Avalanche Forecast
Regions: Cascades - South West.
You’ll have two avalanche problems competing for your attention Friday in the West-South zone, newly forming wind slabs and old deep persistent slabs. Even though these overlapping problems can seem daunting, the solution can be simple. Avoid traveling on open slopes greater than 35 degrees.
Discussion
Snow and Avalanche Discussion
The West-South zone has been the most active area for avalanches in the past few days. Two very notable events occurred near Crystal Mt on Wednesday. Crystal Mt Ski Patrol triggered a very large avalanche on a SE aspect, 6300’ during avalanche mitigation work. This area sees light skier traffic and may be more pertinent to backcountry conditions.
Also on Wednesday, 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.Â
Looking down on the crown of a skier triggered avalanche near Crystal Mt. Photo: Crystal Mt Ski Patrol
The snow in the West- South zone is deep. Deep snow immersion and tree-well hazards are very real and potentially deadly. Keep your partners in sight and be leery of traveling close to small trees.
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
There is plenty of soft snow around the West-South zone available for the wind to move. As the winds increase on Friday, we expect wind slabs to begin to form. As these new slabs get larger they will become easier to trigger and more consequential. Look for signs that the wind affected the snow. Snow drifts, blowing snow, and the snow knocked out of the trees, all indicate that wind slabs are likely forming. When you see observations like these, avoid traveling on open slopes greater than 35 degrees.
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
There isn’t much clearer sign of unstable snow than the recent avalanches near Crystal Mt. Even though this slab-weaklayer combination may not exist in all areas, these recent events highlight the magnitude and potential consequences of finding this lingering beast. Don’t get distracted by the new snow. Continue to avoid large open slopes greater than 35 degrees. Be particularly leery of areas where the snowpack is shallower or the terrain is complex. The only way to observe this weak layer is to dig in the snow. If you do, you may find a layer of buried facets and/or surface hoar just above a thick and firm old snow surface.
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: 1 - 2