Avalanche Forecast
Regions: Cascades - East.
As we enter a storm cycle Thursday, watch for building wind slab on a variety of aspects near and above treeline where there is plenty of low density snow available to transport onto lee slopes. Human triggered avalanches will become increasingly likely as denser but shallow storm and wind slab build over weaker snow during the day. Watch for changing conditions, travel conservatively and rein in your plans if conditions deteriorate faster than expected. The avalanche danger is expected to further increase Thursday evening so plan your exit accordingly.
Detailed Forecast
As we enter a storm cycle Thursday, watch for building wind slab on a variety of aspects near and above treeline where there is plenty of low density snow available to transport onto lee slopes. Â
Human triggered avalanches will become increasingly likely as denser but shallow storm and wind slab build over weaker snow during the day.
Shallow storm slabs may become touchy by the end of the day. Watch for changing conditions, travel conservatively and rein in your plans if conditions deteriorate faster than expected. The avalanche danger is expected to further increase Thursday evening so plan your exit accordingly. See the mountain weather tab for weather forecast details.Â
Keep the 12/15 crust/PWL interface in the back of your mind as we progress through this storm cycle, as we see if gradual loading over the next few days awakens any pockets of deeper instabilities.Â
Early season hazards still exist at some lower elevation locales and especially around creek beds that are not filled in.
Snowpack Discussion
Our last major storm cycle ended 12/19-12/20. Since then a series of weak weather disturbances have delivered several small refreshers to the east slopes of the Cascades, each delivering 1-3" of low density snow. Periods of moderate and shifting alpine winds have caused both wind stiffened surfaces and built shallow wind slabs in exposed terrain above treeline.Â
1.5 to 3 feet of recent snow sits atop the 12/15 crust/persistent weak layer interface in the Washington Pass area and in the central-east Cascades near the Cascade crest. Overall recent snow has been gaining strength leading to a recent stabilizing trend.
Comments from professional guides in the Washington Pass area are included below that help reduce uncertainty surrounding the 12/15 crust/persistent weak layer interface.Â
Snowdepth decreases substantially the further east of the Cascade crest one travels. In many areas below treeline, there is not enough snow to present an avalanche danger.Â
Observations
North
A summary of professional guide reports regarding persistent weak layers for the Northeast zone including Washington Pass:
- In the above treeline zone, small faceted grains are rounding and gaining strength, down 70 cm and below the storm snow that accumulated starting 12/15. They have not been reactive in snowpack tests.
- The upper snowpack on solar aspects near treeline are bonding well to a strong solar crust buried on 12/15. However, they cannot rule out buried surface hoar in some sheltered larch glades around 6000 feet.
- Below treeline, surface hoar that was buried on the 12/15 crust is down 70 cm in flat meadows and creek valleys, but has yet to be found in avalanche terrain.Â
- Aggressive terrain has been skied without direct signs of instability over the last several days.
Central
No recent observations
South
No recent observations
Avalanche Problems
Wind Slabs
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: Likely
Expected Size: 1 - 1
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1