Avalanche Forecast
Issued: Feb 21st, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Avalanche danger will increase during the day on Friday as another round of cold snow with strong winds affects the area. Expect sensitive wind slabs to form by the afternoon. Persistent slabs remain possible, especially further east. Choose conservative routes.
Summary
Discussion
On Thursday at Stevens Pass, skiers were able to easily trigger small soft slab avalanches that ran on a recently buried melt freeze crust, which was the new/old snow interface. These were 6” deep, and on Southeast aspects at 5,500ft on Lichtenberg Mountain. An observer on Tumwater Mountain reported lots of collapsing on Tuesday the 19th.
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.
Problems
Wind Slabs
Expect snowfall and winds to ramp up during the day on Friday, which will increase the avalanche danger. The Salmon la Sac and Teanaway will do better out of this storm than Blewett Pass/Mission Ridge area. As such, more dangerous conditions may be encountered further west. The storm is coming in cold, and staying cold. The current snow surface is weak in many areas, and on northerly aspects it is faceted. As new snow accumulates and the winds redistribute it, expect sensitive slabs to form quickly by the afternoon. On southerly aspects, these slabs may rest over a recently buried melt freeze crust, and may be more reactive. Steer clear of steep, open slopes when the snowfall rates and winds begin to pick up. Cracking in front of your sled or ski tips is a good sign to step back, and seek out lower angled and more sheltered 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: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
The February 8th facet/crust layer can be found from 1-3 feet beneath the surface on all aspects and elevations. This layer appears to be standing its ground for now, as tests continue to indicate the potential for avalanches to occur on it. The shallower snowpack zones closer to the Columbia River may have a weaker structure than nearer the crest. Here, you can find a very weak, almost entirely faceted snowpack on northerly aspects. In this part of the zone, the most dangerous slopes are where you find stiff wind drifts overlying weak facets.
The last big storm came through on the 12th and created lots of avalanches on this layer. Many slopes slid with very wide crowns during the cycle on the 12/13th, but not all did. At this point, the evidence is all but covered up. Continued light snowfall the past week have created what we call an "incremental loading pattern". It all adds up, just slowly over time.
Avoid large, open slopes 35 degrees and steeper, and minimize your exposure to avalanche terrain. Especially steer clear of steeper, unsupported slopes. Stop and re-group in safer terrain, well out from under overhead avalanche paths. Choose conservative up routes and descent options and when dealing with this tricky snowpack. The slopes should be considered guilty until proven innocent. A handful of sled or ski tracks on a slope doesn't mean that it won't go with the 6th, or 10th person.
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
Expected Size
Valid until: Feb 22nd, 2019 10:00AM
