Avalanche Forecast
Regions: Stevens Pass.
There is well over a foot of new snow in the mountains around Stevens Pass. Wind slabs may be reactive on exposed slopes, and deep slabs remain possible in isolated locations. Give the snowpack time to adjust, and avoid big, complex terrain on Sunday. Choose well supported routes with limited exposure to avalanche terrain.
Discussion
Snow and Avalanche Discussion
Over a foot of new snow can be found in the area. Observers reported lots of dry loose activity on Saturday, with some slides big to injure a person. The recent snowfall has buried a mix of surface hoar and near surface facets on shaded aspects, and melt freeze crust on southerly aspects. 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. Observers also noted some collapsing in a stand of old growth trees on Arrowhead on Saturday.
Keep these layers in mind as you plan your tour, and as you are out there on Sunday:
1. Wind slab within the new snow near and above treeline in exposed areas.Â
2. A couple of recently buried melt freeze crusts and near surface facets or surface hoar within the upper two feet of the snowpack.Â
3. The deeper February 8th facet/crust (Found 2-4ft 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
Wind Slabs
Easterly winds should ramp up tonight and ease a bit into Sunday, though there is plenty of low density new snow to move around. These winds may create stiffer and deeper drifts and slabs that will be resting on weak snow. Be especially cautious near ridges and locations exposed to the highway corridor where easterly winds tend to funnel through and form thicker drifts. If there is enough cohesion within the new snow, slab avalanches may be sensitive, easy to initiate, and they could pack a punch. Besides wind slab within the new snow, the hefty load of new snow is sitting on a couple of potentially problematic recently buried layers. These consist of weak, faceted grains on northerly aspects, and melt freeze crusts on southerlies, potentially with facets on the top. 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 deep slab avalanche. Out of the wind zone, the new snow may still be susceptible to easy "sluffing", or loose dry avalanching on steep slopes. These could entrain, and run fairly fast. If the sun pokes out, expect loose wet to become a concern on steep southerly aspects.
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: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Deep Persistent Slabs
The February 8th facet/crust layer can be found about 2-4 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. It can be found closer to the surface in isolated locations, and further east in the Hwy 2 corridor. Recently, this layer is responsible for a few very large skier triggered avalanches near Crystal Mountain.
The "incremental loading pattern" continues, and a large amount of uncertainty remains. The weight of the new snow may be stressing this layer, potentially evidenced by the collapses reported from Arrowhead on Saturday. Avoid large, open slopes 35 degrees and steeper, and minimize your exposure to avalanche terrain. Especially steer clear of recently loaded and steep, unsupported slopes. Stop and re-group in safer terrain, well out from under overhead avalanche paths. Choose conservative up routes and descent options to minimize your risk. 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 thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 2 - 2