Avalanche Forecast
Regions: Cascades - South West.
Be on the lookout for places where the wind affected the snow surface. It’s these wind loaded slopes where you are most likely to find trouble. If you think the wind drifted snow, avoid nearby slopes greater than 35 degrees. Even though it may be difficult to trigger deep, old, weak layers, you should consider the consequences of a deep persistent slab before traveling in large open avalanche terrain.
Discussion
Snow and Avalanche Discussion
On Wednesday, a skier triggered a large hard slab off the NW aspect of Mt McCausland near Stevens Pass and went for a 600ft ride. He was the second skier on the slope, 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 info as it becomes available. While this event is in a different forecast zone, similar conditions make this avalanche involvement extremely relevant.
A very small shot of new snow will not be enough to increase avalanche danger around the West-South zone Thursday. There are two pertinent pieces of information on our minds.
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Recent east winds reshaped the snow surface in many areas. You may find wind affect snow on a variety of aspects and elevations.
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Two shallow weak layers have been reported in the upper snowpack. You may find thin layers of facets and/or buried surface hoar preserved in the upper few feet of the snowpack. So, far there have not been any avalanches confirmed on either of these layers. That may change as winds drift snow into slabs over these weak snow layers.
A snow profile highlighting buried weak-layers in the surface snow. Photo:Â Dallas Glass
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
It doesn’t take much to blow the light, dry surface snow into wind slabs. Over the past few days, the winds impacted the West-South zone from a variety of directions. As a result, you may find reactive wind slabs on several aspects, especially at higher elevations. Keep your eyes open for areas where the wind deposited snow into thicker and firmer drifts. If you see smooth pillow-shaped features, fresh cornices, or find firmer snow surfaces, steer away from nearby slopes greater than 35 degrees.
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
Deep Persistent Slabs
The West-South zone saw the most recent activity on our deep persistent weak layer. However, even that activity is now more than a week old. So, why are we still talking about this avalanche problem? Recent snow profiles from the Crystal, Paradise, and White Pass areas all confirmed the presence a layer of weak sugar facets 3-5 ft below the snow surface. Many snowpack tests continue to show this layer can fail and produce avalanches. During conditions like this, be leery of places where the snowpack is thinner, the terrain is complex, or potential triggers are large. If you are planning to travel in large open avalanche terrain, stop and consider this low likelihood but high consequence scenario.
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: Possible
Expected Size: 1 - 2