Avalanche Forecast
Regions: Cascades - South West.
Tuesday’s storm brought strong winds and wind slabs to some areas (particularly White Pass) and more snowfall than areas further north, keeping the avalanche danger elevated. A very large deep persistent slab avalanche triggered in the Crystal area highlights the low-likelihood - high consequence problem that should continue to affect your route choices, steering you away from slopes capable of producing large avalanches.
Discussion
Snow and Avalanche Discussion
Two recent Avalanches in the 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. 02/20. 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.
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Snow profiles and snowpack tests continue to highlight a weak snowpack structure and reactive snow in some locations, but we can’t say exactly which elevations or aspects you will find this reactivity. Depending on where in the zone you are, the 2/8 crust interface is 3 to 5 feet down. Significant east wind events that scoured windward snow surfaces over the last week have added to the spatial variation surrounding this interface in this zone. The ingredients for a destructive persistent slab avalanche remain in place and this should influence your decision-making.
Other notable hazards on Thursday:
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Snow immersion and Suffocation and tree wells are a very real threat given all the recent low-density snowfall.
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Small loose dry avalanches have been running long distances, particularly below treeline. Don’t let such a slide pull you over a cliff or into a terrain trap where it can bury you deeply.
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If the sun comes out on Thursday, expect aspects facing the sun to produce small Loose Wet avalanches within the surface snow, frequently originating off rocks.
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
Wind Slabs
This avalanche problem is highly variable throughout the zone. Strong westerly winds during Tuesday’s storm likely created large wind slabs that are relatively fresh and easy to trigger. In the Paradise and Crystal areas, lower wind speeds were observed and you will find isolated slabs covered by low-density surface snow. Look for firmer layers that you can punch through with your pole and avoid potentially wind-loaded slopes near ridgelines, particularly in the White Pass area. You can avoid triggering a wind slab by steering around wind loaded 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
In most locations, a layer of buried facets and/or surface hoar may be found 3-5 ft below the snow surface just above a very stout and thick crust. These avalanches are probably very tough to trigger, but recent avalanches have proven to be very large and destructive. On slopes that have already avalanched on this layer, reloading by new or wind transported snow may make it easier to trigger.
You aren’t likely to find obvious signs of instability such as whumphs or shooting cracks. Only digging in the snow can help you locate where this layer may be present. Given recent avalanche activity, choose terrain where you have options to mitigate your exposure to large open slopes greater than 35 degrees where you could trigger one of these lingering persistent slabs.
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