Avalanche Forecast
Regions: Mt Hood.
Watch for recently formed wind slabs on lee slopes above treeline that may extend into the near treeline elevation band. A slowly refreezing upper snowpack should be found at mid and lower elevations.
Discussion
Weak snow formed and buried earlier in the month can still be found in the upper snowpack. On Sunday, Mt. Hood Meadows pro-patrol found buried surface hoar intact in a sheltered location at 5500 ft. While this layer was preserved intact 16" (40 cm) below the snow surface, it was neither reactive in snowpact tests nor involved in any recent avalanche activity. Several rain crusts were found above this layer. We will continue to track this layer as we move through the next storm cycle.Â
Snowpack Discussion
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: Â
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Mt. Baker: 75â€
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Washington Pass: 35â€
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Stevens Pass: 42â€
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Snoqualmie Pass: 36â€
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Paradise: 51â€
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Crystal Mt Base: 29â€
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Mt. Hood Meadows: 21â€
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Olympics: mix of rain and snow, no net gain
The change in the snowpack is pretty dramatic with elevation. The 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 a while. 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
Wind Slabs
Recent winds have likely formed shallow wind slabs on lee slopes near and above treeline. Watch for evidence of recent wind transported snow, such as firm surface snow, deep drifts, and wind scoured areas. When you encounter firm surface snow on steep slopes, be aware that avalanches may break above you.
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