Avalanche Forecast
Regions: Stevens Pass.
Recent large wind slab avalanches should be at the forefront of our minds. The recent wind events, along with cold temperatures have sculpted the landscape. You can trigger wind slab avalanches on open, exposed slopes at all elevations. Very large, deep slabs are not out of the question yet. Seek out sheltered, well-supported terrain and minimize your exposure to large open slopes where avalanches can start.
Discussion
Snow and Avalanche Discussion
A couple of easterly wind events with the most recent one tapering off on the 27th have shaped the landscape and have apparently initiated a round of wind slab avalanches. On Wednesday a skier triggered a large hard slab off the northwest aspect of Mt McCausland and went for a 600ft ride. He was the second skier on the slope. The skier pulled his airbag and ended up on top of the debris. Luckily the skier was ok and got away with just losing a pair of poles. This was in a couloir feature at 5,600ft in the near treeline elevation zone, and the crown was 18-24" deep. We will pass along more information as it becomes available. Another slide was reported in the Stevens Pass backcountry from the 26th on the obs page here. The recent storms buried a mix of surface hoar and near surface facets on shaded aspects, and melt-freeze crust on southerly aspects. These cold temperatures are inhibiting settlement within the upper snowpack.Â
Wind transported snow suspended above fresh drifts. Monday, 2/25, 5,000ft, SSE, Lichtenberg Mountain, Stevens Pass. Photo: Josh Hirshberg
Snowpack Discussion
February 24th, 2019
The Status Quo
As we look at the avalanche conditions the phrase “Status Quo†keeps coming to mind. We use this term in the avalanche industry to note periods of no substantial changes, where observations continue to support the current avalanche danger, and our terrain use doesn’t appreciably change. We’ll look at recent avalanche conditions with this lens.
Avalanches
Following several large winter storms just prior to Valentine's Day, we saw very active avalanche conditions. With a few notable exceptions, most of the avalanche activity occurred more than a week ago. This lack of activity has allowed us to gain some confidence with the persistent weak layer buried on February 8th. You can see this reflected in the progression in the likelihood of persistent slab avalanches from “Likely,†to “Possible,†to “Unlikely.†The timing of this progression has been different depending on the zone.
The recent exceptions were two very large avalanches in the Crystal Mountain area on February 20th. During avalanche mitigation work, Crystal Mountain Ski Patrol triggered a slide on a SE aspect at 6300 ft in a less often skied portion of their ski area. On the same day, a skier triggered a persistent slab on a W aspect at 6300 ft in the backcountry adjacent to the ski area. Both were 3-4 ft deep. It’s hard to ignore these two obvious signs of instability.
Feb 20, 2019: SE 6300’: SS-AE-D3-R3-O. Photo: Crystal Mt Ski Patrol
Snowpack
Snow profiles and snowpack tests have been coming in from nearly every region. This is great! It helps us put together a more complete picture of the snowpack structure. Observers continue to report a layer of facets 2-5 feet below the snow surface and just above a crust. These facets are showing signs of rounding (gaining strength). In the past week, snowpack tests have become more variable. Some tests are indicating triggering an avalanche and crack propagation (necessary for a slab avalanche) are becoming less likely. That said, we are still seeing other tests that indicate that triggering deep persistent slabs remains a possibility. More tracking of this trend is needed to concretely illustrate the trend.
The Feb 8 facet layer is 3-5ft deep in the Snoqualmie Pass area. Â Photo: Susie Glass
Weather
Our recent weather systems have not been big water producers, especially by Cascade standards. However, the colder than normal temperatures have produced light, low-density snowfall. Wind transported snow have been the main driver of slab avalanches in the new snow. These storms have been large enough to keep the avalanche danger elevated, but not enough for major spikes in danger.
Recent low-density snow drifted by the wind to form fresh slabs Photo: Dallas Glass
Looking Ahead
At this point avalanches on persistent weak layers can’t be fully ruled out on specific aspects or elevations in most zones. Your chances of triggering an avalanche on a persistent weak layer are lower than they were on February 13th, but the change from day-to-day will remain slow and incremental. As a result, you’ve seen slow changes in the avalanche danger in most zones. Due to the serious consequences and the uncertainty that these avalanches present, we will continue to operate with a “Status Quo†mindset. Don’t let this lack of change lull you to sleep.
When will the conditions change? Not tomorrow, but possibly within the next week or two. Keep checking the forecast. Make sure to read the Snow and Avalanche Discussion and the Avalanche Problem text so you can stay up to date with any changes as they arise.
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Avalanche Problems
Wind Slabs
The recent skier triggered accidental wind slab avalanche is bullseye information. Continue reading if you'd like advice to help avoid getting into a similar situation... The recent wind events have drifted low density, cold snow into hard wind slabs on open, exposed features. These slabs may be resting on a layer of near surface facets, surface hoar, or facets over a melt freeze crust. It may be easier to find wind slabs than to avoid them at the moment. Expect slabs on open slopes above, near and even below treeline. Areas exposed to, and that funnel east wind over the Cascade Crest are likely to have thicker and more dangerous slabs. Watch for uneven snow surfaces, hard drum-like sounding snow, and lens shaped drifts. Don't get lured out onto a wind slab on a slope steeper than 35 degrees, as they make break above you. Avalanches within upper snowpack layers have the potential to step down and create much wider persistent slab avalanche. Seek out sheltered terrain, thats where the quality powder is, anyway.
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: Possible
Expected Size: 1 - 1
Deep Persistent Slabs
It's now difficult, but not impossible, to trigger avalanches on facets buried in early February. As the weak layer gains strength, the evidence that you can trigger these avalanches is getting harder to find. If you're in the wrong place, you may get more "evidence" than you bargain for. While some people are beginning to "punch it", I'm still holding back and choosing my terrain cautiously.
The most likely place to get one of these to release would be on shallow, unsupported features and large upper elevation start zones that have been recently wind loaded. Stop and re-group in safer terrain, well out from under overhead avalanche paths. Choose conservative uphill routes and descent options to minimize your risk.
The February 8th facet/crust layer can be found about 2-4 feet beneath the surface on all aspects and elevations. On the 20th, this layer is responsible for two very large triggered avalanches near Crystal Mountain. The last reported avalanche on the weak layer in this zone occurred on the 16th in the Tunnel Creek drainage.
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 2 - 2