Avalanche Forecast

Snow and Avalanche Discussion

Parts of this zone are very near the Crystal Mountain area, but all parts of this zone have similar weak snowpack structures which have produced a series of life-threatening avalanches in the Crystal backcountry. Two were triggered by backcountry travelers and were near-misses. Deep or persistent slabs like we are dealing with are not easy to assess. They can lull you to sleep, and the snow can “feel safe.” These avalanches highlight that it is not. Don't let the zone boundary fool you, the same life-threatening avalanches could easily occur in this terrain. We likely lack the usage patterns that lead to more regular triggering in the East South zone.

With new storm snow, you may encounter loose dry avalanche in very steep terrain. Be leery of these easy to manage avalanche problems if you are traveling near slopes where even small avalanches can hurt you.

Snowfall since February 8th incremental loading has brought the snow depth close to 3 ft based on snotels; a slab may be resting on an old weak snow/firm crust sandwich, creating ripe conditions for persistent slab avalanches. Any terrain, even low elevations, that had a shallow snowpack prior to this cycle should be treated as potential avalanche terrain (think northerly aspects with sagebrush). The likelihood of triggering a life-threatening avalanche continues to gradually decrease, but the high consequences should shape your terrain choices such that you choose lower angled and supported slopes.

We do know that weak snowpack structures don’t handle stress well, so when winds increase on Friday and form slabs, this additional loading of certain slopes may incrementally load slopes to near their tipping point or a small wind slab might trigger the large and deep persistent week layer.

In addition to avalanches, the deep snow has hazards include Snow Immersion Suffocation and tree wells. Travel in the mountains with partners and keep them in sight.

Forecast Schedule and No Rating

At this time, we do not have enough specific snowpack information to issue an avalanche hazard rating for the East Slopes South zone. However, even when No Rating is applied, applicable avalanche conditions and backcountry travel advice will be provided throughout the season. When weather systems produce very dangerous avalanche conditions in adjacent zones, NWAC will issue an avalanche warning for this zone as well.

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.