PORTLAND -- La Nina, the name given the cooling of the Pacific Ocean at the equator is being blamed for the cold, wet spring in Oregon. Even a slight change makes a big difference in weather patterns.

You've seen most of your annual precipitation for the year by last month, Oregon State University's Deputy Director of the Oregon Climate Service Kathie Dello said. It kinda aims the storm track right at us, Dello said. Its usually a cold and wet Northwest winter -- and its been that way, she said.

Which begs the question, are we stuck with cold, wet springs in the Northwest?

Not necessarily, Dello said. We have all sorts of natural variability going on. Springs look differently -- we had La Nina starting last year so that possibly added to it. Also the Arctic's a little warmer than normal -- and it's kind of like leaving the refrigerator door open -- and the cold air just spills down, Dello said.

She later explained that typically, extreme cold temperatures at the poles keeps the cold air in one place. Warmer temperatures allow the air to wander bringing the chill to the rest of us.

But what about the Midwest? Why are the tornadoes so extreme? Why are they so deadly this spring?

The jet stream is unusually strong this year, Dello said. And to get a tornado you need strong upper level winds to create wind sheer, she said.

Finally, is global warming to blame for La Nina, the wandering arctic air and the stronger than normal jet stream? It s a little too soon to tie anything to global warming, said Dello. You can t tie one event or one series of events in a year to global warming.

Certainly extreme events may increase as the earth warms but we can t tie these tornadoes to climate change, she said.

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