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CHICAGO (Reuters) - La Nina, a weather pattern characterized by colder ocean temperatures in the eastern equatorial Pacific, may be playing a part in the high number of U.S. tornadoes this spring, according to an AccuWeather meteorologist.
"La Nina typically has a more active southern jet stream. This spring that has played a role in the severe weather," said Mark Paquette, meteorologist for AccuWeather.com.
Another factor may be warmer temperatures in the Gulf of Mexico, which helped contribute to a warm and muggy air mass in the south, Paquette said.
But meteorologists said it was impossible to determine if climate change is responsible for the surge in natural disasters.
Weather experts agree that the deadly nature of this year's tornadoes is mostly due to bad luck and population sprawl -- as some tornadoes have hit densely populated areas in Missouri and Minnesota over the weekend and Alabama in April.
"We have people where there used to be farmland," said Paquette.
This year has seen an unusually high number of tornadoes, with 1,168 as of May 22, compared to an average of about 671 by this time, according to Joshua Wurman, president of the Center for Severe Weather Research in Boulder, Colo.
This year's tornado season has been exceptionally deadly -- the most recent example being the tornado that hit Joplin, Missouri Sunday, killing at least 116 people.
The U.S. is on pace to break the record for deaths from tornadoes this season, the National Weather Service said on Monday.
Tornadoes typically form in the spring months as a result of cool air clashing with warm, humid weather. The conditions this spring have been "very favorable" for tornado formation, noted Wurman. He said that these conditions occur in some years with La Nina, but these conditions also can occur without La Nina.
Wurman said scientists are leery of drawing connections between tornadoes and long-term climate change, for a few reasons. One reason is that if something is attributable to a long-term change in climate, it would have to happen repeatedly. Last year was not a high year for tornadoes.
Scientists also do not have a good feel theoretically for what climate change would likely do to the frequency and intensity of tornadoes, Wurman said. While nearly all scientists agree climate change is occurring and globally average temperatures will probably go up, they do not know what that means for tornadoes.
"It could be climate change might cause more tornadoes, or less tornadoes, or there might be no change," Wurman said.
The tornadoes that hit the south in April were exceptional in their number, according to weather experts. What was unusual about Sunday's Missouri tornado was that it made a direct hit on a small city.
"It's bad luck," said Paquette. "Sometimes you have tornadoes that hit in the cornfields of Kansas or Nebraska or Iowa and the only person affected is that farmer and it doesn't even hit his house. But here we have a tornado that hit a hospital."
The expanding population of the United States, with accompanying suburban sprawl, has created more areas for tornadoes to cause serious damage.
Wurman noted that the tornado could have been worse if it hit an even more populated urban area, like the Chicago suburbs. "A tornado doesn't really care what's underneath it," said Wurman.
Wurman said that while it is easier for tornadoes to cause expensive and deadly damage because of sprawl, warnings also are better than they used to be. Thirty years ago, people only got an average of 3 minutes of warning before a tornado hit, now the average is 13 minutes.
"We'd like to get that up to 30 or 40 minutes," Wurman said.
He said he also would like to get the false alarm rate down because 70-75 percent of tornado warnings are false alarms, so people do not always seek shelter in time.
Writing and reporting by Mary Wisniewski; Editing by Greg McCune