Where and When do Derechos Occur
The occurrence of derechos is divided into two seasons; the "warm" season which is May, June, July and August. 70% of all derechos occur during these four months. The remaining eight month comprise the "cool" season.
Progressive derechos are generally only warm season events and are found moving over the north side of upper atmosphere high pressure ridges (image below). In meteorology, we say they move "over the top" of the ridge.
As the Northern Hemisphere moves from spring to summer, the jet stream moves toward the North Pole along with the storm track. Upper level pressure patterns also move north, with high pressure moving from the tropics to the southern U.S.
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 derechosSerial derechos, while not rare in the warm season, are primarily cool season events. They occur between the upstream trough and downstream ridge (image left). Approximately 40% of all derechos are of the serial type.
The image (left) shows in the air pressure pattern at an approximate altitude of 18,000 feet (5,500 meters). The wind follows the thin black lines from west to east (clockwise around the "high").
Where serial derechos occur depends upon the actual position of the pressure patterns. The atmosphere is in constant motion, and the location of these pressure systems constantly move, albeit rather slowly.
For example, in the cool season, If the high pressure ridge is located off the east coast and the trough of low pressure is located over Central Plains then a serial derecho could possibly occur over the Southeast U.S.
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). 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.
Derechos in the United States typically occur along two axes. One axis extends along the "Corn Belt" from the upper Mississippi Valley into the Ohio Valley. The other warm season axis extends from the southern Plains into the mid Mississippi Valley.
During the cool season derechos are most likely to occur from eastern Texas into the southeastern states. 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.
Also, derechos tend to occur in families or groups. 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.Derechos likely occur in other areas of the world where meteorological conditions are favorable for their development. However, only one such event has been formally documented in recent years.
On July 10, 2002, a serial derecho occurred over eastern Germany and adjacent portions of neighboring European countries. In Berlin and surrounding areas, 8 people were killed and 39 were injured, mainly from falling trees.
In Bangladesh and adjacent parts of India, a type of storm known as a "Nor'wester" occasionally occurs in the spring.
From various descriptions and knowledge of the meteorological environments involved, it appears that some of these storms may be progressive derechos.