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La Niña might bring a better water year in 2021

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La Niña is here, and meteorologists say it may bring a particularly eventful winter to Oregon this year.

The temperature change in the equatorial Pacific Ocean will likely have significant impacts on weather throughout the northern hemisphere during the next few months. And there’s a chance it could result in a better water year for 2021.

Friday and Saturday’s snow and cold temperatures ended a particularly resilient high-pressure ridge that had been keeping southern Oregon warm and dry well into autumn. Systems like this weekend’s are more likely to occur under La Niña conditions, which tend to result in cooler, wetter weather in the Pacific Northwest.

Late in the summer, strong trade winds pushed warm surface water along the equator in the eastern Pacific westward toward Australia and Indonesia, allowing colder, deeper water to replace it. The western Pacific’s abnormally warm water causes it to experience stronger storms, which pump moisture into the upper atmosphere and send it back east across the Pacific, where it cools, sinks and begins the cycle again.

As the air travels east from the western Pacific toward North America, it tends to cause the jet stream to bend over the western part of the continent. That usually results in more extreme winter weather for the Pacific Northwest while the southern U.S. experiences warmer, drier conditions.

The National Oceanic and Atmospheric Administration’s Climate Prediction Center said La Niña conditions have an 85% chance of continuing through January and a 60% chance of extending further into March, and that models suggest it could be a moderate or even strong La Niña.

Pete Parsons, meteorologist with the Oregon Department of Forestry, released his forecast for November through January last month, taking into account NOAA’s predictions. He noted that the next few months are likely to be a departure from the last two winters, during which opposite El Niño conditions resulted in weather that was warmer and drier than normal, and that the first year coming out of an El Niño period tends to see more active winter weather. Parsons also said a cooling trend in the Pacific Decadal Oscillation, which measures water temperatures at the coast, will likely cool down weather systems blowing in off the ocean.

“We’ve got some pretty big changes in store,” he said in his forecast. “We’re looking at a whole different tropical ocean structure than we have the past couple of years.”

Based on prior La Niña conditions, Parsons predicted a milder start to the winter and colder, wetter weather later in December and through January. In January, he forecasted that the South Central climate region of Oregon, which encompasses the Klamath Basin, may receive 40 to 50% more precipitation than normal. He also said there will be a higher chance of windstorms, floods, blizzards and dumps of Arctic air throughout the Pacific Northwest.

“Be prepared for an interesting winter coming up,” Parsons said.

However, the Klamath Basin-specific forecast doesn’t point to a wet winter as definitively as the statewide one. Shad Keene, meteorologist at the National Weather Service office in Medford, said an analysis of local climate and weather patterns since 1950 showed no statistically significant relationship between Klamath Basin’s temperature and precipitation and winter La Niña conditions. In fact, that Basin’s last above-average precipitation year from 2014-2015 occurred during an El Niño period.

Keene did note that the analysis doesn’t factor in the strength of La Niña, given the possibility that this year’s could be particularly strong. But it’s clear that the Basin’s location in the rain shadow of the Cascades shelters it from much of the winter weather systems that dump snow and rain on the Rogue and Umpqua Valleys more reliably during La Niña years.

The silver linings, Keene said, are wind and mountain snow. More constant breezes and wind events are associated with La Niña, meaning that air gets more well-mixed in a Basin that would normally collect settling particles in the air (particularly from wood-fired stoves).

The major change could come up in the mountains, which are expected to receive more snowfall than normal because of La Niña-driven systems. Keene said Crater Lake, a good bellwether for the High Cascades, has a 75% chance of seeing above normal snowfall. If cold conditions hold out, that’ll mean more water entering Upper Klamath Lake and its tributaries next spring. The NOAA’s winter 2020 drought outlook even expects southern Oregon’s dry spell to improve as a result of La Niña.

“You’ll be impacted by the snowpack and water supply,” Keene said. “It’s good news for Klamath Falls in that regard.”