Avalanche Forecast
Regions: Mt Hood.
Overlapping new and old snow avalanche problems on Sunday will translate to dangerous avalanche conditions. Enjoy the new snow by choosing simple terrain and by steering clear of slopes capable of producing large avalanches.
Discussion
Snow and Avalanche Discussion
Professionals digging in the snow and looking for the 2/8 weak snow/crust combination in the Mt. Hood Meadows area are finding it roughly 4 feet down on N-E aspects and in a rather narrow elevation band around 5300’-6000'. While this is very useful and specific information, we can't extrapolate this to the larger Mt. Hood area.Â
With Mt. Hood in the bulls-eye of the storm track, the avalanche danger will largely be driven by new snow concerns over the next few days. However, continue to think about the lingering deep slab potential when choosing terrain. Â
Join those that contribute to the process of backcountry safety and take a moment to submit an observation for the Mt. Hood area, especially for places less traveled.
With all the low-density snow piling up, the chance for Snow immersion and Suffocation continues. Keep your partner in sight if it's deep in the trees.Â
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
The faster and deeper the snow piles up Saturday night, the more likely you’ll find storm slabs Sunday. You are most likely to trigger storm slabs on unsupported and very steep slopes. Storm slabs may fail on instabilities within the new snow or become larger where they bond poorly to old snow interface. Look for signs of natural slab avalanches and use small inconsequential test slopes to gauge the storm slab potential. You can avoid storm slabs by sticking to lower angled and moderate slopes.
In areas that don't have a storm slab problem, loose dry avalanches are possible. Think about sluff management on very steep slopes and what would happen if a small but fast moving loose dry avalanche pushed you into or over a terrain trap.
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
Wind Slabs
Wind slabs formed Friday and Friday night on Mt. Hood near and above treeline. You may still be able to trigger a wind slab avalanche in specific areas on Sunday. Cold temperatures will slow the healing process for this avalanche problem. The other catch is the new snow expected Saturday night and Sunday may fall with light winds, hiding older wind slabs.
Avoid terrain above 35 degrees where wind slabs may have recently formed. If you can't tell if there is a wind slab lurking on a specific aspect, then choose lower angled terrain.
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: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Deep Persistent Slabs
The 2/8 persistent weak layer/crust combination we have been tracking is only getting deeper with each passing storm and there haven’t been any reported avalanches on this layer in over a week. However, the snowpack structure remains the same and with deep persistent slabs, long stretches of time may pass between triggered avalanches.
You are unlikely to experience warning signs like cracking or whumphing given its depth. It will probably take finding a shallower spot in the snowpack where you can trigger an avalanche on this layer or a smaller avalanche that steps down to this interface.
Continue to let this low-likelihood/high consequence avalanche problem shape your terrain choices, steering you away from terrain capable of producing large avalanches.
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.
Elevations: Treeline, Below Treeline.
Likelihood: Unlikely
Expected Size: 1 - 2