Ongoing La Niña conditions in the central tropical Pacific will bring warmer and drier conditions to the Southwest from late fall to early spring. This makes the probability of less-than-normal snowfall at higher elevations likely, according to scientists at the National Oceanic and Atmospheric Administration's Climate Prediction Center. La Niña conditions developed in May-June 1998, and have persisted since that time.

Speaking during the Climate Diagnostics Workshop in Tucson, Ariz., Ants Leetmaa, director of the Climate Prediction Center, noted that La Niña conditions will likely persist into March 2000. He noted that the La Niña will be a weak-to-moderate episode, but strong enough to impact weather patterns across the Southwest later this winter.

"Although this year's La Niña is not of the same strength as last year's, we cannot rule out seeing similarly extreme conditions in the Southwest this winter," said Leetmaa. Last year was the third driest and fifth warmest in 105 years of record keeping.

"We were very concerned about wildfires last winter because of the dryness, but fortunately the wetter-than-normal spring helped to mitigate some of that danger," he added. Last year's monsoon season started abnormally early and was quite active, with the Southwest region experiencing its 15th wettest July - September on record, and Arizona seeing its seventh wettest monsoon season.

La Niña refers to cooler-than-normal ocean waters across the central and eastern equatorial Pacific, whereas El Niño refers to warmer-than-normal ocean waters in these regions. Both La Niña and El Niño affect the weather patterns across the North Pacific and North America by directly changing the character of the wintertime jet stream, which ultimately controls the weather patterns.

Armed with graphic reviews of conditions in the tropical Pacific Ocean, Leetmaa explained the genesis of El Niño and the opposite climate phenomenon, La Niña. "When warmer ocean waters dominate at the equator, you have an El Niño. When
these waters are driven back to the western Pacific by colder waters, you have a La Niña." These events, which are monitored by a network of buoys and satellites operated by NOAA, impact the weather throughout the world.

"This shift in the ocean's sea-surface temperatures from the eastern Pacific to the western Pacific Ocean areas and back again approximately every four to seven years is like a pool of warm water sloshing back and forth in a huge bathtub. These temperature changes impact the location of marine life, prevailing winds and our world's climate. It has been going on for years but our observing capabilities now are getting better and we're learning how to forecast it," said Leetmaa.

According to the latest analyses and forecasts by the Climate Prediction Center, La Niña conditions continue to evolve. The Center forecasts weak to moderate La Niña conditions for the remainder of 1999 and continuing through the winter months. This is consistent with other forecast models being run by research institutions.

For more information on NOAA's Climate Prediction Center and its forecasts, log onto http://www.cpc.ncep.noaa.gov.