Ongoing La Niña conditions in the central tropical Pacific will impact temperature and precipitation patterns across the Pacific Northwest region of the United States during the next six months, 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 at the La Niña Forum during the annual Media and Emergency Management Workshop sponsored by the National Weather Service, Seattle-Tacoma, 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, and strong enough to impact weather patterns across the Pacific Northwest later this winter.

"The Pacific Northwest can expect an increased likelihood of above-normal precipitation during the upcoming winter season. Some areas, including the mountains and intermountain region, may see above-normal snowfalls as a result of an expected
La Niña-related increase in storminess across the region," Leetmaa said. "We also expect an increased likelihood of normal to below-normal temperatures in the Pacific Northwest this winter."

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 advisory issued by the Climate Prediction Center on Sept. 10, 1999, 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.

NOAA's Climate Prediction Center uses statistical tools and computer models to predict future seasonal climate variability. Severe weather can occur regardless of the impact of El Niña and La Niña, Leetmaa cautioned.

"Severe weather happens every season and is not restricted to one part of the country. The National Weather Service urges people to become familiar with the forecasts for the areas where they live, work and vacation. You should be prepared to take steps to protect your life and property. People who try to defy Mother Nature's threats are putting themselves in jeopardy," Leetmaa said.

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