Avalanche Forecast
Regions: Stevens Pass.
As the snow continues to pile up, the snowpack is becoming more complicated. Loose dry, wind slab, storm slab, persistent slab-they can all be found. This is a good time to step back, and choose simple routes with minimal exposure to avalanche terrain.
Discussion
Snow and Avalanche Discussion
On Friday, snowfall began during the day, burying a crop of surface hoar and near surface facets on shaded aspects, and melt freeze crust on southerly aspects. Skiers were able to trigger small loose dry avalanches on steep slopes in the afternoon. On Thursday skiers were able to easily trigger small soft slab avalanches that ran on a recently buried melt freeze crust, which was the new/old snow interface. These were 6†deep, and on Southeast aspects at 5,500ft on Lichtenberg Mountain. A profile on a Southwest aspect at 5100ft showed the February 8th crust/facet layer to be down 3 feet, and tests consistently indicated the potential for propagation.Â
There will be three potential layers of concern to keep in mind on Saturday:
1. The recently buried interface, described above.Â
2. The buried melt freeze crust and facets from February 19th (Found down about 12-18â€).
3. The deeper February 8th facet/crust (Found ~3ft deep).
On the 20th a couple of pertinent deep slab avalanches occurred at and near Crystal Mountain. These ran on the February 8th facet/crust persistent weak layer. Even though there are regional differences in the snowpack, there are enough similarities to make these observations pertinent. Three skiers exited Crystal Mt ski resort traveling west towards Hwy 410. The first skier on the slope triggered a very large avalanche but was able to ski out before getting caught. Crystal Mt Ski Patrol visited the site on Thursday. The avalanche was 3 ft deep, 200 ft wide, and classified as a SS-ASu-D3-R2.5-O. 6300 ft. WSW aspect. Slope angle 36 degrees. The involved party descended to Hwy 410 without incident. Crystal Mt Ski Patrol triggered a very large persistent slab as part of avalanche mitigation measures the same day. This avalanche occurred on a SE aspect at 6300’. This forecast may be updated at 0700 on Saturday.
Looking down on the crown of a skier triggered avalanche near Crystal Mt. Photo: Crystal Mt Ski Patrol
Snowpack Discussion
February 19th, 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 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 4200 ft low in Glacier Creek drainage (Hwy 542). 02/13/19 Lee Lazzara Photo
Variability and Mixed Messages
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. 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 groups' exposure to avalanche terrain. Don’t use them to justify traveling in more consequential terrain.
A natural persistent slab avalanche (D2), likely occurred on 2/12 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 field 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
Storm Slabs
Continued snowfall and winds are adding to the avalanche danger all around. There are two recently buried layers in the upper snowpack that may become problematic. These consist of weak, faceted grains on northerly aspects, and a melt freeze crust on southerlies, potentially with facets over the top. If there is enough cohesion within the new snow layers, slab avalanches may be sensitive, easy to initiate, and they could pack a punch. Be especially cautious near ridges and where the wind has created thicker drifts. Out of the wind zone, the new snow may be susceptible to easy "sluffing", or loose dry avalanching on steep slopes. These could entrain, and run fairly fast. Cracking in front of your ski tips is a good sign to step back, and seek out lower angled and more sheltered terrain. Any slide within the upper layers may step down and create a much larger and destructive persistent slab avalanche.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Persistent Slabs
The February 8th facet/crust layer can be found about 3 feet beneath the surface on all aspects and elevations. This layer appears to be standing its ground, as tests continue to indicate the potential for avalanches to occur on it. Recently, this layer is responsible for a few very large skier triggered avalanches near Crystal Mountain.
Continued light snowfall the past week have created what we call an "incremental loading pattern". It all adds up, just slowly over time. Avoid large, open slopes 35 degrees and steeper, and minimize your exposure to avalanche terrain. Especially steer clear of steeper, unsupported slopes. Stop and re-group in safer terrain, well out from under overhead avalanche paths. Choose conservative up routes and descent options and when dealing with this tricky snowpack. The slopes should be considered guilty until proven innocent. A handful of ski tracks on a slope doesn't mean that it can't or won't avalanche with the 6th, or even 10th person.
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: 2 - 2