Every few years, a new weather term enters the lexicon of the general public, having already traversed from meteorology academics to forecasting professionals to amateur weather geeks.
It seems now is the time for the popularization of "derecho," galvanized in the minds of millions from the Ohio Valley to the East Coast, including Southwest Virginians, who watched trees break, transformers blow and homes turn dark in the throes of a mighty roaring wind Friday night.
While "derecho" trended on Twitter on Friday night, readers of this column were introduced to the term eight years ago, and twice since.
So, what is a derecho? Quoting my June 11, 2004, Weather Journal column: "A long-lived bow-echo MCS that produces damaging straight-line winds over hundreds of miles of terrain is sometimes referred to as a derecho (pronounced 'day-ray-cho'). Derechos frequently produce widespread damage to trees, power lines and some structures."
"MCS" refers to a "mesoscale convective system," which is a fancy way of saying a "storm cluster."
A "bow echo" is a type of storm cluster with an appearance on radar just as it sounds, in a bow shape. When strong winds push the middle of a line of storms out farther than the ends, a bow shape forms.
Only a bow echo that travels hundreds of miles, producing constant outflow winds that cause damage over nearly the entire length of its path, earns the moniker "derecho."
Friday night's storm system produced constant wind damage indicative of winds qualifying as "severe" (58 mph) from the Chicago suburbs to Washington, D.C., as far north as southern Pennsylvania and as far south as northern North Carolina. Numerous wind gauges along the path recorded winds in excess of 70 mph, including the 81 mph gust clocked at Roanoke Regional Airport.
The storms developed along the edge between the extreme "heat dome" to the south and cooler air to the north. They quickly formed a bow and rocketed southeastward, pushed by northwest winds aloft, as well as by a cold pool of air created by the storm's own downdrafts.
Friday night's derecho had some peculiar characteristics as it approached the Roanoke and New River valleys, at least compared with the storms we usually see.
There was little or no rain with the storms, mostly just wind. One reason for that is that the outflow of the winds of a derecho often outrun the storm cells that produce them by 10 or more miles, so the winds hit long before any rain would fall.
In Friday night's case, the storm cells closest to Roanoke were actually weakening as they approached the city. As strange as it may sound, this may have added to the winds, as the cumulonimbus clouds making up the storms collapsed, enhancing storm downdrafts. But it also meant that by the time the cells reached the city, there was not much rain falling out of the clouds, and what little did fall dried up before reaching the surface.
The outflow winds sometimes act much like a cold front, firing new storm cells upon clashing with warmer air. This indeed happened, as storms re-intensified just east of Roanoke, continuing eastward through central Virginia.
The term "derecho" is not a recent invention, nor is the phenomenon a recent discovery. It was known about and named, at least in academic circles, more than 70 years before the more well-known term "supercell."
Quoting my May 9, 2009, Weather Journal column: "According to the Storm Prediction Center, the word 'derecho' was applied in 1888 by University of Iowa physicist Gustavus Hinrichs to distinguish a long-lived straight-line windstorm from the rotating windstorm known by another Spanish-origin term, tornado."
Tornadoes sometimes occur within a derecho, but usually these embedded circulations are not all that remarkable within the widespread field of damage produced by straight-line winds of 60 to 120 miles per hour. Derechos rarely produce the long-lasting or violent tornadoes that occasionally spring from supercells.
Friday night's events show once again that it doesn't take a tornado to deliver misery for millions of people.
A powerful series of storms blew through several eastern states late Friday and early Saturday morning, killing at least nine people and throwing at least 3 million people into the dark.
This unusually damaging system is called a derecho, notes AccuWeather (it's pronounced deh-RAY-cho). According to the National Weather Service, a derecho is a gigantic wind storm coupled with thunderstorms. These are as powerful as tornadoes, but they don't twist; they drive in a straight line. They're described as land hurricanes because they have wind gusts of at least 58 miles per hour and higher.
Meteorologists with the Washington Post say derechos get their power from hot, humid weather. That's exactly what was going on when the storms blossomed above regions setting heat records or getting awfully close to them.
It all started late Friday morning outside Chicago as a cluster of thunderstorms, says the Weather Channel. The bad weather plowed south and east, hitting Indiana, Ohio, Kentucky, West Virginia, Virginia, Maryland, the District of Columbia and New Jersey.
Increasingly violent storms downed power lines, yanked roofs off houses and toppled trees that crushed people beneath them. Several states declared states of emergency and President Obama signed a disaster declaration for West Virginia.
Yesterday's storms didn't cool temperatures — in fact, the Weather Service expects record high temperatures today from the Mississippi River valley up through the Mid-Atlantic states. The biggest threat is from another round of damaging winds that could pass through the same states that got hit yesterday...Click npr.org for more [LINKS]