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Mt Hood

Issued: 6:19 PM PST Wednesday, February 20, 2019
by Robert Hahn

A Tuesday-Wednesday storm created energetic and far-running 1-1.5 ft slabs that you can still trigger on Thursday. Avoid steep open slopes where evidence or suspicion of receiving wind transported snow exists. There remains a potential for a low-likelihood - high consequence persistent slab avalanche that should continue to affect your route choices, steering you away from slopes capable of producing large avalanches.

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Snow and Avalanche Discussion

As recently as Monday, February 18th, Mt. Hood pro-patrol is finding the same weak snow (1-2 mm facets) over a hard crust on northerly aspects below treeline roughly 3 feet below the snow surface. What has changed is that we haven't had an avalanche release down to this depth, human or explosive triggered, in several days over the busy holiday weekend.

The addition of new loading from the current storm, the ingredients for a destructive persistent slab avalanche remain in place, and may make triggering a large avalanche more likely, despite the recent downward trend of these avalanches.

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.

Other notable hazards on Thursday:

  • Snow immersion and Suffocation and tree wells are a very real threat given all the recent low-density snowfall.

  • 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.

  • 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.

Avalanche Problems for Thursday

Wind Slabi

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Moderate to strong WNW winds formed reactive wind affected soft slabs in the Mt. Hood area that were 1 ft or more deep and easily triggered using skis. Given the cold temperatures, these are likely to be reactive another day. Look for slightly firmer snow near the surface as an indication that wind slabs may be in your terrain, but expect them to be widespread near treeline and above (perhaps isolated in exposed terrain features below treeline).

 

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.

 

Persistent Slabi

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The snowpack structure remains intact for persistent slab avalanches around 3' in depth near and especially below treeline, but they have become harder to find and less likely to trigger over time. We haven't had any of the confirming observations that have been reported in other parts of the WA Cascades, like whumpfing as the weak layer collapses or even human triggered avalanches.  

The recent 16” or so of storm snow load may have helped to activate this layer and increase the likelihood of triggering a large and destructive avalanche. The potentially high consequences should shape your terrain choices such that you continue to choose lower angled and supported slopes.

 

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 2/12 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 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.

No Corresponding Mountain Weather Forecast Available