Avalanche Forecast

We expect a weak system to bring up to an inch of snow on Saturday morning with light to moderate winds. Before stepping into aggressive terrain, be sure to evaluate slopes for fresh, wind drifted snow by looking for signs such as textured snow surfaces or freshly built cornices. If these signs are present, be leery of traveling on higher consequence slopes.

While the avalanche danger remains low, we're tracking snow surfaces before the next storm arrives. Near-surface facets and large surface hoar have been widespread throughout the region.

Large surface hoar on Watson Peak, West North zone. 02/21. Photo credit: Andrew Keifer

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.

We have 4+ sunny days under our belt now which have initiated a transition in conditions for our zone from the cold dry snow we received last weekend. Due to solar input, the zone now contains varied snow surfaces. Polar aspects generally hold cold dry snow with breakable crusts likely on E or W aspects. Earlier this week, this transition in adjacent zones has produced both Wet Loose and Wind Slab avalanches, but with cool weather and light snow arriving, Wet Loose activity should be finished. Prior to Saturday, the last wind transport was observed 2/18 in the Stevens Pass zone, so any old slabs should be quite stubborn or unreactive to trigger.

Lastly, give large and overhung cornices a wide berth by not traveling on or below them.

 

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.