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Archived

Avalanche Forecast

Dec 16th, 2018–Dec 17th, 2018
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Snoqualmie Pass.

You can still trigger surprising avalanches at upper elevations. Minimize your exposure to large steep slopes and avalanche paths. If you experience collapsing, cracking in the snow, or see recent avalanches, avoid open slopes 35 degrees or steeper.

Discussion

Avalanche Summary

The most dangerous slopes remain at upper elevations where new snow and wind drifting have added to existing slabs. Use extra caution in areas where you see wind drifts, pillows, and variable height of snow. While new snow, strong wind, and rain have all affected the snowpack, the main concern is triggering avalanches due to old, weak layers. At low elevations, open creeks and shallow snow cover pose a hazard to travel. Expect wet snow conditions for Monday. Watch for elevated danger on Tuesday as another storm makes its way into the Cascades.

Snowpack Discussion

Updated Regional Synopsis 20181216

Dangerous and fickle avalanche conditions remain as active weather continues into the week. Thick slabs of new snow (2-5 feet) are perched above a weak layer of faceted crystals. Avalanches triggered on this layer could be very large and life threatening. Reports continue to come in of very large natural and triggered avalanches in the northern and eastern zones. For perspective, several of these slides have been classified as D3, or large enough to destroy a house. You can find similar snowpack structure responsible for these avalanches in many other locations throughout the Cascades, including Stevens and Snoqualmie Passes. Anyone accessing alpine areas should limit their exposure to areas where avalanches start, run and stop. In some places the weak snow will talk to you by whumpfing and cracking. In other places, the heavy blanket of new snow is thick enough that it can give a false sense of stability while it masks the dangerous layering below. Approximate snow totals from 12/10 - 12/16: 

  • Mt. Baker: 75”

  • Washington Pass: 35”

  • Stevens Pass: 42”

  • Snoqualmie Pass: 36”

  • Paradise: 51”

  • Crystal Mt Base: 29”

  • Mt. Hood Meadows: 21”

  • Olympics: mix of rain and snow, no net gain

The change in the snowpack is pretty dramatic with elevation. Height of snow decreases rapidly below 4500’ at Baker and Washington Pass, 5500’ at Crystal/Rainier. The Passes have better low elevation coverage, but it's still pretty thin below 4000’. With additional warm storms in the forecast, this pattern is expected to continue for awhile. Mt. Hood and Hurricane Ridge have low snow coverage below 5000’.

If you're out in the mountains, please let us know what you see.

Be cautious and get home safe.

Avalanche Problems

Persistent Slabs

While avalanches have become more difficult to trigger, they could be big enough to bury or kill you. There's uncertainty about these avalanches. Persistent slabs are difficult to predict and often break much larger and wider than you expect. Minimize your exposure to avalanche terrain:

-Limit the amount of time you spend near slopes 35 degrees and steeper.

-Put an extra buffer of terrain between where you travel and where avalanches could start.

-Stay well out from under large avalanche paths and identify safer areas to stop and regroup.

If you dig in the snow, you will find a layer of sugar-like, faceted snow and feather-like surface hoar buried 2-3 feet below the surface. Where this snowpack structure exists, you may see signs of instability such as collapsing (a whumpfing sound), recent avalanches, snowpack tests that show propagation, or cracks that travel away from where you impact the snow. If you see these warning signs, avoid avalanche terrain.

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: All aspects.

Elevations: All elevations.

Likelihood: Possible

Expected Size: 1 - 1