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Avalanche Forecast

Archived

Dec 16th, 2018–Dec 17th, 2018

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

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.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.