Avalanche Forecast
Regions: Snoqualmie 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. Stay off of large slopes steeper than 35 degrees where it is difficult to minimize your exposure to avalanches.
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 Sunday the 17th, a snowboarder was partially buried in a persistent slab avalanche on a north aspect of Roaring Ridge at 4,600ft. On Saturday and Sunday, Alpental patrol triggered several persistent slab avalanches in their back bowl terrain. A few of these of these occurred at a slightly lower elevation (4000’) and slope angles (30-35 degrees). Snow profiles and snowpack tests from this zone continue to highlight the weak snow structure.
Many zones in the Cascades contains a similar potentially dangerous snowpack setup. Another compelling piece of information we have concerning the persistent slab problem is from Stevens Pass. On Saturday, two snowboarders triggered a persistent slab avalanche in Tunnel Creek outside the boundary of Stevens Pass ski area. Both were caught, carried and partially buried, but no injuries were reported.
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.
A snowboarder triggered persistent slab avalanche on a north aspect of Roaring Ridge. February, 17th
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
Deep Persistent Slabs
Don’t confuse the lack of obvious signs of instability with safe avalanche conditions. Deep slabs often don't provide any warning before they are triggered. Though avalanches are difficult to trigger, they can still be dangerous. Snow profiles and snowpack tests show the persistent weak layer is gaining strength but still could result in a large and destructive avalanche. The culprit has been a layer of buried facets and surface hoar 3-5 feet below the snow surface and typically above a crust.
The snowpack structure is widespread on all aspects and elevations. In areas where the layer is deeply buried, you may not find obvious signs of instability such as whumphs or shooting cracks. Digging in the snow can help you locate where this layer may be present. This could be a good time to simply stay out of large, open slopes greater than 35 degrees where you could trigger one of these lingering deep persistent slabs. Stick to simple to challenging terrain where you have options to minimize your exposure or avoid avalanche paths altogether.
Prioritize information that indicates a potential for triggering avalanches over observations that point at relative stability. On Sunday at neighboring Stevens Pass, 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 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