Avalanche Forecast
Regions: Cascades - West.
Reactive storm slabs are expected where higher snowfall rates occurred in areas of convergence, creating a widespread avalanche problem at all elevations, which may be further elevated where recent winds have stiffened the snow. A persistent slab likely remains more sensitive in the southern parts of this zone. Minimize your risk in a complex avalanche picture by avoiding open slopes greater than 35 degrees.
Discussion
Snow and Avalanche Discussion
While storm slabs are the greatest concern in areas of convergence, winds likely transported significant low-density snow on Friday. The weather station at Mt Baker ski area recorded 20 hrs of moderate or stronger average wind speeds and the mountain loop area probably experienced similar conditions. That’s plenty fast to efficiently move snow.
Be leery of very steep slopes and any areas where small avalanches could bury you deeply such as gullies.
The snow from Friday will continue to add to the slab over a layer of buried surface hoar, facets, and/or crusts. At this point, this weak layer is likely nearing 1 ft deep in some areas. So far, we have not heard of any avalanches on this layer. Will that change? It’s tough to say. As the snow continues to stack up, keep this layer on your mind. You may encounter it in some more unusual locations such as lower and mid-elevation areas.
Other areas in the Washington Cascades are still suffering from a similar persistent slab problem. There have been three recent very large avalanches reported in the Crystal area. While that may seem like a significant distance away, they serve as a reminder to not totally let the persistent weaklayer drop from your radar.
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
Storm Slabs
Bands of heavy convergence zone snow have been focusing on the Cascades East Central zone. These bands produce the higher snowfall rates that can quickly produce a storm slab that you will be able to trigger. If you find new snow greater than 8” deep, expect this snow to be reactive on slopes steeper than 35 degrees or unsupported slopes. These avalanches have significant snow to entrain, so even 8” of snow can produce a large avalanche.
You might also find a layer of wind-affected snow under the lower-density snow that may create a bed surface for storm slabs or may release as a wind slab. Pole probes will help determine whether a wind layer is in your terrain. If you find this layer, seek wind-sheltered terrain or trees.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
Persistent Slabs
The persistent weak layer has been quiet this week. That’s good news and gives us some confidence. However, there are still enough pieces of evidence pointing towards weak snow layers that this problem is still on our minds. You aren’t likely to see any obvious signs of instability. The only way to see this problem is to dig in the snow. You may find a layer of facets and/or surface hoar 2-4ft below the snow surface. If you suspect this layer is present, it’s best to simply stay off of large open slopes greater than 35 degrees.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1