Last year, Missouri logged 101 tornadoes. The annual average between 1961 and 1990 was 27, he said.

On Monday, Sandtorf installed the Striegels’ storm shelter after installing one near Columbus, Kan. Both were for residences.

“It generally takes about 2 1/2 hours to dig a hole, put the shelter in the hole and clean up,” he said. “A unit typically starts at $2,350. It depends on the job after that. It costs more if we hit rock while digging, if we have to go far to put one in or if it’s in a tight spot that might add more labor.”

La Niña

When compared with last year, 2009 is off to a slow start when it comes to tornadoes. But, the ingredients are there for another stormy spring.

New research has shown a connection between the occurrence of a La Niña in the Pacific Ocean and volatile weather in the Midwest. La Niña, a periodic cooling of waters in the Pacific Ocean, is the flip side of the better-known El Niño phenomenon.

La Niña shifts the polar jet stream into a position in which storms from the west collide with moisture from the Gulf of Mexico. When that happens, the jet stream — a shifting river of air at high altitudes — brings an abundance of warm, moist air from the Gulf of Mexico into the Midwest. That, coupled with high winds and a storm system with cooler air from the west, provides the primary ingredients for violent weather.

The research found that tornadoes during a La Niña are stronger and track greater distances.

Greg Carbin, warning coordination meteorologist with the Storm Prediction Center, said the La Niña-El Niño cycle was in a neutral phase until January, when a new La Niña started to show its influence. Last year’s La Niña was the strongest in a decade.

“We have a La Niña that’s ongoing right now, but it’s not quite as pronounced as last year,” Carbin said. “It is expected to weaken this spring. That connection between La Niña and severe weather in the Plains is tenuous. But it was certainly the case last year. It was a busy year all around.”

Unlike last year, the country so far this year has experienced relatively quiet conditions for severe weather.

“Thunderstorm activity is expected this spring,” Carbin said. “It gets more active regardless of whether there’s a La Niña in place. Another ingredient that is a factor, and it’s closer to home, is the sea surface temperature in the Gulf of Mexico.

“When that temperature is above normal, it helps the return of moisture to the Plains states. It provides the fuel for thunderstorm activity. Those temperatures are now slightly above the long-term normal.”