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East Slopes South - South of I-90 to Columbia River

Issued: 6:19 PM PST Wednesday, February 20, 2019
by Robert Hahn

This area received more new snow and wind than areas further north, elevating the risk of slab formation near and above treeline. You can also 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. If you experience cracking in the snow, collapsing, or snowpack tests indicating that you can trigger avalanches, stay off slopes steeper than 35 degrees.

Danger Scalei
  • No Rating (Info Avail)
  • Low (1)
  • Moderate (2)
  • Considerable (3)
  • High (4)
  • Extreme (5)

Snow and Avalanche Discussion

Two recent Avalanches in the neighboring Crystal Mountain area remind us that while you may not see evidence of recent avalanche activity in your terrain, a low-probability high-consequence deep slab threat is very real.

On Wednesday, Crystal Mountain Pro Patrol triggered a very large (D3) 3-4 ft slab avalanche in an infrequently skied chute on the SE side of the King . The slab released on facets sandwiched between two crusts.

The King, SE Aspect, Crystal Mountain Area. Photo Credit: Crystal Mountain Ski Patrol.

Another recently triggered avalanche occurred Saturday in the Crystal Lakes backcountry near Crystal Mountain. This avalanche was triggered by the third skier on the slope and released on the persistent layer above the 2/8 crust. Fortunately, no injuries occurred.

Moderate winds in some locations were forecast to build fresh wind slabs with 3-6” of snow fallen overnight Tuesday. While shallow new wind slabs will form the most recent avalanche problem, you should be aware that a small avalanche is the easiest way to trigger a large and deep persistent week layer.

Snowfall since February 8th is estimated to be 2-3 ft based on area Snotels and 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.

In addition to avalanches, the deep snow has hazards of its own such as Snow Immersion Suffocation, tree well hazards, and roof avalanches. 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 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 2/12 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 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.

No Corresponding Mountain Weather Forecast Available