Derechos and heat waves
Some of the most intense summer derechos, especially those of the progressive type (see below), occur on the fringes of major heat waves. Examples include the July 1983 “I-94” derecho in the upper Mississippi Valley, the Mid-July 1995 derechos in New York and Canada, and the more recent Ohio Valley / Mid-Atlantic derecho of June 2012. The relationship is more than statistical. It turns out that the meteorological conditions favorable for large-scale heat waves often also are conducive to derechos. In the United States, this is especially true from the Upper Mississippi Valley and Upper Great Lakes into the Ohio Valley and Northeast.
The primary link between heat waves and derechos is the presence of an elevated mixed layer, or EML. An EML is a layer of mid-tropospheric air that originates over the arid, elevated terrain. Because of their origin, EMLs exhibit sharp decreases in temperature with height. The large vertical temperature differentials (or “steep” lapse rates) in EMLs are analogous to those observed over black-topped roofs and parking lots on sunny days. Such thermal stratification encourages the formation of strong updrafts that can lead to the development of thunderstorms. In fact, the frequent presence of an EML on days otherwise favorable for thunderstorm formation to a large extent accounts for the intensity of the storms commonly encountered over the Great Plains.
During a typical heat wave over the central and eastern United States, a large, stationary upper-level high pressure area usually is present over the south-central states. Persistent westerly winds on the poleward side of the high allow EMLs generated over the Rocky Mountains to extend eastward into the Ohio Valley and Northeast, well beyond their usual range over the Plains. Warm air aloft associated with the base of the EML acts as a “cap” or “lid” that prohibits thunderstorm development along much of the extent of the EML, southward into the heat wave-associated “high.” But on the northern fringe of the EML, where low-level uplift frequently is focused along a stationary front marking the northern edge of the heat wave, updrafts that form in the strongly heated air near the ground may breach the cap, resulting in an explosive release of instability. If other conditions are favorable (e.g., low-level moisture is abundant along the front, winds are largely unidirectional, parallel to it, and increase with height), additional storms may erupt in concentrated fashion along the boundary, yielding a band of downstream-developing storms and, on occasion, a full-blown derecho.
The illustration below illustrates the idealized, large-scale meteorological environment favorable for progressive derechos on the northern fringe of a quasi-stationary high pressure cell associated with a major heat wave over central and eastern United States. The location of the EML in such situations is approximated by the brown-tinted band behind the word “Progressive,” with the mean mid-tropospheric flow depicted by the arrow marked “Airflow,” and the high pressure center by an “H.” The region most susceptible to serial-type derechos in such a pattern also is shown.
