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Tornado risk hard to predict

BY CINDY GARCIA | MAY 14, 2015 5:00 AM

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With two tornadoes hitting the state this month and summer fast approaching, the risk is unclear for Iowans.

Tornado-risk prediction is inherently difficult, especially months or weeks in advance, officials say, because conditions that lead to tornados can develop in just one day.

Greg Carbin, the warning coordination meteorologist at the National Oceanic and Atmospheric Administration Storm Prediction Center, said that tornadoes, especially significant ones, are the result of climatic “ingredients” coming together.

“So there’s moisture, there’s instability, there’s something to light the air to produce a thunderstorm, and then ultimately, the wind shear produces persistent thunderstorm updrafts that go on and produce possible tornadoes,” he said.

He said to get these “ingredients” together, a strong storm system must be present.

“If you’re in the right position of the cyclone, the low-pressure system, the funnel system, and thunderstorms develop coincident with that low-pressure system, there’s a good chance that some of those thunderstorms could contain a tornado or even large hail and damaging winds,” he said.

The preliminary tool used to gauge the chances for tornadoes is whether the trend of weather indicates it is an El Niño or La Niña year.

This is an El Niño year.

“Generally speaking, this has lots of exceptions, but about Iowa southward, there are 10 and a half fewer tornadoes in El Niño years than what the long-term average would be and South Dakota, North Dakota, northward has a bit of the opposite tendency, so perhaps better allows for tornadoes in El Niño years,” said Harry Hillaker, the Iowa state climatologist.

El Niño is a large-scale atmospheric and oceanic phenomenon in which equatorial Pacific waters are warmer than normal.

“What it does is it increases the potential for strong and deep thunderstorms to occur over the equatorial Pacific, which then have an impact on the jet stream, which then has an impact downstream over the North American continent,” Carbin said.

However, experts acknowledge that an El Niño year is not a foolproof way to predict whether there will be more or fewer tornadoes in Iowa.

Carbin said research done on severe weather across Iowa and Nebraska has yielded some connections with El Niño, but correlations are not strong.

“I can tell you the linkages are relatively weak,” he said. “There’s more to it than just what El Niño is doing.”

Another meteorologist echoed this.

“Well, it’s a weak El Niño, so really it usually doesn’t affect something like that,” said John Haase, a meteorologist with the National Weather Service. “It wouldn’t affect it as much. It’d be more of a typical tornado season.”

Hillaker indicated that the long-term average of tornadoes in Iowa is 46 per year. Four tornadoes have hit Iowa this year, making it a relatively quiet season so far. However, June is usually the busiest in regard to tornadoes.

Even though the conditions that can create a tornado can be on short notice, possible damage inflicted on civilians and property is still dependent on how severe a tornado is.

Weak tornadoes, Haase said, may develop quickly so there may be no warning or a short lead-time before it hits. However, weak tornadoes inflict minor property damage and usually don’t cause fatalities.

“The stronger tornadoes are usually the ones the radar would be able to pick up on and see the strong rotations, so you’d be able to have a longer lead time, which is an average of about 11 minutes,” said Haase.

Officials are wary of predicting tornado risk.

“With tornadoes, there’s a lot of variability. A quiet year in Iowa is to have 15 to 28 tornadoes. A really active year, around 100 tornadoes,” Hillaker said. “So a huge amount of variability from year to year, and so it makes it hard to generalize, when you have that much of a range of possibilities.”


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