WASHINGTON (AP) -- Weather from nearly all parts of the country combined to brew this week's killer tornadoes.
To get tornadoes -- especially the big deadly kind -- everything has to come together in just the right way and it hadn't been doing that lately, said meteorologist Greg Carbin at the Storm Prediction Center in Norman, Okla.
Until the weekend, there had been relatively few significant twisters this year across the United States -- just 20 and no deaths.
But the conditions were right on Sunday in the central U.S.
Dry, cool air swooped off California's Sierra Madre and southern Rocky mountains. That sat on top warm, moist air from the Gulf of Mexico, creating thunderstorms. And the jet stream brought in wind shear, which helps provide rotation.
Cook that all with daytime heating and it makes a tornado outbreak, meteorologists say.
What makes this outbreak unusual is that it is essentially stalled, Carbin said. The slow-moving jet stream plunging from the Northwest is keeping a large, high-pressure system off the East Coast. And that's preventing the tornado-prone weather from moving east and weakening.
That could mean more storms in across the South, maybe into today.
In the past few decades, the U.S. has averaged about 1,250 tornadoes a year. Last year, which also had a slow start, ended with 908 tornadoes that killed 55 people.
"You expect to see one or two outbreaks like this each spring and certainly we were due," said Jeff Masters, meteorology director of the private Weather Underground.
Storm Prediction Center: http://www.spc.noaa.gov/