March 3, 2014

Researcher finds possible clue to some tornadoes’ paths

Tornado Alley weather officials say they’ll be watching supercell thunderstorms closely this year for a potential clue indicating a potential tornado’s path.

Tornado Alley weather officials say they’ll be watching supercell thunderstorms closely this year for a potential clue indicating a potential tornado’s path.

That clue lies in the speed of winds at the middle levels of the atmosphere.

If some preliminary data is confirmed by additional research, meteorologists may be able to more readily convince people near but not in a tornado’s expected path to take shelter.

Baffled by the unpredictable paths of two large tornadoes last year – near Bennington in north-central Kansas on May 28 and El Reno in central Oklahoma on May 31 – noted weather researcher Jon Davies studied their characteristics more closely. He noticed that both tornadoes had fairly weak winds between 10,000 and 30,000 feet above the surface.

“Bennington ... was really unusual,” said Davies, who presented some of his research at the national storm chaser convention in Denver last month.

The large tornado was virtually stationary for nearly an hour, moving slowly in a large loop. The tornado destroyed one house and damaged four others in the rural area, as well as killing scores of cattle and damaging farm equipment.

A storm motion algorithm that many veteran chasers use indicated the storm should move east at 12 to 15 mph.

“Bennington didn’t do that,” Davies said. “The storm didn’t move.”

Three days later, in Oklahoma, storm chasers focused on a storm that had many of the same characteristics. But the El Reno tornado changed direction, size and speeds dramatically over the course of its life – with deadly consequences. Eight people were killed, including four storm chasers.

“El Reno’s case was really weird,” Davies said.

Lessons from El Reno

Tornadoes most commonly take a southwest-to-northeast path. But El Reno drifted southeast, then straight east, then expanded and accelerated rapidly as it took a hard turn to the north – even looping back to the west at one point, catching and killing veteran chasers Tim Samaras and Carl Young and Samaras’ grown son Paul.

The tornado accelerated from about 30 mph to more than 60 mph, Davies said, which explains how a storm chase crew from the Weather Channel was caught and tossed off U.S. 81.

Like the Bennington tornado, El Reno had fairly weak midlevel winds, Davies said.

“It suggests an area to research further,” he said.

Davies has been studying other tornado-producing supercell thunderstorms. Tornadoes that took a traditional southwest-to-northeast track were spawned by thunderstorms with strong midlevel winds.

That suggests the winds pull the storm, and the tornado, along with them.

But the weaker winds appear to leave tornadoes vulnerable to other influences closer to the surface, Davies said. One deadly tornado with a bizarre path struck Jarrell, Texas, on May 27, 1997.

Like the Bennington tornado, the Jarrell storm had just enough wind flow aloft to generate a supercell thunderstorm. The storm ran into a boundary between two air masses that stretched to the south-southwest and simply followed that line, moving the opposite direction of most tornadoes. The tornado killed 27 people.

Kansas weather officials said they’ll be tracking midlevel winds with strong thunderstorms this season to see if they can correlate them with tornado paths.

“I think it would be an interesting concept to see if we could get some reading on it,” said Larry Ruthi, meteorologist-in-charge of the Dodge City branch of the National Weather Service. “It’d be interesting to see how it plays out this year. I think it’s worth looking at.”

Since forecasting models are proficient at projecting midlevel wind speeds, Ruthi said, forecasters could potentially get a bearing on whether a supercell thunderstorm’s tornadoes will be erratic.

Midlevel winds’ role

Suzanne Fortin, meteorologist-in-charge of the weather service’s Wichita branch, said she was intrigued by the role weak midlevel winds might play in affecting a tornado’s path.

A storm with a strong upper-level wind flow helps create “a very structured environment” within which a tornado moves, she said.

“When you start deviating from a very structured environment, that’s when you’re going to get these small-scale deviations” in a tornado’s path, she said.

Fortin used analogies from the Winter Olympics to illustrate her point. Tornadoes that develop from storms with strong upper-level winds are like snowboarders performing in the half-pipe.

“They stay mainly within that controlled environment,” she said.

Tornadoes that form from supercells with relatively weak midlevel winds could theoretically be like skiers on the free slope.

“They can be all over the place,” she said.

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