These clusters of storms can eventually evolve into a single strong storm. What is a derecho? 3) Wind reports should show a continuous and nonrandom pattern of occurrence. The image (left) shows in the air pressure pattern at an approximate altitude of 18,000 feet (5,500 meters). Regardless, derechos tend to produce widespread damage, unlike tornadoes, which produce isolated damage. The winds in the troposphere, or the layer of the atmosphere where our weather occurs, typically become relatively strong and unidirectional. However, in this case, warm air ahead of the system can actually refuel and energize the thunderstorm complex. National Oceanic and Atmospheric Administration. That’s what happened on August 13, 2011, during the Indiana State Fair Stage collapse. National Weather Service During the cool season derechos are most likely to occur from eastern Texas into the southeastern states. Have you ever felt a cool wind prior to a thunderstorm approaching your area? During this time of year, derechos are mostly found in the Midwestern United States and the U.S. The downdraft winds can create a cool pool along the surface. Derechos occur mainly across the central and eastern United States, where many locations are affected one to two times per year on average. However, only one such event has been formally documented in recent years. In this post, we’ll look at how a derecho is formed, and where they occur. In Bangladesh and adjacent parts of India, a type of storm known as a "Nor'wester" occasionally occurs in the spring. In fact, a derecho’s path of damage is at least 240 miles long. Although derechos are extremely rare in that portion of the United States west of the Great Plains, isolated derecho events have occurred in the interior portions of the western United States, especially during spring and early summer. These windstorms tend to form in the late spring and summer. As a result, the winds form mesoscale convective system (MCS) in the upper divergence level of the pattern of the wind. They can produce significant damage to … Read more from NOAA about how derechos develop. The other 30% occur during the cool season. 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. Derechos in North America form predominantly from April to August, peaking in frequency from May into July. The wind follows the thin black lines from west to east (clockwise around the "high"). Derechos are associated with bow echoes, or bow-shaped bands of severe thunderstorms. 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. Monster sunspot AR2786 swings into better view. Where derechos are likely to occur NOAA SPC website/Dennis Cain NOAA defines a derecho as, “(pronounced similar to "deh-REY-cho" in English, or pronounced phonetically as "") a … 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. Of course, for it to be classified as a derecho, it has to meet the above-mentioned criteria. They generally affect states east of the Rocky Mountains. Serial derechos, while not rare in the warm season, are primarily cool season events. These tornadoes are generally weak and are below EF-2 strength. In Berlin and surrounding areas, 8 people were killed and 39 were injured, mainly from falling trees. All the weather we experience at ground level is governed by the weather pattern in the atmosphere, and derechos are no different. 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. Questions? The strong-to-violent winds typically move ahead of the main system, as the outflow from the storms becomes more concentrated. Derechos happen when the right conditions for downbursts occur over a wide area. Derechos in the United States typically occur along two axes. With that said, no more than three hours can elapse between successive wind damage events. 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). Fair officials knew a storm was coming, but it had not arrived yet. In weather systems that have this shape, winds become stronger ahead of the system and can affect a large area with wind damage. With this is mind, derechos are serious storms. Anyone living east of the U.S. Rocky Mountains can see derecho events, particularly in the spring and summer months. As more storms develop, it can help strengthen and elongate the cold pool at the surface. They … Nearly half of fatalities attributed to derechos occur either in vehicles or boats. In meteorology, we say they move "over the top" of the ridge. Shelf cloud bringing rain on Ormond Beach, Florida, on June 1, 2018, via Rita Addison. It’s definitely difficult to determine exactly where storms will form a day or two in advance. Derechos occur mainly across the central and eastern U.S., where many locations are affected one to two times per year on average. 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). The occurrence of derechos is divided into two seasons; the "warm" season which is May, June, July and August. Image via NOAA. People can also experience derechos across the U.S. Southeast, the Great Lakes region, and the Ohio River Valley. Derechos mostly occur in the summer, especially in June and July, both in the day or night. Image and caption via Wikimedia Commons. Derechos typically form in late spring and summer. Derechos might include microbursts, downbursts and downburst clusters. This was the leading edge of the derecho of June 11, 2012, as it pushed into this area. Derechos can consist of bow echos (which are shaped like an archer’s bow) or a squall line that tends to bow. In boats, the vessels overturned due to the high winds and the occupants drowned. 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. Approximately 40% of all derechos are of the serial type. This is precisely what occurred on April 4-5, 2011 when a serial derecho produced over 1,200 high wind reports (blue dots on image right). Derechos start with thunderstorms whereby frontal zones outrun cold fronts. Wind damage is typically directed into one direction and can create its vast damage along a relatively straight path. Image via National Weather Service meteorologist Samuel Shea. Image via Mike Wilhelm. Although winds do not rotate during derechos such as in tornadoes, and do not reach the incredible intensity that some violent tornadoes do, they can still cause extensive damage. A musical event was allowed to go on. These systems are also called mesoscale convective systems, or MCSs. Also, derechos tend to occur in families or groups. Check out the images in this post. A shelf cloud in 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. Well-defined and photogenic shelf clouds like these tend to occur with derechos. Image via NOAA. 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 atmosphere is in constant motion, and the location of these pressure systems constantly move, albeit rather slowly. Evolution of a bow echo/derecho. 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. In the mid-summer, derechos occur further north in north western Ontario as well as Manitoba. The stage roof structure collapsing toward spectators at the Indiana State Fair. As the system continues to organize, the squall line can bow out and eventually become a derecho. In other parts of the world – for example, parts of Europe and India – derechos are fairly rare events. The winds head eastwards towards the equator where there is a warm sector. As the performance progressed, officials did decide to cancel the show and were putting together an evacuation plan for those in the audience. They can produce significant damage to … Formation of Derecho Storms. JetStream, Comments? Progressive derechos are generally only warm season events and are found moving over the north side of upper atmosphere high pressure ridges (image below). If you hear one is coming, consider taking shelter immediately, as falling trees and power lines may be likely. It’s a violent storm system that can produce widespread wind damage, usually associated with a rapidly moving band of showers and thunderstorms. 2) The area must have a major axis length of 248.5 miles (400 km). One axis extends along the "Corn Belt" from the upper Mississippi Valley into the Ohio Valley.
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