Friday, February 23, 2018

La Niña winter update

Winter Storm Oliver brings February snowpack relief


Skiers and snowboarders had a lot to like about the recent President’s Day weekend weather. Winter Storm Oliver not only was significant nationally, it brought an impressive snowfall to the Washington Cascades. At the same time, the white stuff that closed mountain passes is helping to narrow the level of uncertainty we might have about this summer’s water supply.

When paired with forecasted cooler temperatures, conditions for late winter appear to be improving considerably. The more localized storm this past weekend (Feb. 23-25) appears to be further evidence the season is normalizing, particularly at Snoqualmie Pass.


What a difference the second half of February is shaping up to be.

How much snow did our mountains receive?

Generally, SNOTEL sites across the Cascades received about 10 percent of the annual total during Winter Storm Oliver.

A few exceptional highlights:
  • Snow-measuring sites near Snoqualmie Pass accumulated between 14 and 22 percent of the median annual total snowpack. Meadows Pass, located in the Cedar River watershed, improved dramatically by receiving 22.4 percent of annual snow water equivalent (SWE).
  • In the north Cascades, the Wells Creek site for the north fork of the Nooksack River added 11 percent of the annual total, finishing at 107 percent of annual peak snowpack.
  • The Potato Hill site near the north side of Mount Adams finished the storm at 94.8 percent of the annual peak, adding 16.7 percent of SWE over the long weekend.



Meadows Pass is a SNOTEL station at approximately 3200’ in elevation in the Snoqualmie Pass area. Snow falling at this location drains into the Cedar River, supplying Seattle Public Utilities’ reservoir at Chester Morse Lake.  It is also adjacent to the headwaters of the Yakima River Basin.



Early February was a different story

Just a few weeks ago in this blog, I characterized the winter snowpack as a “fits-and-starts” variety inconsistent with the cold and snowy winters typical of La Niña years. Day and nighttime temperatures well above normal and, in some cases, record-breaking warm, were a concern.

Signs portending an early spring seemed to be ramping up. Seattle had above-normal temperatures for the first nine days of the month of February, on many days 5-to-10 degrees above normal.

Even more startling, Yakima had several days with high temperatures near 70 degrees. The Paradise ranger station on Mount Rainier had nine days with high temperatures, 32 degrees or above, in the first 20 days of the month. On four of those days, the average temperature for the entire 24-hour period was at or greater than freezing.

It is certainly nice for folks to haul out their skis instead of their bikes now that temperatures are cooler and the snow levels are improving. But water managers may suffer headaches if warm winter days return and trigger an early melt-off of the snowpack.

Water distribution systems in the Pacific Northwest rely on the slow but steady runoff of melted mountain snow to replenish our reservoirs and rivers, and provide the water used during our dry summers. An early snowmelt leaves less for rivers, communities, and farms during the dry season.

Cool high-country temperatures late in the snow year, even as few as a couple of weeks of cooler temperatures, can go a long ways toward providing seasonal water security in the region. As it stands right now, the unseasonably warm weather of a few weeks ago appears to be more of a short-term perturbation, rather than a harbinger of a very early spring. This is good news.

Is spring canceled?

“Winter is not over—the story is still being written,” a colleague of mine adroitly put to me. The spring-like temperatures of early February are not something we are contending with for the time being. This doesn’t necessarily mean spring is far away, but we're no longer seeing Arizona-like winter temperatures threatening to drive an early snowmelt.

The return of winter weather also means that basins with below-normal snowpack have less catching up to do before winter begins to glide into its denouement.
  • The Stevens Pass SNOTEL site, for example, currently has 94 percent of peak annual snowpack in inches of SWE.
  • Rainy Pass, a site in the north Cascades at the headwaters of the Skagit and Stehekin rivers, also has 94 percent of the median total annual snowpack.
  • White Pass’ Pigtail Peak SNOTEL—a key indicator for Yakima River Basin water supplies—added 11 percent of the peak annual snowpack during the most recent storm, improving from 64 percent to 75 percent of the median maximum annual SWE.
We’ll need to see continued cold weather to ensure this snow stays in place until it is most needed.

What's next?

The outlook by NOAA’s Climate Prediction Center sees normal precipitation and much-below-normal temperatures for both the six-to-10 day and 8-to-14 day forecasts. Forecasts also indicate near-normal levels of precipitation for late spring and summer.
So, stay tuned – there’s more to come. And this winter remains one to watch!

For more information check out our statewide water conditions page.


By Tyler Roberts
Watermaster for Ecology's Office of Columbia River

2 comments:

Yakima River Ranch LLC said...

Thanks for all the good work Ecology does, including connecting with water users.I didn't realize White Pass accumulation was a key indicator. I also would like to know more about the role of Ecology in flood control, if any. Or is this solely a Reclamation function? In any case, thanks again for the news.

WA Department of Ecology said...

Thanks for the comment. The Yakima River reservoirs are managed by the U.S. Bureau of Reclamation for water supply and flood control. When it comes to flood control activities across the state, the Department of Ecology’s role includes providing technical assistance to our local government partners. We also work with cities and counties on funding proposals for projects designed to prevent or reduce flood-related damage to public resources such as levies, roads, bridges, and public waters. In addition, we work closely with the Federal Emergency Management Agency to assist local governments with the National Flood Insurance Program. Finally, we also help city and county governments promote safety from floods by providing guidance about local development in frequently flooded areas. Local governments usually incorporate our guidance and assistance into their local land-use laws and rules called critical areas ordinances. You can learn more here: https://ecology.wa.gov/Water-Shorelines/Shoreline-coastal-management/Hazards/Floods-floodplain-planning