Derechos: The Facts

You've probably heard the term "derecho" before, and perhaps wondered what it was, how they form, and what type of damage they can cause. A derecho, by definition is: a widespread, long-lived, and violent convectively induced 'straight-line' windstorm. It is a type of mesoscale convective system (MCS), and is associated with a fast moving band of severe thunderstorms that usually takes the shape of a bow echo (see the last image). To be classified as a derecho, a MCS must have a swath of damaging winds (greater than 58 mph) for at least a path of 240 miles, with sporadic gusts to 75 mph or higher also observed.

Favorable Conditions for Development and Maintenance of Derechos:

1) Enough instability (CAPE) in the atmosphere for multiple thunderstorms to form
2) Atmospheric winds that increase significantly with height and little to no change in wind direction with height (uni-directional)
3) An Elevated-Mixed-Layer (EML) which creates an area of rapidly decreasing temperature with height in the atmosphere
4) A Stationary front along which storms can form, and track (this is for progressive derechos)
5) Abundant low level moisture

Formation:

Derechos form when all the above atmospheric parameters come together to initialize multiple thunderstorm formation. One of the main ingredients is strong, uni-directional winds throughout the lower atmosphere. These strong winds help to develop an intense gust front ahead of the "pool" of rain cooled air (see cross section below). As the pool of rain cooled air continues to expand it allows the gust front to maintain its strength. The cold pool of air then elongates and a rear inflow jet develops. This causes the updraft to tilt toward the rear of the storm, permitting the storm to expand and conserve its strength over a longer distance. As new storm cells continue to form along the leading gust front the process repeats itself, and the entire Mesoscale Convective System (derecho) can track unimpeded for hundreds of miles causing extensive damage.

(Above Left) Typical ideal setup for derecho development. Many of the derechos that eventually affect the East Coast are progressive derechos that 'ride' the ridge and are maintained by the Elevated Mixed Layer; the area north of the upper level ridge is therefore often referred to as "the ring of fire". (Above right) Conceptual depiction of a mature derecho, showing the rain cooled air, gust front, updraft and typical radar reflectivity pattern

Typical Damage and Climatology:

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). 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. Derechos must be respected, as many people have been injured or even killed as a direct result of their damage. Derecho damage is usually easily distinguishable from tornadic damage, as most trees and other debris will fall in the same direction when they succumb to the powerful wind speeds. This is the opposite of what occurs in a tornado, when debris is typically "twisted", shredded and spread in all directions.


(Above Left) Derecho, straight-line wind damage pushes over trees all in the same direction, June 27th 1998. Credit: Todd Shea WCM, US National Weather Service La Crosse Wisconsin. (Above Right) Tornadic damage to trees and a shed, June 11th 2008. Credit: US National Weather Service Sioux Falls

 

 

 Example 1: Labor Day 1998 Derecho