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La Nina Portends Warm, Dry Times Ahead for S. Arizona

December 4, 2010

By Tony Davis, The Arizona Daily Star, Tucson

Dec. 04--Southern Arizona streams and washes that brimmed with water last winter probably will look a lot more parched this winter.

The desert Southwest is heading for what could be one of the strongest La Nina winters in the past 60 years, University of Arizona researchers reported Friday.

To the uninitiated, that means the drought isn`t over here.

Temperatures are also likely to be warmer than normal.

But so far, despite our dryness, the Upper Colorado River Basin -- where much of Tucson`s drinking water now comes from -- is so far having a good snowpack season.

Here are some questions and answers about the winter weather lying ahead for Southern Arizona:

What is La Nina?

La Nina conditions have unusually cold ocean temperatures in the equatorial Pacific Ocean, compared to El Ninos, which have unusually warm ocean temperatures, says the National Oceanic and Atmospheric Association. Last winter was an El Nino that brought Tucson far more winter rainfall than normal. La Ninas can last as long as two years, which some NOAA researchers say could happen this time around.

What does that mean for us?

Typically, a La Nina brings unusually dry weather to the Southwest and unusually wet weather to the Northwest. Based on past La Nina events, a dry Southwestern winter is likely this year, particularly in southern portions of Arizona and New Mexico, researchers at UA`s Institute for the Environment said Friday.

In fact, dry conditions are expected to stick around long after the winter ends, maybe as late as May, with the highest chances for dry conditions predicted for the February-April period.

What about temperatures?

While La Ninas don`t always mean warmer weather, this one likely will bring warmer-than-average temperatures to the Southwest, the new report said. The researchers didn`t predict how much warmer.

What will drier weather mean?

Desert grasslands, trees and shrubs grow more vulnerable to wildfires in late spring and early summer. Ranchers and farmers may have trouble getting water for cattle and crops. Phoenix-area reservoirs will run low if La Nina brings less spring runoff to the Salt and Verde rivers.

What about our Central Arizona Project drinking water supply from the Colorado River?

While Lake Mead at the Arizona-Nevada border remains extremely low, the precipitation and snowpack in many areas of the Upper Colorado River Basin states of Colorado, Wyoming and Utah where CAP water ultimately comes from are running above normal so far.

But it`s too early to know for sure how that will affect the river`s spring runoff, said Zack Guido, a scientist for the UA institute. Except for northwest Arizona, scant snow has fallen in the state so far, however.

What about the long-term forecast?

Dry winters can set the stage for longer-term drought conditions, Guido said. A dry, La Nina-based, 1998-99 winter essentially kicked off the drought we are still in, he said.

Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746.

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Copyright (c) 2010, The Arizona Daily Star, Tucson

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