Avalanche Forecast

New snow amounts from Friday night/Saturday vary widely across the region. The Salmon la Sac appears to have won that round with 8" of snow and 0.6" of snow water equivalent. More snow is likely to be found at higher elevations in that area. I found about 4" at 4,000ft in Icicle Creek, and Blewett Pass/Mission Ridge came in at just a couple of inches. A fair amount of uncertainty exists with the weather forecasts for Sunday, but in any case the snow should begin to pile up. On Friday, I went into the hills above Number 2 Canyon, finding a poor snowpack structure on northerly aspects in sagebrush country. The large facets near the ground have changed little since my last time in similar terrain about 10 days ago. Slab avalanches remain possible on this weak basal structure, and a recent avalanche was observed in Swakane Canyon on a North aspect at 1,900ft on this same layer (see photo below). On Thursday at Stevens Pass, skiers were able to easily trigger small soft slab avalanches that ran on a recently buried melt freeze crust, which was the new/old snow interface. These were 6” deep, and on Southeast aspects at 5,500ft on Lichtenberg Mountain. This same recently buried crust is likely to be found in the western part of the zone in areas such as the Salmon la Sac, Teanaway, and Icicle. The last reported collapses in the area came from an observer on Tumwater Mountain on the 19th, though a few were reported from Arrowhead in the eastern Hwy 2 corridor on Saturday. 

D2 Persistent Slab avalanche in Swakane Canyon that ran on basal facets near the ground. North at ~1900ft. Unknown when it occurred, perhaps remotely triggered by a skier ascending the rib nearby. Observed on 2/22. Photo: Steven Gnam

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.