Avalanche Forecast
Regions: Cascades - South West.
Recent snow and strong recent easterly winds will maintain dangerous avalanche conditions Tuesday. Wind slabs formed over the past few days will be slow to settle. You can trigger wind slabs on steep open slopes on a variety of aspects. Choose conservative, simple terrain with minimal exposure to slopes capable of producing large avalanches where new and old snow avalanche problems may overlap.
Discussion
Snow and Avalanche Discussion
The most recent observation on Mt Rainier Monday shows the nature of the wind slab problem and some of the potential weak layers and bed surface that exist in the upper snowpack.
There are two persistent weak layers, buried surface hoar and/or facets, roughly 10 and 20 inches below the snow surface respectively. While we haven’t heard of any avalanches on these layers, that may simply be due to a lack of a slab. These layers were identified in the White Pass area on Monday in snow pits above 5000'. Strong easterly winds and light snow on Monday have continued to build a firmer slab over these weak layers. Â
In very steep non-wind affected terrain, think about the consequences of small but fast-moving loose dry avalanches near terrain traps. With all the low-density snow recently, the risk of a Tree well/Snow immersion and Suffocation accident continues.
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
Strong E-NE winds Monday and new snow easily transported low-density snow, forming fresh and sensitive wind slabs. Wind slabs may have formed in open terrain below treeline. The most recent wind slabs have likely formed in unusual aspects so watch for signs of wind transported snow on all open slopes. Wind slabs may be hidden below shallow new snow, so best to look for stiff wind slabs below.
Best to choose simple terrain as these wind slabs will be slow to heal under this cold weather pattern. Avoid slopes steeper than 35 degrees where wind slabs have likely formed, using visual clues like textured snow, blowing snow and fresh drifts. Feel for firm or hollow snow. If you can't tell which slopes have been loaded, choose lower angled 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: Likely
Expected Size: 1 - 1
Deep Persistent Slabs
The West-South zone has been the most active avalanche area over the last week regarding this avalanche problem. There have been several very large avalanches reported from the Crystal area. Don’t let the lack of recent avalanche observations from other areas fool you. A very similar snowpack structure has been reported from Paradise, the Tatoosh, and White Pass areas. You aren’t likely to see obvious signs of unstable snow. The only way to observe this deep weak layer is to dig. Snow profiles and snowpack tests may highlight a layer of buried facets just above a very stout crust 3-5 ft below the snow surface. When conditions are like this, its best to continue to avoid large open slopes greater than 35 degrees.
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