Avalanche Forecast
Regions: Stevens Pass.
You can still trigger large and dangerous avalanches on a deeply buried weak layer at all aspects and elevations. Watch for recent wind loading on slopes above treeline. If you experience cracking in the snow, collapsing, or snowpack tests indicating that you can trigger avalanches, stay off slopes steeper than 35 degrees.
Discussion
Snow and Avalanche Discussion
About 6 inches of new snow accumulated on Wednesday with westerly wind at upper elevations. Expect clearing skies on Wednesday. Small loose wet avalanches may run on sunny slopes.
On Saturday afternoon, a party of two snowboarders triggered a D2 avalanche at 5,400 ft on a southeast aspect in Tunnel Creek. Both members were partially buried but were able to self-rescue without injuries. An observer reported a rumbling collapse on a northwest aspect at 4,200ft on Sunday near Lanham Creek. Also on Sunday, a rider was partially buried on Snoqualmie Pass after triggering an avalanche at 4,600ft on a north aspect. On Friday the 15th, a skier remotely triggered a slab from approximately 50ft away at 4,800ft in an open area between old-growth trees near Lanham Lakes. The slide took out the entire clearing, edge to edge, and ran on facets over a crust. Many tests and profiles from the past weekend indicate a potential for triggering avalanches.Â
In addition to avalanches, the deep snow has hazards of its own such as Snow Immersion Suffocation, tree well hazards, and roof avalanches. Don't linger beneath roofs, travel in the mountains with partners and keep them in sight.
Snowboard triggered slide in Tunnel Creek. Photo:Â Dan Veenhuizen 2/17/2019
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Snowpack Discussion
February 20th, 2019
Recap
We’re now over a week out from a major winter storm and avalanche cycle that left a string of school cancellations and avalanche near misses in its wake. As with snowfall amounts, the avalanche cycles have been similar, but not identical in all regions. The further we’re getting from the peak of the cycle, the more variation in avalanche conditions we’re seeing between regions and even within individual zones. Variable snow totals from storms this week are further adding to the range of conditions you will encounter. In some places, these storms may add stress to existing weak layers.
In the days after the natural cycle, it was obvious that you could trigger an avalanche. Large crowns were visible and you could feel and hear collapses in many zones. Managing your risk was easy. Avoid avalanche terrain. Since the natural avalanche cycle of the 11-12th quieted down, the main concern for avalanches has focussed on the February 8th facets in regions where the weak layer is problematic.
A natural persistent slab (D2) on a north aspect at 4200ft low in Glacier Creek drainage (Hwy 542). 02/13/19 Lee Lazzara Photo
Variability, Complexity, and Manag
As the time moves on and the snowpack structure changes, we’re seeing the potential for triggering avalanches change as well. The February 8th layer is rounding (strengthening) and the likelihood of triggering an avalanche on it is decreasing. so much so that the problem is trending to unlikely in some regions. Unfortunately, the consequences (size and destructive potential) remain the same if you do trigger an avalanche on this layer.
These conditions are commonly described as "low probability - high consequence" scenarios. Under these circumstances, common clues may paint a conflicting picture and snowpack tests become even more difficult to interpret (snowpack tests often don’t give us a clear “go or no-go†answer, if such a thing exists).
Q: How do we manage our risk when observations are contradictory and difficult to interpret?
A: When avalanche conditions are complicated, defer to less consequential and simpler terrain to manage your risk. Prioritize obvious clues, like recent avalanches, shooting cracks, or collapses. Focus on other observations that indicate a potential to trigger avalanches. Snowpack tests are just one piece of the decision-making puzzle. Lean on them as reasons to reduce your group's exposure to avalanche terrain. Don’t use them to justify traveling in more consequential terrain.
A natural persistent slab avalanche (D2), likely occurred on 2/12Â on southwest through southeast aspects of Windy Mountain at 5,400ft in the Tye River drainage. Photo: Dan Veenhuizen.
Case Study
On the 17th I dug a profile, east of Stevens Pass on a north-northeast aspect at 4,127ft. I found the February 8th facets (0.5-1.5mm) rounding and buried 59cm from the surface. After much investigation, I found the following results at the February 8th interface: CTH (SP), ECTN28, PST 45/100 (END), 5 yellow flags (structural indicators). Later that day, about 2000 linear feet away from the profile site at the same elevation and slightly different aspect, we experienced a massive rumbling collapse.
All this crypto snow-speak means that some of the observations I made indicated that triggering an avalanche was likely, but some did not. Depending on your interpretation, some results could be argued either way. Confusing, right?
With all of this data in my filed book, it was the collapse that stuck out. It was enough evidence for me to avoid slopes steeper than 35 degrees. That was a more obvious answer than all the other data I gathered and it’s the easiest to interpret. Without the collapse, I would have prioritized the test results that indicated I could have triggered a slide.
Avalanche Problems
Persistent Slabs
Continue to use caution when traveling in the mountains and choose terrain where you have options to minimize your exposure to avalanche terrain. Avoid large, open slopes 35 degrees and steeper. Stop and re-group in safer terrain, well out from under overhead avalanche paths. Professional guides and astute backcountry travelers are still avoiding big, complex terrain and unsupported slopes. When dealing with this tricky snowpack keep your decisions conservative and simple. These avalanches should be considered guilty until obviously innocent.
You’ll find the February 8th facets buried 2-3 feet below the surface with an underlying crust. The snowpack structure is widespread on all aspects and elevations. Watch for shallow drifted slabs above treeline that formed on Tuesday night. Watch for signs like collapses (whumphs) or tests indicating propagation/triggering. Prioritize observations that indicate a potential for triggering avalanches over observations that point at relative stability.
As we get further from the last avalanche cycle, signs of instability are becoming less frequent and sometimes less obvious. But they are there. On Sunday near Lanham creek, I poured over the snowpack identifying grains and performing many tests. After emerging with contradictory test results, we experienced a large rumbling collapse 2000ft from the profile site. The collapse was all the information I needed to avoid avalanche terrain.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1