Avalanche Forecast
Issued: Dec 21st, 2018 10:27AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Deep Persistent Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
We aren’t out of the woods yet, but it’s becoming harder to trigger a very large deep slab avalanche. Large loads like a cornice failure or finding shallow spots in the snowpack are exceptions to think about. Any avalanche failing on a deeply buried weak layer is likely to be very large, destructive and deadly. Limit your exposure in terrain where avalanches start, run and stop.
Summary
Discussion
Several large or very large natural, skier, and explosive triggered avalanches occurred in the Crystal area Monday through Wednesday. Many of these avalanches acted in surprising ways including wrapping around terrain features, running long distances, and responding to relatively small triggers.
Thursday’s storm brought rain and high winds to West Slopes South zone. On Friday, professional observations from the Crystal backcountry and the Tatoosh Range (south of Paradise) revealed no new significant avalanche activity following Thursday’s storm. We don’t have any information about this layer in the White Pass area.
A supportable rain crust was found at higher elevations while the snowpack was still refreezing below treeline. While the 12/9 persistent weak layer was identifiable in snowpits in both areas, it was rounding and gaining strength. Several intermittent crusts could be found above the 12/9 layer in the upper snowpack. Near treeline, only shallow wind slabs were found on isolated lee slopes. However, recent winds have caused wide variations in snow depths - something worth noting when thinking about deep persistent slab avoidance.
Snowpack Discussion
Regional Synopsis: December 19, 2018
Why has the avalanche danger been so high for so long? Two reasons: A very active and wet weather pattern combined with a widespread persistent weak layer.
For perspective, the approximate snow totals from 12/9-12/19 are:
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Mt Baker: 102”
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Washington Pass: 55”
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Stevens Pass: 76” mid-mountain
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Snoqualmie Pass: 68” mid-mountain
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Crystal Mountain 70” Green Valley
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Paradise: 78”
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Mt Hood Meadows: 44” mid-mountain
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Olympics: 48”
In many areas, a layer of buried surface hoar and/or weak sugary facets was buried on December 9th. This layer has been the cause of numerous natural, explosive, and skier triggered avalanches. As this layer gets deeper it gets harder to assess. In short, the scenario is tricky and getting more dangerous by the day.
Higher snowfall totals along the Hwy 542 corridor/Mt Baker area have driven several avalanche cycles during this period. Loading from recent storms has been more incremental to the east and south of the Mt. Baker area, so we have not yet experienced a widespread cycle in other areas.
When will we reach the breaking point? It’s hard to say. What we do know is we have a deep weak layer, reports of very large explosives triggered slides at Mission Ridge and Crystal Mountain, and more storms on the way.
Be patient and continue to stick to lower angle slopes with nothing above you. This is a good time to avoid areas where avalanches can start, run, and stop.
We’d like to thank all of you who have sent NWAC your observations. If you are out in the mountains, let us know what you see.
Problems
Deep Persistent Slabs
As recently as Wednesday, a persistent weak layer consisting of buried surface hoar and/or facets has been the culprit in several very large avalanches in the Crystal area. This layer is 3-6 feet below the snow surface in many areas. You are most likely to find the buried persistent weak layer above 6000 ft in this region. While these avalanches are likely to start higher in the terrain, they are running down to much lower elevations.
Recent observations strongly suggest it’s becoming less likely for a human to trigger a deep persistent slab avalanche. Larger loads, like a cornice failure, and all bets are off. Winds have moved a lot of snow around recently, so shallow spots in the snowpack are potential trigger points.
When choosing terrain, remember deep persistent slabs are tricky, don’t show signs of obvious instability and are difficult to assess. Any avalanche failing on this layer is likely to be very large, destructive and deadly.
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: Alpine, Treeline.
Likelihood
Expected Size
Wind Slabs
Watch for evidence of recent wind transported snow, such as firm surface snow, deep drifts, and wind scoured areas. Human triggered wind slab avalanches are possible on unsupported wind loaded slopes and terrain features 35 degrees and steeper.
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
Expected Size
Valid until: Dec 22nd, 2018 10:27AM
