SEATTLE - Remember the winter of 2007/2008? I-5 through Chehalis was under water, clearing snow at Snoqualmie Pass was literally wearing out plows and graders. That was the start of our last La Nina, the wetter, colder weather phenomenon that's the opposite of the more common and warmer El Nino. At least that's what it all means for the Pacific Northwest.
La Nina occurs when the surface water near the equator tends to be cooler than normal in the eastern Pacific Ocean. El Nino is the opposite, as warm water pushes up against central and south America. While the engine of these weather effects seems a long way off, their effects are global.
On Thursday, NOAA's Climate Prediction Center issued a new advisory about La Nina, saying it strengthened during August.
"Nearly all models predict La Nina to continue at least through early 2011," the report's authors said, adding, "La Nina will begin to exert an increasing influence on the weather and climate of the United States. These impacts include an enhanced chance of above-average precipitation in the Pacific Northwest."
For other parts of the nation, La Nina is likely to bring less precipitation to the Southwest and portions of the middle and lower Mississippi Valley. It also amps up the Atlantic hurricane season.
Scientists who study this, many of whom work at NOAA's Pacific Marine Laboratory in Seattle, analyze water in the upper 300 meters of the ocean. That water is monitored by a belt of buoys strong across the Pacific, along and north and south of the equator.
But scientists are quick to point out that the effects from La Nina and its warmer cousin El Nino are about averages. According to the Climate Prediction Center, "It is likely that the peak strength of this event will be at least moderate... to strong."
But while some weather records recall the winter of 2008/2009 a neutral year, leading NOAA Research Scientist Mike McPhaden says it was a second La Nina Winter.
"In fact, the last three La Ninas have been double dippers. They've gone on for two winter seasons," says McPhaden.
2008/2009 winter saw the December snow storms that impacted the City of Seattle, and helped to cost former mayor Greg Nickels his job.
In January 2009, record levels in the Howard Hanson flood control reservoir in the Cascades damaged part of the ground that abuts the Howard Hanson Dam. At that point, the U.S. Army Corps of Engineers said another storm might have forced them to release water, flooding the Green River Valley from Auburn to Lake Washington. At that point the chance of a flood was placed at one in three.
The Corps saying extensive work on the dam has reduced the chance today to one in 60. The goal is to have the dam back up to full strength to a flood chance of one in 140. The Corps says it's ready to handle the flood threat as best it can this winter.
The valley got a break and more preparation time last winter, which was a warmer and drier El Nino.
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