Avalanche Forecast
Regions: Mt Hood.
715 AM Update: Forecast Below Treeline raised to Considerable. Continued wind combined with fresh snowfall will create larger and more dangerous slabs than Friday and where more than 8” of snow falls, snow could be reactive on all aspects. A deep slab lurks as a low-likelihood - high consequence threat, rounding out a complex avalanche picture. A simple solution is to avoid open slopes greater than 35 degrees.
Discussion
Snow and Avalanche Discussion
On Thursday, Pro observer Nick Asher looked for persistent weak layers in below treeline terrain (5300 ft) on a N aspect. He found facets below a crust (but not above) down almost 4 ft that were stubborn to trigger, but were reactive with a significant forcing.
On Wednesday, Mt. Hood Pro Patrol found advanced facets below the crust, as observed in bounds near and below treeline on sheltered northerly terrain. The facets were between 2 and 10 cm thick over a crust just over 3 ft below the surface. Some of these facets survived heavy skier traffic.
Nick also found some reactivity on a layer of preserved stellar crystals (18†down) and a zipper crust (12†down). Otherwise, Nick found a relatively right-side-up snowpack above the crust.
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.
New low-density snow increases the threat for Snow immersion and Suffocation and tree wells.
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
Wind Slabs
715 AM Update: Forecast Below Treeline raised to Considerable. Winds and snowfall were heavy enough Friday night that wind slabs have likely formed on open slopes below treeline. Look for recently wind-stiffened snow and stick to lower angled and treed terrain.
Winds intensified Friday morning and will continue to transport recent and fresh snow through the day Saturday. These slabs are expected to be large and dangerous, particularly near and above treeline. Where 8” or more of storm snow falls in wind-protected terrain below treeline, storm slabs can be triggered on any aspect.
Where wind slabs linger near treeline and overlap with the persistent slab potential, don't thread the needle between these avalanche problems and stick to slopes 30 degrees or lower. A triggered avalanche may step down into deeper weak layers to create deadly avalanches, so think seriously before testing larger slopes.
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: Likely
Expected Size: 1 - 1
Deep Persistent Slabs
The crust/facet layer is getting deep, but just because it is out of sight does not mean it should be out of mind. Fresh snow and wind continues to incrementally load the weak snow. If or when it will fail, we don't really know. You may find this layer in your terrain by digging a deep snow pit or if it releases on you. You are unlikely to experience signs of reactivity (cracking, whumphing) until it is too late. This low-probability, high-consequence problem remains best managed by avoiding large avalanche terrain and steep slopes.
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: Possible
Expected Size: 1 - 2