or small area. The illustration below might help you get these terms into context. These tornadoes are generally weak and are below EF-2 strength. Regardless, derechos tend to produce widespread damage, unlike tornadoes, which produce isolated damage. During the cool season derechos are most likely to occur from eastern Texas into the southeastern states. In meteorology, we say they move "over the top" of the ridge. Derechos typically form in late spring and summer. Image via Matt Mitchell. Wind damage is typically directed into one direction and can create its vast damage along a relatively straight path. Derechos usually exhibit "straight-Line" wind damage, which simply means that the damaging winds all generally occurred in the same direction (i.e., without rotation). They occur between the upstream trough and downstream ridge (image left). Fair officials knew a storm was coming, but it had not arrived yet. If you hear one is coming, consider taking shelter immediately, as falling trees and power lines may be likely. These systems typically occur across Oklahoma, Kansas, Missouri, and Arkansas. With that said, no more than three hours can elapse between successive wind damage events. These systems are also called mesoscale convective systems, or MCSs. They can produce significant damage to … They … Derechos in North America form predominantly from April to August, peaking in frequency from May into July. However, in this case, warm air ahead of the system can actually refuel and energize the thunderstorm complex. 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. Bottom line: Derechos are very powerful, destructive wind storms that typically occur during the warm late spring and summer months. Derechos occur mainly across the central and eastern U.S., where many locations are affected one to two times per year on average. Photo taken by Doug Berry of the very impressive shelf cloud on the leading edge of the "People Chaser" Derecho of May 27, 2001 near Fort Supply, Oklahoma. They occur between the upstream trough and downstream ridge (image left). When the derecho tore through Iowa on Aug. 10, ... winds greater than 74 mph are likely to occur within 25 miles of any given point an average of once or more per year. The downburst cluster family, in descending order of the size of affected area, consists of derechos, downburst cluster, downburst, microburst. The derecho initially starts as a cluster of storms that forms a squall line. Also consider that outflow from strong thunderstorms may send the storm’s strongest winds before the thunderstorm – with its heavy rain, lightning or hail – actually hits you. However, only one such event has been formally documented in recent years. Please Contact Us. National Oceanic and Atmospheric Administration. In the mid-summer, derechos occur further north in north western Ontario as well as Manitoba. The winds head eastwards towards the equator where there is a warm sector. A self-described "big weather and music geek," Matt has a passion for helping to keep people safe when severe weather strikes and says if you don't have a NOAA Weather Radio ... you should get one. The other warm season axis extends from the southern Plains into the mid-Mississippi Valley. Read more from NOAA about how derechos develop. From various descriptions and knowledge of the meteorological environments involved, it appears that some of these storms may be progressive derechos. Bottom line: Derechos are very powerful, destructive wind storms that typically occur during the warm late spring and summer months. A musical event was allowed to go on. This line of storms can eventually show a bowing structure, indicating stronger storms and more concentrated winds in those areas. Derechos are often referred to as inland hurricanes due to the hurricanelike conditions, in terms of ferocious wind and torrential rain, which are spawned by this weather phenomenon. Have you ever felt a cool wind prior to a thunderstorm approaching your area? The stage roof structure collapsing toward spectators at the Indiana State Fair. In weather systems that have this shape, winds become stronger ahead of the system and can affect a large area with wind damage. Some derechos can form small, quick spin-up tornadoes – sometimes called gustnadoes – that only last a minute or two. As the Northern Hemisphere moves from spring to summer, the jet stream moves toward the North Pole along with the storm track. Derechos mostly occur in the summer, especially in June and July, both in the day or night. 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 (97 kph). Interior Highlands most commonly from Oklahoma and across the Ohio Valley. It’s definitely difficult to determine exactly where storms will form a day or two in advance. Approximately 40% of all derechos are of the serial type. Even if you’re not a weather buff, chances are you’ve heard the terms squall line or possibly bow echo. Illustration modified from one by Dennis Cain via NOAA. As the system continues to organize, the squall line can bow out and eventually become a derecho. What is a derecho? They generally affect states east of the Rocky Mountains. Only 15 minutes after that decision – before the evacuation was announced – a wind gust (possibly a gustnado) hit the stage structure, causing it to collapse. Derechos occur mainly across the central and eastern U.S., where many locations are affected one to two times per year on average. Forecasting the formation of squall lines – and derechos – can be tricky and difficult. The other 30% occur during the cool season. 5) Derechos are typically continuous and can sustain themselves for hours. 4) In the storm reported areas, at least three reports, separated by 39.8 miles (64 km) or more, should include wind gusts greater than 64 knots, or 74 mph (119 kph). Image via NOAA. Image via NOAA. Derechos occur mainly across the central and eastern United States, where many locations are affected one to two times per year on average. Another shot of the leading edge of the June 11, 2012 derecho, North Jefferson County, Alabama. Derechos are often referred to as inland hurricanes due to the hurricanelike conditions, in terms of ferocious wind and torrential rain, which are spawned by this weather phenomenon. Serial derechos, while not rare in the warm season, are primarily cool season events. In some systems, the downdrafts in the storms can weaken the systems as cooler air pushes towards the surface and stabilizes the atmosphere. 3) Wind reports should show a continuous and nonrandom pattern of occurrence. Upper level pressure patterns also move north, with high pressure moving from the tropics to the southern U.S. This was the leading edge of the derecho of June 11, 2012, as it pushed into this area. The image (left) shows in the air pressure pattern at an approximate altitude of 18,000 feet (5,500 meters). 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. Also, derechos tend to occur in families or groups. Derechos might include microbursts, downbursts and downburst clusters. Wind reports of 60+ mph (100+ km/h) with gusts to 90 mph (145 km/h) felled thousands of trees and caused widespread power outages in addition to 68 tornadoes. The type of derecho that occurs is a good indicator of the weather pattern and also is partly dependent on the time of the year. Typically, a cluster of storms develops during the late spring and summer time period. The wind follows the thin black lines from west to east (clockwise around the "high"). It’s a violent storm system that can produce widespread wind damage, usually associated with a rapidly moving band of showers and thunderstorms. Seven people were killed, and 58 others were injured. Questions? During this time of year, derechos are mostly found in the Midwestern United States and the U.S. 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. That’s what happened on August 13, 2011, during the Indiana State Fair Stage collapse. Michigan, Minnesota, and North Dakota also experience derecho storms during this period. Progressive derechos are generally only warm season events and are found moving over the north side of upper atmosphere high pressure ridges (image below). Derechos likely occur in other areas of the world where meteorological conditions are favorable for their development. Simon Toogood in Tasmania – an island south of the Australian mainland – told us that stormy weather and shelf clouds like these are common in his part of the world, too. Anyone living east of the U.S. Rocky Mountains can see derecho events, particularly in the spring and summer months. The downdraft winds can create a cool pool along the surface. In vehicles, some fatalities occur when 18 wheelers are blown onto their sides, trees are blown onto automobiles, and when vehicles are driven into trees. With this is mind, derechos are serious storms. Derechos in the United States typically occur along two axes. Approximately 40% of all derechos are of the serial type. Evolution of a bow echo/derecho. Derechos happen when the right conditions for downbursts occur over a wide area. In other words, a rain-cooled downdraft (cool winds, moving down) occurs in the thunderstorm and hits the Earth’s surface, spreading horizontally and pushing outward. They are violent storm systems that can produce widespread wind damage across a large area. This means that when the first one occurs, there is a good chance that more derechos will occur within the following few days, although not necessarily in the same location. The occurrence of derechos is divided into two seasons; the "warm" season which is May, June, July and August. As the performance progressed, officials did decide to cancel the show and were putting together an evacuation plan for those in the audience. Some of the regions which experience derechos during the summer are the Midwestern US in the Ohio Valley and Oklahoma. The strong-to-violent winds typically move ahead of the main system, as the outflow from the storms becomes more concentrated. Derechos in the U.S. generally occur through out the southern Plains, the Mississippi Valley, and the Ohio Valley. Check out the images in this post. It has a very specific definition in meteorology. Areas most likely to experience derechos across the United States. Derechos are associated with bow echoes, or bow-shaped bands of severe thunderstorms. In this post, we’ll look at how a derecho is formed, and where they occur. The other parts of the world which experience derecho winds include Bangladesh and Brazil, especially the Amazon Basin among other places. In this position, the ridge of high pressure is over the northern plains and upper mid-west region of the country, and it is this location that experiences the majority of derechos each year. Serial derechos, while not rare in the warm season, are primarily cool season events. US Dept of Commerce As more storms develop, it can help strengthen and elongate the cold pool at the surface. Where serial derechos occur depends upon the actual position of the pressure patterns. Derechos start with thunderstorms whereby frontal zones outrun cold fronts. 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