Avalanche Forecast
Regions: Snoqualmie Pass.
New snow and wind will build fresh slabs on Wednesday and further stress deeper weak layers. Recreationists and avalanche workers have triggered slab avalanches on Snoqualmie Pass in the past three days. Stay out of large avalanche paths and put lots of space between yourself and large, open slopes 35 degrees and steeper.
Discussion
Snow and Avalanche Discussion
Expect around 1 foot of low-density snow Tuesday night with moderate southwest to northwest wind, strong enough to form fresh wind slabs. You may be able to trigger small dry loose avalanches below treeline where you find more than 6 inches of new snow.Â
Don’t confuse the lack of obvious signs of instability with safe avalanche conditions. On Sunday the 17th, a snowboarder was partially buried in a persistent slab avalanche on a north aspect of Roaring Ridge at 4,600ft. On Saturday and Sunday, Alpental patrol triggered several persistent slab avalanches in their back bowl terrain. A few of these of these occurred at a slightly lower elevation (4000’) and slope angles (30-35 degrees). Snow profiles and snowpack tests from this zone continue to highlight the weak snow structure.
Many zones in the Cascades contains a similar potentially dangerous snowpack setup. Another compelling piece of information we have concerning the persistent slab problem is from Stevens Pass. On Saturday, two snowboarders triggered a persistent slab avalanche in Tunnel Creek outside the boundary of Stevens Pass ski area. Both were caught, carried and partially buried, but no injuries were reported.
In addition to avalanches, the deep snow has hazards of its own such as Snow Immersion Suffocation, tree well hazards, and roof avalanches. Don't linger beneath roofs, travel in the mountains with partners and keep them in sight.
A snowboarder triggered persistent slab avalanche on a north aspect of Roaring Ridge. February, 17th
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 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.
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
New snow and moderate winds will form fresh slabs on leeward slopes overnight. The storm will taper off early on Wednesday, but the chances of triggering a wind slab avalanche will remain.
This isn't a time to trigger avalanches in the new snow. While avalanches due to new snow and wind may typically be more predictable, today they could easily step down to a much more dangerous persistent slab. This isn't a gamble I would be willing to take. Instead of just steering around the wind features, I would fully avoid any freshly wind-loaded slope. Give any slope even more space than you would under normal wind slab conditions. Watch for wind slabs on leeward slopes near ridges where you see thickly drifted and stiffer snow on the surface.
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
New snow and wind may be just enough to "re-activate" persistent slabs, making them easier to trigger. Snow profiles and snowpack tests show the persistent weak layer is gaining strength, but still could result in a large and destructive avalanche. The culprit has been a layer of buried facets and surface hoar 3-5 feet below the snow surface and typically above a crust.
The snowpack structure is widespread on all aspects and elevations. Another round of snow and wind in the next 48 hours will build more fresh slabs. In areas where the layer is deeply buried, you may not find obvious signs of instability such as whumphs or shooting cracks. Digging in the snow can help you locate where this layer may be present. This could 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.
These avalanches should be considered guilty until obviously innocent. Prioritize information that indicates a potential for triggering avalanches over observations that point at relative stability. On Sunday at neighboring Stevens Pass, I poured over the snowpack identifying grains and performing many tests. After emerging with contradictory test results, we experienced a large rumbling collapse 2000ft from the profile site. The collapse was all the information I needed to avoid avalanche terrain.
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