Earth

Forecasters are predicting that the odds favor a La Nina forming next fall, a climate phenomenon linked to large snowpacks and good irrigation seasons in most Northwest basins.

The National Oceanic and Atmospheric Administration issued a La Nina watch on July 8, indicating conditions are ripe for it to develop in the next six months.

A La Nina has a 66% chance of prevailing in November, December and January, according to forecasters with NOAA’s Climate Prediction Center and at Columbia University’s climate research center.

The forecast conflicts with most climate models. Averaged together, 25 models reviewed by forecasters predict only a 34% chance of a La Nina next winter.

Forecasters, however, are using their human judgment. A La Nina reigned last winter and historically one La Nina follows another, according to a statement by NOAA.

The La Nina watch also has support from a group of climate models developed in the United States and Canada.

“I trust humans informed by the models,” Washington State Climatologist Nick Bond said.

La Nina generally brings cool and wet winters to the northern tier of the U.S. La Nina winters are generally warm and dry in the southern tier of the U.S. Another La Nina could worsen the drought in the Southwest U.S.

“We like La Ninas up here for the snow,” Bond said. “When there are back-to-back La Ninas, the second one is usually weaker than the first one.”

Washington’s snowpack on April 1, after a weak to moderate La Nina, was 131% of average, according to the Natural Resources Conservation Service. The melting snow has helped irrigators weather an extraordinarily dry spring and early summer.

In Oregon, snowpacks were up to 152% of normal in northern basins, but as low as 77% of normal in southern basins. Snowpacks in Northern California were generally lower than in Southern Oregon.

There have been two back-to-back La Nina winters since 2010-11. Every year, Washington’s snowpack was at least 113% of average on April 1.

Sea-surface temperatures, taken in the mid-Pacific Ocean along the equator, are expected to stay normal through the summer.

If the models, uninfluenced by human judgment, are correct, the ocean likely will stay neutral through the winter, too, robbing forecasts of their best clue to seasonal outlooks.

The models and forecasters see little chance of an El Nino, linked to low snowpacks in the Northwest.

“The odds of an El Nino are low, so at least we won’t have that,” Bond said.

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