Avalanche Forecast
Regions: Cascades - East.
Happy Holidays! If you find an area that has the combination of a thick recent slab and presence of weak older snow below it, you can trigger a deep and dangerous avalanche. Be most cautious above 6,000ft, and on steep slopes that face the north half of the compass.
Discussion
Happy Holidays from all of us at NWAC! The recent atmospheric river event left 3ft of settled storm snow on the ground in many places near the crest, while the Wenatchee Mountains east of Blewett Pass received less than 1ft of new snow. Observers at Holden Village and in the Icicle Creek drainage reported a cycle of large, natural avalanches on the 20th and 21st. These avalanches were easily big enough kill a person (up to size D2.5). Some broke widely across the terrain, ran on weak old facets, and may have been triggered remotely. On the 23rd, Mission Ridge Ski Patrol got results with explosive work below the Microwave Tower. The slab was observed to have failed on a layer of facets over a crust, which coincides with the same layer of concern we have closer to the crest. Watch this video I put together from my field day in the Icicle drainage on the 23rd for more info.
A sometimes breakable, sometimes supportable, but now frozen melt freeze crust can be found up to mid 5,000ft throughout much of the zone, making travel difficult at these lower elevations.Â
Crown of an explosive triggered slide at Mission Ridge. NW at 6,700ft on December 23,2019. Jacob Peterson photo.
Snowpack Discussion
New Regional Synopsis coming soon. We update the Regional Synopsis every Thursday at 6 pm.
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Avalanche Problems
Persistent Slabs
The snowpack still needs time to adjust to it's hefty new load of snow from the recent storm. A layer of weak, faceted snow that formed in late November is often found resting on top of a stout crust. You can trigger dangerous and surprising avalanches on this layer from below, or on adjacent slopes. Any triggered avalanche could break widely across the terrain and would be large. Put a wide buffer of space between you and any slopes steeper than 35 degrees. I would especially avoid thin rocky terrain, steep convexities, and unsupported slopes that face the north half of the compass.
Feel for weak snow near the ground (or 2.5 to 3.5ft down) with your probe. Listen for signs of instability such as whumphs, and look for shooting cracks. Dig down to identify this weak layer near the ground. Snowpack tests can help confirm the presence of this layer, but you will likely be able to identify them with a simple hand hardness test.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 2