All the weather we experience at ground level is governed by the weather pattern in the atmosphere, and derechos are no different. The type of derecho that occurs is a good indicator of the weather pattern and is partly dependent on the time of the year.
The occurrence of derechos is divided into two seasons: the "warm" season – May, June, July, and August – during which 70% of all derechos occur, and the "cool" season, comprising the remaining eight months.
Progressive derechos
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, they are said to 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 derechos
Serial derechos, while not rare in the warm season, are primarily cool season events. They occur between the upstream trough and downstream ridge. Approximately 40% of all derechos are of the serial type.
The image below 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 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.
Derechos tend to occur in families or groups. This means that once 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.
While relatively uncommon, derechos also occur outside North America. Only a few, however, have been formally documented, and all of those after 2000. In Europe, a serial derecho occurred on July 10, 2002 over eastern Germany and adjacent portions of neighboring countries. The Berlin area was hardest-hit, with eight people killed and 39 injured, mainly from falling trees. Derechos in Finland, Spain and France, and Bulgaria also have been described. The derecho in Finland was the first to have been documented poleward of 60 degrees latitude.
In Bangladesh and adjacent parts of India, a type of storm known as a "Nor'wester" occasionally occurs in the spring, bringing high wind and very heavy rain. From various descriptions and knowledge of the meteorological environments involved, it appears that some of these storms may be progressive derechos.
