Avalanche Forecast
Regions: Cascades - South West.
A storm overnight Tuesday will build new wind slabs on all terrain exposed to strong winds. 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.
Discussion
Snow and Avalanche Discussion
The most recent triggered avalanche in West-South zone occurred Saturday in the Crystal Lakes backcountry near Crystal Mountain. See photo one. This avalanche was triggered by the third skier on the slope and released on the persistent layer above the 2/8 crust. Fortunately, no injuries occurred.
Tests on this layer show a trend towards strengthening and the distribution is not widespread. However, snow profiles and snowpack tests continue to highlight a weak snowpack structure. Depending on where in the zone you are, the 2/8 crust interface is 3 to 5 feet down. Significant east wind events that scoured windward snow surfaces over the last week have added to the spatial variation surrounding this interface in this zone. So the ingredients for a destructive persistent slab avalanche remain in place, but are becoming harder to find and to trigger over time, notwithstanding the incoming storm!
Snowpack Discussion
February 19th, 2019
Recap
We’re now over a week out from a major winter storm and avalanche cycle, February 9-13th, 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 different zones and even within the individual zone. 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.
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. In the days after the natural cycle all observations, including rumbling collapses, remote triggering, and snowpack tests screamed, “avalanche!â€
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 and Mixed Messages
As the facets and surrounding snowpack structure changes, we’re seeing the potential for triggering avalanches change, as well. Now, the likelihood of triggering persistent slab avalanches is decreasing. In some regions, the problem is trending to unlikely. Meanwhile, the consequences (size and destructive potential) remain significant, if not the same.
Time has helped round the February 8th facets. No longer will every clue give a resounding answer as to whether or not you can trigger an avalanche. To complicate things, observations like snowpack tests can be notoriously difficult to interpret, requiring a lot of time practicing good snow-craft. Snowpack tests often don’t give us a clear “go or no-go†answer, if such a thing exists.
Q: How do we interpret observations that are contradictory, when some point at the potential to trigger avalanches and others indicate better stability?
A: Focus on the observations that show the potential to trigger avalanches. Look for obvious clues, like recent avalanches, shooting cracks, or collapses. Prioritize observations that indicate triggering (initiation) and propagation.
A natural persistent slab avalanche (D2), likely occured on 2/12Â on southwest through southeast aspects of Windy Mountain at 5,400ft in the Tye River drainage. Photo: Dan Veenhuizen.
Case Study
In a recent profile, east of Stevens Pass, I found the February 8th facets (0.5-1.5mm) rounding and buried 59cm from the surface. The results of the profile were:
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 pointed towards triggering an avalanche was likely, but some did not. Confusing, right?Â
With all of this data in my head, it was the collapse that stuck out. That was enough evidence for me to avoid slopes steeper than 35 degrees. That was a more obvious answer than all the other pieces of 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. If only snowpack tests would give you the sinking feeling of almost triggering an avalanche that you get from a rumbling collapse...
Avalanche Problems
Wind Slabs
Increasing winds and snow above treeline will build fresh wind slabs overnight Tuesday. Observe where winds are transporting new and recent snow, looking for textured snow surfaces, drifting, or fresh cornices. If you see these features, expect wind slabs on nearby slopes. You can avoid triggering a wind slab, by steering around wind loaded slopes greater than 35 degrees. Storm snow will be easily transported by any winds and may form wind slabs capable of being triggered even below treeline on Wednesday.
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: Likely
Expected Size: 1 - 1
Persistent Slabs
The most recent evidence indicates the slab/weak layer combination needed for a persistent slab avalanche may not widely exist throughout the West-South zone.
Its spotty distribution is part of what makes this layer so difficult to manage. The other is depth. In most locations, a layer of buried facets and/or surface hoar may be found 3-5 ft below the snow surface just above a very stout and thick crust. These avalanches are probably very tough to trigger, but any avalanche on this layer would be very large and destructive. However, on slopes that have already avalanched on this layer, reloading by new or wind transported snow may make it easier to trigger.
You aren’t likely to find obvious signs of instability such as whumphs or shooting cracks. Only digging in the snow can help you locate where this layer may be present. As we enter a new storm cycle and we put more stress on the snowpack, this continues to be a good time to simply stay out of large, open slopes greater than 35 degrees where you could trigger one of these lingering persistent slabs.
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 - 2