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"Central Oklahoma is a hot spot and there’s a good reason for it," Houston said. "There’s this perfect combination where the jet stream is strong, the instability is large and the typical position for this juxtaposition climatologically is central Oklahoma."
And the timing has to be perfect. Earlier in the year, there’s not enough warm moist air, but the jet stream is stronger. Later, the jet stream is weaker but the air is moister and warmer.
The hot spot is more than just the city of Moore. Several meteorologists offer the same explanation for why that Oklahoma City suburb seemed to be hit repeatedly by violent tornadoes: Bad luck.
Of the 60 EF5 tornadoes since 1950, Oklahoma and Alabama have been struck the most, seven times each. More than half of these top-of-the-scale twisters are in just five states: Oklahoma, Alabama, Texas, Kansas, and Iowa. Less than 1 percent of all U .S. tornadoes are this violent — only about 10 a year, Brooks said.
The United States’ Great Plains is the "best place on Earth" for the formation of violent tornadoes because of geography, Markowski said. You need the low pressure systems coming down off the Rocky Mountains colliding with the warm moist unstable air coming north from the Gulf of Mexico.
Scientists know the key ingredients that go into a devastating tornado. But they are struggling to figure out why they develop in some big storms and not others. They also are still trying to determine what effects, if any, global warming has on tornadoes. The jet stream can shift to cause a record number of tornadoes — or an unusually low number of them.
Early research, much of it by Brooks, predicts that as the world warms, the moist energy — or instability — will increase, and the U.S. will have more thunderstorms. But at the same time, the needed wind shear — the difference between wind speed and direction at different altitudes — will likely decrease.
The two factors go in different directions and it’s hard to tell which will win out. Brooks and others think that eventually there may be more thunderstorms and fewer days with tornadoes, but more tornadoes on those days when twisters do strike.
"Tornadoes are perhaps the most difficult things to connect to climate change of any extreme," said NASA climate scientist Tony Del Genio. "Because we still don’t understand all the factors required to get a tornado."
The National Weather Center; http://www.nwc.ou.edu/
Seth Borenstein can be followed at http://twitter.com/borenbears
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