Derechos: The Facts

Posted: June 9, 2016, 11:55 am by mmurphy

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

 


(Above) Map showing where relative maxima & minima number of derechos occur. Derechos are most common over the South-Central US, with more than 75% of all derechos nationwide occurring between the months of April--August. Courtesy of (SPC derecho facts)

 

 Example 1: Labor Day 1998 Derecho

 

(Above) Radar image of the Labor Day 1998 derecho bearing down on the Syracuse, NY region. Credit: SPC Case Study

 Facts/Stats: 

- There were two separate derechos on this day, the first tracked from near Buffalo to Albany, NY and finally moved into the Boston, MA area
- The second derecho formed over southern Michigan and tracked along a line from Cleveland-Pittsburgh-Harrisburg-Allentown-NYC and ended over eastern Long Island
- Peak wind gusts of 89 mph and 77 mph were reported at Rochester and Syracuse, NY respectively. Maximum gusts along its path were estimated up to 115 mph
- All told, about 80 people were injured and 7 were killed, mainly from fallen trees and overturned boats
- At least 130 houses/buildings were damaged and 300,000 people lost power during the storm (More info here)

 Example 2: June 2012 Derecho


(Above) Hourly radar images along with ASOS wind reports from the June 29th, 2012 derecho. Credit: SPC Case Study

 Facts/Stats:

- Tracked from Eastern Iowa to the Mid-Atlantic coast (NC, VA, MD, DE & S. NJ) which is more than 700 miles
- Wind gust reports: Fort Wayne, IN 91 mph... Atlantic City, NJ 87 mph...Dayton & Columbus, OH 82 mph...Roanoke, VA 81 mph... Dulles, VA 71 mph
- Numerous houses and buildings were damaged along the storm's path
- Nearly two dozen fatalities were reported, mainly from fallen trees. 4 million customers lost power, some for up to a week
- More information on this devastating derecho can be found here: NWS Service Assessment & SPC Case Study

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