Avalanche Forecast
Regions: Cascades - East.
Large avalanches remain possible in areas where weak, old snow existed prior to the recent storm. Avoid steep slopes that have thick wind pillows on them. Limit your exposure to sunny slopes, as loose slides may run with daytime warming, and be mindful of overhead exposure to cornices.
Discussion
The storm and winds tapered away on Monday, but we were left with anywhere from 6" to 16" of new snow over the old snow interface. Winds were strong, blowing through the day, and created deep slabs in some areas. Mission Ridge reported some results with control work, slides were 12" deep and 30' to 80' wide. Prior to 2/22, a range of snow surfaces existed- most notably pockets of large surface hoar on sheltered and shaded slopes near and below treeline. West to north to southeast aspects held near surface faceted grains at upper elevations. Sunny slopes held thin sun crusts. We still have very limited information in the zone on how the new snow is reacting, especially near the crest. If you get out, suspect that the new snow is resting on a weak interface of some sort.Â
Large surface hoar (5-8mm) found preserved in a profile on a northwest aspect at 5,200ft in neighboring Stevens Pass zone, near Skyline Lake. February 24, 2020. Matt Primomo photo.Â
Snowpack Discussion
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.Â
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.Â
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.Â
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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 RiggsÂ
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.
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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.Â
Avalanche Problems
Wind Slabs
Strong winds have continued to form cohesive drifts and slabs over weak old snow layers. Look for signs of instability such as shooting cracks, whumphing collapses, and recent avalanches. These may be very deep in some areas. You may or may not observe signs of instability, but because they may be resting over faceted grains and surface hoar, they could still fracture in surprising ways. Slabs may break widely across the terrain, and could be triggered from below in some places. Avoid areas that appear heavily drifted, or thick looking pillows of snow on steep, open slopes.
Surface hoar is most likely to be a problem in somewhat sheltered, shaded pockets from 5,000ft to 6,500ft in elevation. Some of these slopes may be lingering in the balance, waiting for the right trigger to come along and provoke it. Slides on buried surface hoar have been known to run on slope angles in the low 30 degree range.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Loose Wet
As the daytime warms up, watch for loose snow slides to run on steep, sunny slopes. Loose snow slides can entrain a significant amount of snow on sustained steep slopes, and may become large. If the sun is out, look out for fresh rollerballs, sticky wet snow, or fan shaped avalanches. These are signs that tell you it is time to move on to a different slope aspect.
Cornices have gone through another growth spurt this past storm, and may be sensitive to breaking off in the sun. Be aware of these and plan your routes to steer clear of these overhead hazards. If traveling along ridgelines, give them a wide berth, as they may break from further back than you could anticipate.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1