An ongoing La Nina enhances chances that late winter and early spring in Washington will be cool and wet, the federal Climate Prediction Center forecasted Thursday.
The outlook for March, April and May bodes well for building and preserving snowpacks that already are deeper than normal. La Nina's affect was relatively slight until recently, Washington State Climatologist Nick Bond said.
"It took its sweet time," he said. "We hadn't seen the atmospheric weather patterns that tend to come together with La Nina, but that changed this month."
A cooler than average Pacific Ocean along the equator triggers a La Nina, the opposite of an El Nino. In La Nina years, winters in the northern tier of the U.S. are generally colder and wetter.
In the southern tier, La Nina winters tend to be warmer and drier. Oregon and Idaho south of the Panhandle fall in between those bands.
Basin snowpacks in Washington were mostly well above average on Thursday, led by the Olympic Mountains at 135% of normal. The Upper Columbia basin was 126% of normal, and the Lower Snake basin was 114% of average.
Statewide, basin snowpacks averaged 118% of normal. There is one note of caution. Although the next three months are expected to be wetter than normal, March is expected to be about average, according to the prediction center.
A warm wind could still dry out snowpacks, Natural Resources Conservation Service water supply specialist Scott Pattee said.
Snowpacks, however, look strong right now and are nearing the point they only need "maintenance snow," he said.
"We're looking pretty good. If we hold onto what we have, we should have a good water supply," he said.
The five reservoirs that irrigate Yakima Valley farmland held 109% of their average volume on Thursday. Rainfall has been above average at the reservoirs, and the snowpacks that melt into them are above average, too.
"The conditions right now are looking very favorable for an adequate water supply," said Chris Lynch, Bureau of Reclamation river operations engineer.
The Climate Prediction Center, part of the National Weather Service, forecasts that sea-surface temperatures will warm to an average range — or neutral state — by late spring.
For now, La Nina should continue to influence the weather, Bond said.
"I would think it will for at least another month, month and a half," he said.
Even with a La Nina, 11.5% of Washington is in moderate or severe drought, the U.S. Drought Monitor reported Thursday. The drought covers much of south-central Washington.
Some level of drought is more extensive in Oregon (73%), Idaho (24%) and California (84%).
Looking ahead, the prediction center forecasts Washington's summer will be warmer and drier than usual. The forecast is based on a trend toward warmer summers.
Lynch said the snowpack probably can withstand some warm spring weather and early melting. "My sense is we're in a good spot where we'll probably have no drought," he said.
The prediction center forecasts that the Idaho Panhandle also will be cool and wet for the next three months.
The odds are equal for above or below average temperatures and precipitation in Oregon, Northern California and most of Idaho.
Looking much farther ahead, climate models suggest a 50% chance that another La Nina will form next winter, with a 50% chance that conditions will be neutral or an El Nino, according to the prediction center.
It's too early to predict 2021-22 winter weather, Bond said. "We won't have a clue about that until the summer," he said.