Avalanche Forecast
Regions: Snoqualmie Pass.
Wind slabs formed by east winds and lingering weak layers of are keeping avalanches possible. You can trigger wind slab avalanches on open slopes at upper elevations, and large deep slab avalanches remain possible throughout the zone. Choose well-supported terrain and avoid large open slopes where avalanches can start.
Discussion
Snow and Avalanche Discussion
The Snoqualmie Pass area picked up about 11 inches of new snow since the 22nd. East wind is drifting snow at upper elevations. Observers reported loose dry avalanches on steep slopes over the weekend.
There are two layers in the upper snowpack worth keeping on your mind as you travel. On SE-SW aspects there is a thin sun crust surrounded by small weak facets just below the recent snow. In sheltered terrain, we’ve found a layer of buried surface hoar and/or facets down 18-24 in. Beyond the point releases, we haven't received reports of avalanches on these layers, but they could result in wind slab (or lingering loose dry) avalanches being more reactive than in other locations.Â
Wind transported snow suspended above the snow surface. 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
Expect continued east wind near Snoqualmie Pass, and along the Cascade Crest on Tuesday. Watch for drifts and pillows of snow on leeward sides of ridges. Avoid triggering avalanches in the upper snowpack, as they could step down deeper layers and result in a dangerous deep persistent slab avalanche.
East wind squeezed through the pass on Sunday and Monday. You may find minor drifting on open slopes near the pass level. The most likely place to trigger a wind slab avalanche will be near high elevation ridges on the leeward slopes.
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: Possible
Expected Size: 1 - 1
Deep Persistent Slabs
The February 8th facet/crust layer can be found about 3-4 feet beneath the surface on all aspects and elevations. It can be found closer to the surface further east in the I90 corridor. On the 20th, this layer is responsible for a few very large triggered avalanches near Crystal Mountain. While there have been few recent avalanches on this layer near Snoqualmie Pass, tests continue to show that triggering avalanches is possible.
Avoid large, open slopes 35 degrees and steeper, and minimize your exposure to avalanche terrain. Steer clear of steep, unsupported slopes, large avalanche start zones, and upper elevation slopes that received recent wind-loading. 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.
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: 2 - 2