Avalanche Forecast

Archived

Jan 23rd, 2020–Jan 24th, 2020

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

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 unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Sea To Sky.

An intense storm will continue to create very dangerous avalanche conditions on Friday.

Confidence

High - We are confident the snowpack will rapidly lose strength with the arrival of the forecast weather.

Weather Forecast

THURSDAY NIGHT: Heavy flurries continue overnight, especially near the coast, overnight accumulations of 15-30 cm above 1200 m, strong wind from the southwest, alpine temperatures around -3 C.

FRIDAY: Snow continues throughout the day with another 15-25 cm, moderate wind from the southwest, freezing level drops to 1200 m with alpine temperatures around -3 C.

SATURDAY: Cloudy with isolated flurries and 5-15 cm of snow, light wind from the south, alpine high temperatures around -3 C.

SUNDAY: A brief period of calm and clear conditions in the morning before the next system arrives in the afternoon bringing moderate accumulations of new snow.

Avalanche Summary

Preliminary reports from Thursday morning suggest large (size 2) storm slabs were easy to trigger with explosives, producing 40-60 cm thick slabs with wide propagation. Explosive control on Wednesday resulted in a very large (size 3.5) deep persistent slab on a west aspect the alpine, suggesting the deep persistent slab will likely remain reactive during this storm. With more loading on the way natural storm and deep persistent slab avalanches are serious concerns.

Snowpack Summary

A frontal system crossing the region has already delivered 40-60 cm of snow by Thursday afternoon and another 30-50 cm is expected by Friday afternoon, resulting in very thick storm slabs at higher elevations. Thinner deposits of new snow are expected below 2000 m due to a mix of rain and snow during the storm. Wind over the past few days has been strong to extreme from the south, resulting in highly variable conditions in open terrain.

Prior to the storm there were reports of a 50 cm deep rain crust that had been the bed surface for small storm slab avalanches below 2000 m. Weak faceted grains and crusts near the base of the snowpack continue to be a concern in inland parts of the region such as the Spearhead Range.

Terrain and Travel

Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
Avoid traveling through runout zones during stormy periods when large avalanches are likely.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.