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NW Fishletter #374, October 2, 2017
 Current Predictions Foresee Colder, Wetter Weather in the Northwest
Forecasters are now predicting an increasing chance of at least a weak La Niña this fall and winter in the northern hemisphere.
Climatologists with the NOAA's Climate Prediction Center (CPC) and the International Research Institute for Climate and Society (IRI) issued a La Niña update watch Sept. 14.
The forecast for a 55-60 percent probability of a La Niña in 2017-2018 means a good chance the winter in the Pacific Northwest will resemble last year's, when Idaho, Montana, Oregon and Washington had weather that was wetter and colder than average.
The La Niña could occur sometime in the next three months, according to the CPC.
Recent cooling of surface and sub-surface temperatures in the equatorial Pacific Ocean are, in part, the basis for the predictions. Weather forecasting is also more reliable this time of year, the IRI said.
A Sept. 18 blog entry by Rebecca Lindsey, science writer for NOAA's climate.gov, explained how El Niño and La Niña "trigger a cascade of changes in tropical rainfall and wind patterns that echo around the globe."
The changes can vary with each El Niño or La Niña event, and what happens during one of these events is a probability and not a certainty, she emphasized.
Swinging back and forth about every 3-7 years, El Niño and La Niña alternately warm and cool large areas of the tropical Pacific, Lindsey said.
As the world's largest ocean, the Pacific can significantly affect temperature and precipitation across the U.S. and other parts of the world.
She compared the disruption of atmospheric circulation patterns that alter jet streams across the globe to "a boulder dropped into a stream."
"For the United States, the most significant impact is a shift in the path of the mid-latitude jet streams," she said. "These swift, high-level winds play a major role in separating warm and cool air masses and steering storms from the Pacific across the U.S."
The Pacific Northwest and parts of Alaska tend to be cooler and wetter than average and the southern tier of the U.S., including California, tends to be warmer and drier than average during a La Niña.
El Niño and La Niña have the strongest impact in the U.S. during December-February but sometimes that influence extends into early spring, Lindsey said.
The bottom line is a trend toward at least five months of colder, wetter weather in the Northwest.
The next CPC forecast is due Oct. 12. -L. B.
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