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Until an outbreak of tornadoes in the past week, this year had been a relatively quiet one for twisters in the Midwest and Plains states.

The reason, weather experts said, had much to do with a weather phenomenon that also caused much of the East Coast to shiver through colder-than-normal temperatures this spring: The high-altitude winds known as the jet stream brought Arctic air farther south, and for longer, than in a typical year. In the central United States, that prevented warm moist air from the Gulf of Mexico — a key ingredient in the formation of tornadoes — from moving north.

“The jet stream was stuck in place,” said Jeffrey Masters, director of meteorology for the website Weather Underground. “It kept funneling cold air down.”

The jet stream finally started shifting north this month. “The pattern broke, and then wham,” Masters said.

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Like the most destructive and deadly tornadoes, the one that hit Oklahoma on Monday came from a rotating thunderstorm. The thunderstorm developed in an area where warm moist air rose into cooler air. Winds in the area caused the storm to rotate, and that rotation promoted the development of a tornado.

To get such an uncommon storm to form is “a bit of a Goldilocks problem,” said Pennsylvania State University meteorology professor Paul Markowski. “Everything has to be just right.”

For example, there must be humidity for a tornado to form, but too much can cut the storm off. The same goes with the cold air in a downdraft: Too much can be a storm-killer.

“Everything was ready for explosive development yesterday,” said Colorado State University meteorology professor Russ Schumacher, who was in Oklahoma launching airborne devices that measured the energy, moisture and wind speeds Monday. “It all just unleashed on that one area.”

An EF5 tornado has the most violent winds on Earth, more powerful than a hurricane. The strongest winds ever measured were the 302 mph reading, measured by radar, during the EF5 tornado that struck Moore on May 3, 1999, according to Masters.

The EF Scale (for advanced Fujita Scale, named for Tetsuya Theodore Fujita, the Japanese-American meteorologist who developed it) is used to assign a tornado a ‘rating’ based on estimated wind speeds and related damage.

Even though a tornado like the one that struck Moore on Monday was 1.3 miles wide, with a path of 17 miles long, in meteorological terms it was small, hard to track, rare and even harder to study. So tornadoes are still more of a mystery than their hurricane cousins, even though tropical storms form over ocean areas where no one is, while this tornado formed only miles from the very National Weather Service office that specializes in tornadoes.

Unlike hurricanes, which forecasters can fly through in planes and monitor with buoys and weather stations, usually over a period of days, tornadoes form quickly and normally last only a matter of minutes. While meteorologists and television hosts chase tornadoes and try to get readings, it’s not usually enough. Monday’s storm lasted 40 minutes — long for a regular tornado but not too unusual for such a violent one, said Harold Brooks, a research meteorologist at the National Severe Storms Laboratory in Norman, Okla.

Still, the conditions needed to form such a violent and devastating tornado were there and forecasters knew it, warning five days in advance that something big could happen, Brooks said.

‘By Monday morning, forecasters at the National Weather Center, home of the storm lab and storm prediction center, knew “that any storm that formed in that environment had the potential to be a strong to violent tornado,” he said.

“This is a pretty classic setup,” Brooks said.

Tornadoes have two main ingredients: moist energy in the atmosphere and wind shear. Wind shear is the difference between wind at high altitudes and wind near the surface. The more moist energy and the greater the wind shear, the better the chances for tornadoes.

But just because the conditions are right doesn’t mean a violent tornado will form, and scientists still don’t know why they occur in certain spots in a storm and not others, and why at certain times and not others.

On Monday, the moist energy came up from the Gulf of Mexico, the wind shear from the jet stream plunging from Canada.

“Where they met is where the Moore storm got started,” Brooks said.

If you look at the climate history of tornadoes in May, you will see they cluster in a spot, maybe 100 miles wide, in central Oklahoma, said Adam Houston, meteorology professor at the University of Nebraska, Lincoln. That’s where the weather conditions of warm, moist air and strong wind shear needed for tornadoes combine, in just the right balance.

“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.”

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 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.

The burst of deadly weather began last week, when more than a dozen twisters touched down in North Texas late Wednesday, and one of them, in Granbury, killed six people and destroyed or badly damaged 90 percent of the homes in a single subdivision. That tornado was rated 4 on the 5-point EF Scale, with winds estimated to have been 166 to 200 mph. Then on Monday, an EF-5 twister, with winds exceeding 200 mph, hit Moore, Okla., just south of Oklahoma City, killing at least 24 people.

Before mid-May, there had been only three tornado-related deaths this year, according to the National Weather Service’s Storm Prediction Center, in Norman, Okla.

Most of 2012 was similarly calm, with only 10 deaths after March. While experts said that the number of tornadoes can vary greatly from year to year, drought conditions in the Midwest last year may have played a role.

“If you don’t have moisture around, and you don’t have the trigger for thunderstorms, you’re not going to get tornadoes either,” said Kenneth Kunkel, a professor at North Carolina State University.

By comparison, in 2011 — which is ranked as the fourth-deadliest year in history for tornadoes, with more than 550 fatalities — 363 people were killed in April alone.

Robert Trapp, an atmospheric scientist at Purdue University, said that models of climate change suggested that, as the planet warms, there should be more warm moist air that could contribute to the formation of tornadoes.

But the models also suggest that wind shear — the difference in wind speeds by altitude, which provide the force that causes air masses to start rotating and become tornadoes — is projected to decrease.

The increase in warm air may have more of an effect than the decrease in wind shear, he said, so there may be more severe weather in general. “We would project an increase in intensity of storms,” he said. “But whether that relates to tornadoes — it’s hard for us to say.”

Compiled from The New York Times, Bloomberg News and The Associated Press

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