Avalanche Forecast for the Idaho City Mountains

Bottom Line

An extended period of snow drought ended with rain below 6500' and a NEW load of dense snow exceeding 20 mm of SWE (snow water equivalent) by Monday morning.

The new snow has been deposited on many snow surfaces, from near-surface facets, surface hoar, rain crust, solar crusts, and wet snow, depending on aspect, elevation, and topographical configuration, whereas fragile faceted and surface hoar crystals were preserved.

Propagating (and persistent) weak-layer avalanche releases in the Idaho City Mountains are likely.

At elevations above 7500', sensitive wind slabs rest on variable surfaces from recristallized snow (near-surface facets) and varying crust types (rain, solar) undermined by facet development under the melt-freeze crust.

Avalanche Problem #1:

New snow over propagating weak crystals (PWL).

Reactivity: Reactive

Spatial Distribution: Widespread 

Avalanche release likelihood: Very Likely

Avalanche Problem #2:

Wind-driven snow drift and preferential snow deposition over variable surface facets or melt-freeze/facets interfaces.

Limited snow available for transport has limited the extent of this problem. This is the result of rather wet and heavy new snow.

Reactivity: Stubborn

Spatial Distribution: Specific 

Avalanche release likelihood: Possible

Forecast Confidence

Moderate confidence.

Highly variable old snow surfaces.

The presence of liquid water is a source of uncertainty in transition zones near the freezing level where rain has occurred.

Snow and Riding Conditions

Wet and dense snow up to 8000 feet. Sicky wet snow at or below 7000'. Wax your riding tools and your skins!

ASG Snowpack Development Chart

ASG Technical Tip

Cognitive words are important when communicating critical information. When referring to buried facets or surface hoar, the term persistent weak layers is often used. The term "persistent" refers to the general case in which instability lasts for long periods. 

But when using the term "propagating weak layer," it reminds us of one attribute that makes buried facets or surface hoar dangerous: its ability to propagate fracture failure across terrain, usually involving most connected terrain up to 30 degrees in steepness.

In contrast, particularly in some geographies and snow climates, instability persistence is a short-term problem. But while in the short term the instability sensitivity is reactive or stubborn, it can propagate extensively across terrain. The propagating term is essential in effectively communicating the heightened danger. Thus, ASG and alumni refer to PWL as PROPAGATING WEAK LAYERS, rather than persistent weak layer.