Avalanche Forecast for the Idaho City Mountains

Bottom Line

The Idaho City Mountains have received about 30 cm of new snow. Unsettled weather on Wednesday, and an approaching cold front will add another 10 cm of snow. The two storm totals are expected to be between about 40 cm of NEW snow in 48 hours. New snow over an uncertain distribution of reactive, propagating weak layers requires conservative terrain selection. Complex terrain should be avoided.

Avalanche Problem #1:

Last week's storm buried propagating weak crystals (PWL). Conditions have favored the rounding and sintering of these propagating weak layers. It is possible that buried surface hoar and near-surface facets persist in various sheltered areas.

Reactivity: Reactive. The loading has exceeded the critical 25 mm of SWE.

Spatial Distribution: Specific to sheltered slopes that preserved the PWL (propagating weak layers of buried surface hoar and near-surface facets).

Avalanche release likelihood: Likely by Wednesday

Avalanche Problem #2:

Wind drifted snow over week-old snow surfaces.

Reactivity: Reactive

Spatial Distribution: Specific. 

Avalanche release likelihood: Likely

Forecast Confidence

Medium confidence in the anticipated avalanche problem #1.

Up to 30-40 mm of total SWE from a new load is concerning, but the load is distributed between two storms over 48 hours, and the spatial distribution of rounded/sintered propagating crytals is uncertain.

Snow and Riding Conditions

Return of POW skiing!

ASG Snowpack Development Chart

ASG Technical Tip

When the snowpack is capable of initiating a fracture, and/or capable of sustaining crack propagation, and/or sliding after fracture initiation, the snowpack is considered reactive.

There are four levels to describe snowpack sensitivity: touchy, reactive, stubborn, and unreactive.

Touchy reactivity is associated with natural avalanches and snowpack tests that fail during preparation.

Reactive test results are associated with natural and human-triggered avalanches, and test results for fracture initiation and propagation are positive.

A stubborn snowpack is characterized by moderate or hard fracture initiation tests or inconclusive crack propagation results. Stubborn reactivity is often observed during periods of Moderate avalanche danger.

An unreactive snowpack provides no evidence of instability. Reactivity tests are negative for fracture failures.