Avalanche Forecast

Archived

Jan 14th, 2021–Jan 15th, 2021

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 unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

South Coast Inland.

A bit of new snow and wind will keep storm slabs fresh Friday. They will be especially reactive in wind loaded lees and where they sit over a persistent weak layer.

Where overnight precip falls as rain in the south, conditions will be crusty and avalanche danger a step lower.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Thursday night: Rain/snow, 5-10 mm/cm, strong to extreme southwest ridgetop wind, freezing level 1300 m in the north, 2500 m in the south.

Friday: Mix of sun and cloud, moderate to strong westerly ridgetop wind, freezing level 1300 m.

Saturday: Mix of sun and cloud, light southwest wind, freezing level 800 m.

Sunday: Flurries, strong southwest wind, freezing level 1300 m.

Avalanche Summary

A widespread natural avalanche cycle size 2-3 occurred throughout the region on Wednesday. Avalanche character was predominantly storm slab in the snowy north of the region and wet loose or glide slab in the rainy south. Northwest of Pemberton, several stepped down to buried weak layers resulting in very large (size 3-4) persistent slab avalanches.

Snowpack Summary

40-60 mm of recent precipitation fell as snow in the north and rain in the south. At latitudes and elevations that received snow, strong winds have formed reactive storm slabs. These slabs may be extra sensitive where they overlie surface hoar in sheltered terrain or a crust on solar aspects. Lower elevations and latitudes will see a firm surface crust as temperatures drop Friday.

A melt-freeze crust from early December is found around 100 to 200 cm deep in the snowpack. This crust may have surface hoar or sugary faceted grains sitting above it. This persistent weak layer is most prevalent in the North (e.g., Duffey Lake, Hurley) and large avalanches have failed on this interface recently. This interface doesn’t seem to be a concern in the South (Coquihalla) and little information is known for the North Cascades or far South in Manning Park.

Terrain and Travel

Make conservative terrain choices and avoid overhead hazard.
Be aware of the potential for large avalanches due to the presence of buried weak layers.
Pay attention to cornices and give them a wide berth when traveling on or below ridges.
When a thick, melt-freeze surface crust is present, avalanche activity is unlikely.

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.

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.