Until an outbreak of tornadoes in the past couple of weeks, 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," Mr. Masters said.
The burst of deadly weather began earlier this month, when more than a dozen twisters touched down May 15 in North Texas, 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 Enhanced Fujita scale, with winds estimated to have been 166 to 200 mph. Then 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 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.
The Moore tornado's funnel cloud was estimated to be about 2 miles wide, which is about as large as twisters get. Powerful tornadoes are not uncommon, however -- there are about 10 EF-4 tornadoes in a typical year, said Harold Brooks, a research meteorologist at the Storm Prediction Center.
It is less common for powerful tornadoes to hit the same populated area more than once. Moore, which has about 55,000 people, was hit by another EF-5 twister in 1999, killing 44 people. But Mr. Brooks said there was nothing that makes the city, or any other location, a tornado magnet. "It was just bad luck," he said.
It is not possible to draw a connection between climate change and the frequency or intensity of tornadoes, experts said -- the year-to-year variability is too great to draw any useful conclusions. Mr. Brooks pointed out that from June 2010 to May 2011 there were more than 1,000 tornadoes, a record for a 12-month period, but from May 2012 to April 2013 there were slightly more than 200 -- a record low. "It's hard to argue the climate of the planet changed all that much over that time" to account for the difference, he said.
Purdue University atmospheric scientist Robert J. Trapp said 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."