Avalanche Forecast

Archived

Feb 10th, 2020–Feb 12th, 2020

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

North Rockies.

Danger will rise rapidly during storms expected on Tuesday and Thursday because of a buried surface hoar layer.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

Monday night: 15-25 cm new snow with strong westerly winds. Treeline temperatures around -6C.

Tuesday: 10-15 cm new snow with strong northwesterly winds. Treeline temperatures around -6C.

Wednesday: Mostly dry with a mix of sun and cloud. Treeline temperatures around --10C.

Thursday: 10-20 cm new snow with strong southwesterly winds. Treeline temperatures around -6C.

Avalanche Summary

There were no slab avalanches reported in this region between Feb 6 and Feb 10.

I expect storm slab avalanches will rapidly develop on Tuesday and remain sensitive to human triggering through this week.

We are aware of an avalanche that occurred Sunday Feb 2 in the Upper Burnt area that is presumed to involve a snowmobiler. Preliminary information indicates that avalanche was a size 2.5 on a southeast aspect at 1600 m with a fracture line depth of 130 cm.

Snowpack Summary

A layer of surface hoar has become buried by light snow. Until it is buried approximately 30 cm or so it poses little hazard in sheltered areas. However, in the lee of ridge lines, where fresh wind slabs are likely to have formed, it may make avalanches easier to trigger.

Approximately 40-60 cm recent storm snow sits above a thin rain crust from the warm, wet storm on Feb 01. This rain crust has been reported to exist up to treeline elevations around 1700 m. Initially this layer was described as reactive and was the focus of a number of avalanches. However, it now appears to be gaining strength and probably won't end up being a significant long-term weak layer.

In the mid-pack there may still be a layer of surface hoar buried in late December. It's gaining strength, but should remain a concern since snowpack tests still demonstrate this layer has the potential to slide, albeit in isolated areas and/or with very large triggers.

Terrain and Travel

Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
Cornice failure may trigger 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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.