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NW Fishletter #389, January 7, 2019

[13] Forecasters Await El Nino, Predict Below-Normal Runoff

Forecasters say this year's April through September water supply could be well below normal in Oregon, factoring in current conditions, precipitation forecasts over the next 10 days, and the likelihood that El Nino will influence weather over the next three months.

But predictions in a Jan. 3 briefing by NOAA's Northwest River Forecast Center also point to near-normal runoff in the upper Columbia River basin, and above-normal runoff in the Canadian portions of the basin.

The webinar was the first water supply forecast for 2019, with monthly predictions to continue through June.

"Many of the mid- and lower-Snake tributaries are well below normal," said Forecast Center senior hydrologist Kevin Berghoff, showing the forecast for this year's water supply. "The upper Snake is looking reasonably OK at this point--somewhere in the 85- to 90-percent range--but many of the mid- and lower-Snake tributaries are dropping down below the 75-percent range," he said.

Part of the problem is a poor start to snowpack this winter. Washington State Department of Ecology Director Maia Bellon said in a Dec. 31 tweet, that after suffering one of the slowest starts to the snowpack in 30 years, snowpack jumped to 94 percent of normal thanks to recent storms. But Oregon and parts of the Snake River basin haven't been as snow-fortunate.

"Oregon--while it's looking better than it did last year--it's still well below normal even after fairly decent snowpack accumulation over the last week," Berghoff reported. Since Oct. 1, the upper Columbia and Snake river basins have seen below-normal precipitation, while Oregon has been well below normal. The snow-water equivalent is also below normal across the upper Columbia and Snake river basins, and well below normal in the Oregon Cascades.

Berghoff said current three-month forecasts for January through March show an increased chance of below-normal precipitation throughout most of the Columbia Basin, although there's no clear signal for precipitation levels in the Snake River Basin. In addition, he said, there's an increased chance for above-normal temperatures across the Columbia and Snake river basins "during the snow-building months."

Berghoff noted that's partly because El Nino is still very likely to form this winter and continue into spring.

For two months now, forecasters with NOAA's Climate Prediction Center have reported a strong chance that El Nino will form in the Pacific Ocean, and bring warmer-than-average conditions to the Pacific Northwest.

But despite a continued strong likelihood, neutral conditions for the El Nino-Southern Oscillation (ENSO) continued through November and December, even as sea surface temperatures in the Pacific Ocean near the equator continued to be above average.

In early November, forecasters gave El Nino an 80 percent chance of developing. It's now up to 90 percent, the agency reported Dec. 13. The chance of it continuing into the spring is still about 60 percent, the report said.

"Typically, if it's going to develop, it would show its hands soon," Jeremy Wolf, meteorologist at the National Weather Service in Spokane, Wash., told NW Fishletter. Wolf explained that, while surface water in the Pacific Ocean is already warm enough off the central and eastern equatorial coast, the atmosphere has not yet responded.

"They haven't seen that develop yet," Wolf said, adding, "That's the one missing piece, the atmospheric response." When that happens, the jet stream often shifts south into Oregon and California, which can move the storm track south, he said.

Still, Wolf said, even under El Nino conditions, the precipitation amounts in the Northwest are uncertain. However, because temperatures are usually warmer, some precipitation in the winter will fall as rain instead of snow.

"Each El Nino is different," he said. In some, the jet stream moves south, and the Columbia Basin gets less precipitation. Averaging every El Nino year since the 1950s, Wolf said, shows that "wet anomalies are to our south, and we tend to be drier" in the Columbia Basin. "But some El Nino winters have actually brought wetter conditions up here. There's never a guarantee."

A blog post by NOAA research scientist Emily Becker explained that the atmospheric response forecasters are looking for may not have occurred due to the Madden-Julian Oscillation, which affects global circulation in the atmosphere in a matter of weeks, compared to ENSO, which impacts the weather for months. So instead of settling into a seasonal pattern, the atmosphere is still being affected by this short-term circulation pattern.

"Why are forecasters confident that these two are destined for each other?" Becker asked, and then answered, "Most climate models predict that sea surface temperatures will remain higher than the El Nino threshold ... Adding support to this is that the amount of warmer-than-average water under the surface is still quite high, although slightly decreased from last month (the November average tied for sixth highest since 1979). This will provide a source of warmer-than-average water for the surface over the next few months." -K.C. Mehaffey

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Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: K.C. Mehaffey
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