Avalanche Forecast

Archived

Jan 29th, 2021–Jan 30th, 2021

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

North Rockies.

Triggering avalanches will gradually become more likely as new snow and wind builds reactive slabs. Closely watch for whumpfing, cracking and natural avalanches.

Confidence

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

Weather Forecast

A weak storm will cross the region on Sunday night.

FRIDAY NIGHT: Partly cloudy, moderate southwest wind, treeline temperatures around -18 C.

SATURDAY: Mix of sun and cloud, moderate southwest wind, treeline temperatures around -10 C.

SUNDAY: Cloudy with some isolated flurries and trace accumulations of snow, moderate south wind, treeline temperatures around -10 C.

MONDAY: Scattered flurries with 5-10 cm of snow, light west wind, treeline temperatures around -10 C.

Avalanche Summary

No avalanches have been reported in the past week, however some notable shooting cracks and whumpfs were reported around Pine Pass on Thursday. They were both linked to buried surface hoar layers, one 30 cm deep underneath a fresh wind slab (here) and the other 65 cm deep underneath old snow (here).

Wind slabs developing above surface hoar is a good example of the primary problem we expect to see over the weekend. The deeper surface hoar layer appears to be a very isolated problem on specific slopes in the north of the region.

Snowpack Summary

Open terrain is likely being impacted by moderate wind, with wind slabs forming on lee terrain features. Sheltered areas have 10-20 cm of soft faceted snow. Underneath the soft snow and new wind slabs, you will likely find a hard melt-freeze crust up to roughly 1600 m, perhaps weak and feathery surface hoar crystals, and otherwise wind packed snow. Wind slabs developing above surface hoar could be particularly reactive.

There is a spotty weak layer of surface hoar 30 to 65 cm deep in sheltered treeline terrain. This layer has not produced any avalanches to date, but our field team reported some large whumpfs on this layer at Pine Pass on Thursday.

There are presently no deeper concerns in the snowpack.

Terrain and Travel

Closely monitor how the new snow is bonding to the old surface.
Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
Exercise caution on steep, unsupported slopes.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.