ROBERT SIEGEL, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

Major snowstorms are set to bury the Mid-Atlantic States this weekend after record snowfalls in December. Last month, California was awash in rain and the Gulf States too have seen heavy weather lately. This is not just a run of bad luck.

As NPR's Christopher Joyce reports, much of this winter's nasty weather can be explained in two words: El Nino.

CHRISTOPHER JOYCE: Scientists knew last summer that this was going to be an El Nino year. And last December, Californians learned the hard way that it was no ordinary El Nino.

Unidentified Man: It was a wet and wild commute for drivers across California as road crews tried to keep ahead of a massive winter storm.

JOYCE: That report from the Associated Press was one of a litany on California television as back-to-back rainstorms inundated the state. The Mid-Atlantic got much the same treatment though it came down as snow.

The strong El Nino and the subsequent precipitation is a result of something that started thousands of miles out in the Pacific Ocean.

Mr. MIKE HALPERT (Deputy Director, Climate Prediction Center, National Oceanic and Atmospheric Administration): Ocean temperatures across the equatorial and tropical Pacific Ocean are somewhere upwards of two degrees Celsius above average, so we've had what we would characterize as a strong El Nino.

JOYCE: That's Mike Halpert, deputy director of the Climate Prediction Center at the National Oceanic and Atmospheric Administration.

Scientists at the center say this is the strongest El Nino since the 1997-98 winter. What happens is that unusually warm surface temperatures in the Western Pacific move east. That changes the heating pattern of the atmosphere, which in turn pulls the Pacific jet stream farther south. A jet stream is a fast and narrow current of air that travels high up in the atmosphere.

Mr. HALPERT: And that jet stream is where we see a lot of storminess typically. And then we saw a very classical case of that, a super strong jet extended all the way across the Pacific with storms impacting California one after the other.

JOYCE: A strong El Nino also alters another jet stream nearer to the equator and brings more storms to the Gulf area. In fact, a strong El Nino plays havoc with weather from Indonesia to the Atlantic, but in different ways. Indonesia gets unusually dry weather, and in fact fires in parts of Indonesia have been frequent recently. Peru, on the west coast of South America, tends to get what California gets - flooding rains.

Halpert says storms are local events and no single storm can be attributed directly to the phenomenon.

Mr. HALPERT: One way to think of it is that El Nino conditions the atmosphere for these types of storms.

JOYCE: Halpert also points out that scientists don't know yet whether climate change is influencing the frequency or strength of El Ninos.

Scientists say El Nino will likely persist another month or two. That usually means drier than usual weather in the Pacific Northwest and Ohio Valley, wetter in the west and southwest, and colder in the southeast.

El Nino has done some good - in the southwestern U.S., for example.

Mr. HALPERT: Nobody can remember seeing such a small amount of drought on the map. So this El Nino has taken a pretty big bite out of some pretty severe drought conditions that had developed last summer.

JOYCE: And by altering jet streams, El Nino apparently also helped moderate last year's hurricane season in the Atlantic.

Christopher Joyce, NPR News.

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