I see that I missed the Severe Weather Preparedness Week, which was last week. I was on vacation to South Korea and left a couple of articles to be used in my absence, and the week escaped my mind. I will be covering those topics during the summer.
In my continuing discussion on spring and summer weather and preparedness topics, I want to discuss a weather event that came through the Brookings County on May 12, 2022.
What is a derecho?
A derecho (pronounced similar to “deh-REY-cho” in English) is a widespread, long-lived windstorm associated with a band of rapidly moving showers or thunderstorms variously known as a squall line, bow echo, or quasi-linear convective system. Although a derecho can produce destruction like that of a tornado, the damage typically occurs in one direction along a relatively straight swath. As a result, the term “straight-line wind damage” is sometimes used to describe derecho damage.
How strong are derecho winds?
By definition, a derecho must include wind gusts of at least 58 mph (50 knots or 93 km/h) or greater along most of its length. While derecho winds typically are less than 100 mph, gusts as high as 130 mph have been recorded — equivalent to those with strong EF2 tornadoes.
What are the dimensions of a derecho?
A derecho wind damage swath must extend either continuously or intermittently more than 400 miles with a width of at least 60 miles. This criterion is used to eliminate more common, shorter-lived, and generally less-organized wind-producing convective systems.
What is the origin of the term “derecho?”
The word “derecho” was coined by Gustavus Hinrichs, a physics professor at the University of Iowa, in a paper published in the American Meteorological Journal in 1888. “Derecho” is a Spanish word meaning “direct” or “straight ahead;” Hinrichs coined it to distinguish straight-line wind damage from that produced by tornadoes. While the term was used in the meteorological community for a short time during the late 19th century, it disappeared from use for nearly 100 years until it was resurrected by severe weather meteorologists in the mid-1980s.
Can derechos be recognized by characteristic cloud formations or the general appearance of the sky?
The atmospheric conditions (moisture, temperature, and winds) that determine how and when clouds form and what shape they take vary markedly in derecho environments. As a result, the appearance of an impending derecho-producing storm typically varies from one event to the next, and over time during the life of a given event. Variations in the background lighting by the sun (determined by time of day, season, degree of atmospheric clarity, etc.) also affect the appearance of approaching storms. Many derechos are, however, preceded by ominous looking “shelf” clouds that form in the zone of rapidly rising air on the leading edge of the storm system’s outflow winds. In addition, derecho-producing storms tend to appear darker than most, especially when viewed from the ground during the late afternoon or early evening.
Who is most at risk from a derecho?
Those outdoors are most at risk in a derecho. Campers or hikers in forested areas are vulnerable to being injured or killed by falling trees, and people at sea risk injury or drowning from storm winds and waves that can overturn boats.
Occupants of cars and trucks are also vulnerable to falling trees and utility poles. High-profile vehicles such as semi-trailer trucks, buses, and sport utility vehicles may be blown over. At outside events such as fairs and festivals, people may be killed or injured by collapsing temporary structures and flying debris.
Even those indoors may be at risk for death or injury. Mobile homes may be overturned or destroyed, while barns and similar buildings can collapse. People inside homes, businesses, and schools are sometimes victims of falling trees and branches that crash through walls and roofs; they also may be injured by flying glass from broken windows. On occasion, severe structural damage to buildings (for example, the removal of a roof) poses danger to those within.
What can one do to prepare for a derecho?
If planning to be outdoors for a significant length of time, be aware of the weather forecast — especially if you will be well-removed from sturdy shelter. Stay “connected” via television, radio, NOAA Weather Radio, or social media. Derechos rarely occur without warning, although warning lead times may be comparatively limited during the early stages of storm development.
Because protracted and extensive electrical and communication disruptions may occur, set aside emergency water and food supplies, can openers, batteries, and flashlights. As with a hurricane, be aware that hotel and motel facilities may be impacted after the event and that gasoline availability may be limited over a broad area.
What can one do if caught out-of-doors?
Falling debris is the most serious hazard posed by a derecho to those without shelter; cloud-to-ground lightning strikes are an additional hazard. Lie flat and face down on low ground, protecting the back of your head with your arms. If possible, avoid trees; even relatively small branches can become lethal when blown by storm winds.
What can one do after a derecho?
If you suffer a power outage from a derecho, do not assume that the utility company responsible for your area is aware of the problem. Although some utilities are equipped with automated devices that relay information regarding the location and duration of outages to operational centers, many service providers are not so equipped. If possible, contact your utility company with information about the outage.
In the meantime, avoid any downed power lines and, especially, contact with lines in wet areas; the wires could still be “live.” Also watch for broken glass, upstanding nails, partially fallen tree limbs, and other hazardous debris. A sizable number of injuries and deaths related to derechos occur during clean-up operations long after the storm has passed.
Be safe out there, and always keep an eye on the sky.
Leave a Reply