Avalanche Forecast
Regions: North Rockies.
Uncertainty remains around a buried weak layer in the upper snowpack.
This is a good time to make conservative terrain choices, particularly when warm temperatures and solar radiation are testing the snowpack.
Confidence
Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast. Uncertainty is due to the timing or intensity of solar radiation and its effect on the snowpack.
Weather Forecast
A building ridge of high pressure will usher in clear skies and warm temperatures.Â
Thursday overnight: Clearing. 40-90 km/h westerly winds easing to 20-60 km/h in the morning. Freezing level dropping to valley bottom.Â
Friday: Mainly clear. Ridgetop winds 20-80 km/h from the northwest. Freezing level rising to 1000 m in the afternoon.
Saturday: Mainly clear. Ridgetop winds 20-60 km/h from the west. Freezing level rising from 700 m to 2000 m in the afternoon.
Sunday: Partially cloudy. Light westerly winds. Freezing level rising to 2500m.
Avalanche Summary
On Thursday, several loose wet avalanches were observed on all aspects at treeline and below from warm temperatures and rain on snow.Â
On Monday, a small skier-triggered wind slab avalanche was reported and several slab avalanches released naturally on the buried surface hoar.Â
On Sunday, a deep persistent slab avalanche was likely triggered naturally by intense warming and sun and released on depth hoar or the November crust. It occurred just south of the region (see this MIN report).Â
Snowpack Summary
A 1-5 cm breakable rain crust exists at all elevations making for unwelcoming riding conditions. Warm temperatures and strong solar radiation may cause this crust to break down throughout the day.Â
Below the crust, 30-50 cm of recent storm snow buried a weak layer of surface hoar crystals, which is now down 30-90 cm in the snowpack. Reports suggest that the surface hoar is likely most prominent in sheltered openings at and below treeline. Example terrain features to treat as suspect include the lee side of protected ridges, openings in the trees, cut blocks, and burns.
The lower snowpack is generally strong and well-bonded. The base of the snowpack is expected to be weak and faceted in shallow, rocky slopes east of the divide.
Terrain and Travel
- Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet
- Avoid open slopes and convex rolls below treeline where weak layers may be preserved.
- Surface hoar distribution is highly variable. Avoid generalizing your observations.
- Uncertainty is best managed through conservative terrain choices at this time.
Avalanche Problems
Persistent Slabs
A layer of buried surface hoar exists in the upper meter of the snowpack. This layer has been relatively spotty and hard to find but has produced large avalanches in the past week. The most suspect slopes would be sheltered, shaded aspects at treeline and below.
Aspects: All aspects.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1.5 - 2.5
Loose Wet
Solar radiation compounded by strong solar radiation may weaken the upper snowpack in the afternoon, creating wet loose avalanches on steep solar aspects.
Aspects: South, South West, West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 2