Avalanche Forecast

Archived

Feb 21st, 2020–Feb 22nd, 2020

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 unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Northwest Inland.

The new snow may be more reactive than expected to human triggers; especially on slopes where it has been wind loaded and is sitting on a layer of surface hoar.

Confidence

Moderate - Uncertainty is due to the limited number of field observations.

Weather Forecast

FRIDAY NIGHT: 2-5 cm. snow, strong west wind, alpine temperature -7 C.

SATURDAY: 2-5 cm snow, moderate west wind, alpine temperature -3 C.

SUNDAY: Mostly cloudy, light southeast wind, alpine temperature -5 C.

MONDAY: Mix of sun and cloud, light west wind, alpine temperature -7 C.

Avalanche Summary

Some very large and destructive avalanches have been sporadically running on deeply buried weak layers near the bottom of the snowpack, predominantly on north through east aspects in the alpine. On Tuesday, a natural size 2.5 was observed. It is suspected to have started as a wind slab in a thin rocky area before stepping down. On Monday, two deep persistent slab avalanches size 2-3 were triggered from thin snowpack areas by very large loads pushed by snowcats.

Snowpack Summary

10 cm. of new snow and strong southwesterly winds have redistributed the new snow in exposed alpine and treeline areas, over extensively wind affected old surfaces. In sheltered areas, the new snow falls on a layer of surface hoar, and onto a sun crust on solar aspects.

An older, patchy layer of small surface hoar crystals a sun crust both from early February may still exist under previous storm snow, 30-60 cm deep.

A couple of weak layers that formed in January are burried in close proximity to one another 80-140 cm below the surface and continue to give hard results in snowpack tests. Below, an early season crust lurks at the base of the snowpack. Some large avalanches have been triggered on these layers, usually during periods of rapid loading by new snow or wind and by large loads or from thin, rocky areas.

Terrain and Travel

Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
Approach steep, open and/or sparsely treed slopes cautiously.
Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
Storm slabs in motion may step down to deeply buried weak layers resulting in very large avalanches.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.