Severe Weather Outbreak Cougar: Some Tornadoes, But Mostly A Derecho


Home / Severe Weather Outbreak Cougar: Some Tornadoes, But Mostly A Derecho
A derecho in 2008 in Illinois. Courtesy of Brittney Misialek

A derecho in 2008 in Illinois. Image courtesy of Brittney Misialek

The severity of the weather outbreak in the midwest and plains yesterday and today caught Decoded Science’s meteorologists by surprise.

On the theory that late is better than never, we have named the outbreak Cougar.

What Caused This Severe Weather Outbreak?

A sharp trough (dip) in the jet stream is responsible for this violent weather. The atmosphere is very sensitive to vertical motions. Because what goes up must come down, certain conditions can lead to violent up and downdrafts, on scales that range from a few feet to hundreds of miles.

Theoretical considerations lead to the conclusion that the area on the east side of a jet stream trough should be a place where updrafts occur. Fronts are usually found under this part of the curve.

When a front moves gently, the lifting is gradual and is normally accompanied by light to moderate precipitation. However, when the front is sharp, the lifting can be sudden and violent, with ensuing overturning of a large section of the atmosphere.

The exact nature of severe weather associated with a sharp front depends on the vertical wind structure. Frequently the lifting is scattered, and small cells, no more than ten miles in extent roll forward as thunderstorms. Think of a coffee cup without a handle spinning backwards as it moves: on the front side the air is lifted and on the back side it subsides. Heavy rain, lightning, and hail can occur, yet over a limited area.

If the vertical wind structure features rotational wind shear (a change of wind direction with height), it can turn the cell from horizontal to vertical. If the shape narrows, a tornado will form: The coffee cup has turned upright and deformed into a narrow, rapidly-spinning vortex.

Cougar reaches the east coast by Thursday. Forecast courtesy of NOAA

Cougar reaches the east coast by Thursday. Forecast courtesy of NOAA

Cougar Was Mainly A Derecho

Derecho is a Spanish word that roughly means ‘straight.’ It refers to a convective wind that blows in a straight line (rather than in a circle like a tornado).

The National Weather Service’s formal definition of derecho involves several factors: Length in the direction of motion of 240 miles; Width of 50 miles; Wind gusts of 58 miles per hour or greater recurring frequently over the entire length.

The derecho associated with Cougar began in north-central Nebraska and moved east-southeast through Nebraska and into Iowa on Tuesday and Tuesday night. It is continuing through Iowa and will be identifiable into the Ohio Valley.

The Weather Channel has had a lively discussion about whether this is a true derecho which kept its identity, or whether it kept reforming, in which case they prefer the term ‘serial derecho.’

The precise name is of little consequence. This was a long-lived, straight-line, convective storm of large extent and with winds that met all the derecho criteria. It quacked; it’s a duck.

Cougar Also Produced Tornadoes And Hail

A number of tornadoes were reported in northwest Nebraska as Cougar pounced on the state. After the initial rotations had petered out, the derecho formed and dominated the weather. Hail reportedly damaged hundreds of cars at dealerships, and the high winds, reportedly up to 100 miles per hour, tore roofs from buildings and knocked over grain silos and high-profile vehicles.

Cougar Versus Aardvark And Beaver

This year’s previous severe weather outbreaks, Aardvark and Beaver, were different animals, so to speak. They were caused by cutoff lows in the jet stream. Because they spun off on their own, they moved slowly; they were associated with more vertical rotation and so produced more tornadoes. They were also spread out over larger areas and the violent weather was scattered.

Cougar is the result of a short, sharp, open wave in the jet stream. It is moving fairly rapidly, and is characterized by extreme convection but not so much rotation: this is what spawns a derecho.

Cougar’s Future And The Next Possible Outbreak

Cougar will continue to cause bad weather as it moves east through the Ohio Valley today, reaching the mid-Atlantic coast by Thursday and then departing. There will be more violent outbursts, and the daytime heating the next two days could lead to some tornadic activity, but these systems have a limiting feedback mechanism: The clouds associated with the derecho will dampen daytime heating.

With the jet stream still unusually far south and abnormally strong, there is the possibility of a relative of Cougar showing up at any time, though none is visible at present.

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