Avalanche Forecast

Up to an inch of new snow may arrive late Friday night and Saturday morning with generally light to occasionally moderate winds. This may create some very minor wind slabs near ridgetops, but more importantly, it has the potential to bury weak snow surfaces. We're tracking snow surfaces before the next storm arrives. Polar aspects and sheltered slopes, surface hoar (up to 1 cm) sit on top of around 12” (30cm) of faceting older snow above the stout 2/13 crust. If you travel early Saturday and find weak surfaces still out there, please support your avalanche center with an observation. As February storms come more intermittently, the weak surfaces developed between storms create the potential for large and surprising avalanches such as this one from Stevens Pass on 2/16 if they become buried. We could see similar avalanches on Sunday/Monday.

An extended period of high pressure has brought sunny weather to Hurricane Ridge since Monday. Temperatures warmed up significantly on Wednesday, such that Wednesday through Friday each brought daytime highs into the low 40's with nighttime lows near freezing.

NWAC/NPS staff on Thursday and Friday found varied surface conditions but little in the way of new or recent avalanches.  Low relative humidities have generally allowed cold, dry snow to persist on polar aspect. Thickening crusts on southerly aspects generally prevented loose wet avalanche activity. On E or W aspects or near ridgelines, you might encounter breakable crusts. There are no layers of concerns below the 2/13 crust. 

Along and just below ridgelines, winds earlier in the week scoured the surface to an icy crust. Expect challenging travel conditions and either avoid or be prepared to self-arrest if you slip on steeper slopes. 

About 50' of slick crust stands between the ridgeline and softer snow below. Photo: Matt Schonwald Feb 20th, 2020

Forecast Schedule

For the 2019-20 winter season, avalanche danger ratings will be issued for the Olympics every Friday through Sunday and during mid-week holidays. During the week, No Rating will be issued but forecasts will include expected conditions and relevant travel advice.  If you are out in the Olympics, share your backcountry observations.

February 20, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

Intermittent Storms

January’s non-stop pattern of storms extended into the first week of February. Since then, we’ve transitioned to more intermittent storms with notable stretches of high pressure and dry weather. We now see more variability through the region in the snowpack layering and avalanche conditions than in January. 

Significant periods of calm weather in February have allowed avalanche danger to decrease between storms. During the clear periods, strengthening late winter sun and gradual warming has brought some loose wet avalanche cycles to sunny slopes. Even so, from February 1st-20th there were 10 days when one or more zones were forecasted at all Low danger. In comparison, the month of January had zero days with any zone forecasted at all Low avalanche danger. 

Surface hoar near Snoqualmie Pass. February 19th. Photo: Ely Gerbin

Looking Forward

As we look forward, there are a number of considerations that are pertinent to most zones. The pattern of storms separated by clear periods may form new weak layers and interfaces to monitor. Many zones hold variable surfaces that warrant consideration as a travel hazard. In the Mt Hood Meadows area, two fatalities were related to falls on slick surfaces over President’s Day weekend. Another important consideration is the cornice growth that occurred in the past month from predominantly west winds. Very large cornices loom on ridges in most zones, except for possibly the Olympic Mountains. Future warming could be the added ingredient needed for cornices to fall. A close call with a cornice-triggered avalanche near White Pass on February 12th is a reminder of the potential hazard that cornices can pose. 

 

A party of three triggered this avalanche from below on a southeast aspect at 5400ft near Skyline Lake, Stevens Pass on February 16th. Two members of the party were fully buried and one was partially buried. The party self-rescued and no injuries were sustained. Photo: Dustin Riggs 

Weak Layers

So far this season, we’ve had limited persistent weak layers to deal with. February’s periods of calm weather developed a couple of weak layers and problematic interfaces that later got buried in the snowpack. On a whole, weak layers have been short-lived and have generally resolved within a couple of days of being buried. While the following weak layers are no longer major concerns, some readers may want a more in-depth understanding of them before venturing into the mountains.

In the Stevens Pass and East Central zones, the February 4th interface consisted of a thin layer of small (0.5-1.0mm) facets sitting on the stout February 1st melt forms (crust). The crust was widespread on all aspects up to at least 7500ft and the facets were most prominent on the northern half of the compass (west through north through east aspects). A major storm and avalanche cycle February 4th-7th totaled over 60 inches of snow. The February 4th interface was responsible for widespread avalanching including slides up to size D3 at most elevations. As direct action avalanches subsided, the February 4th interface was deeply buried and didn’t produce further triggered avalanches. Interestingly, as recent as February 18th, this interface was found in a profile and produced some mixed and head-scratching test results. While this interface is not a concern for triggered avalanches, you may still be able to find it 4-6 feet below the surface, especially on sheltered, shaded slopes at upper elevations.

 

A profile from 6460ft on a northeast aspect in the Chiwaukum Range on February 18th shows the February 4th layer. Small column tests produced sudden planar results while a Propagation Saw Test did not indicate propagation.

A more elusive and problematic layer for triggered avalanches was the February 13th interface. This was also most prominent in the Stevens Pass and East Central zones in addition areas of the West Central and Snoqualmie Pass zones. At Stevens Pass a combination of large (2.0-4.0mm) surface hoar and small (0.5mm) near-surface facets developed on a variety of surfaces and were buried on the 13th. The weak layer was most problematic on east through south aspects between 4,500-6,000ft, where the grains were preserved as they rested on a thin crust. It seemed that outside of this aspect and elevation range either 1) the weak grains did not persist enough to be triggered by travelers or 2) the underlying layer, or bed surface, wasn’t hard enough for the weak layer to be reactive. A number of parties reported natural and triggered avalanches on southeast aspects during the storm cycle on Presidents Day weekend. This included a party of three that were uninjured after all being caught, with two full-burials, in a surprising avalanche near Skyline Lake on the 17th. As of February 20th, the interface is 1-3 feet below the surface and has become difficult to trigger. The February 13th interface is still visible in the snowpack and we continue to monitor it. It will likely become inactive before the end of the month.

In summary, February has been a great time to explore the mountains and enjoy the longer days for recreating in this season’s robust snowpack. Though, not without complexity and variability. Continue to monitor changing conditions and check the daily avalanche forecasts for the most up to date information.