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Avalanche Forecast

Archived

Feb 2nd, 2020–Feb 3rd, 2020

Alpine
Natural avalanches possible, human triggered probable.
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 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.

Regions

South Coast Inland.

While those north of Pemberton continue to tiptoe through mountains plagued by touchy deep persistent avalanche problems, avalanche danger in the south of the region will be MODERATE at all elevations.

Confidence

Moderate - Recent weather patterns have resulted in a high degree of snowpack variability within the region. Uncertainty is due to the fact that deep persistent slabs are particularly difficult to forecast.

Weather Forecast

Sunday night: Flurries bringing trace to 5 cm new snow. Moderate northwest wind. Freezing level valley bottom.

Monday: Sunny. Light to moderate northwest wind. Alpine high -9 C. Freezing level valley bottom.

Tuesday: Increasing cloud with flurries starting. Southwest wind building to strong, becoming extreme at ridgetop. Alpine high -8 C. Freezing level valley bottom.

Wednesday: Flurries bringing 5-10 cm new snow. Strong west wind, extreme at ridgetop. Alpine high -4 C. Freezing level rising to 700 m.

Avalanche Summary

Numerous size 3-4 deep persistent slab avalanches were reported Saturday in north parts of the region including the Lillooet River valley north of Pemberton and near Goldbridge. Start zones included all aspects 1900-2300 m and avalanches ran to valley bottom, taking out mature timber and blocking waterways. 

Explosive work conducted Saturday also produced deep persistent size 2-3 in these areas as well as in the Duffey. In some cases, these large touchy slabs were triggered not by detonation, but as the explosive product landed on the slope.

A widespread size 2-3 natural storm slab cycle was observed in most areas toward the tail end of the storm Saturday.

Snowpack Summary

Above 1800 m, weekend storm totals for the north of the region are in the range of 30-50 cm, while the Coquihalla received around 100 cm and the Manning Park area received around 35 cm. Extreme southwest wind during the storm has scoured windward aspects, formed deep loaded pockets in alpine lees and contributed to rapid cornice growth. Below 1800 m, snow overlies a crust and tapers with elevation.

In the north part of the region (ie. Goldbridge/Whitecap/Duffey/Hurley), a few deep instabilities exist in the snowpack, including a weak basal facet crust complex. This has been the failure plane in ongoing reports of very large, deep persistent avalanches.

The south of the region, including the Coquihalla and Manning areas, currently have no concerns about deep instabilities.

Terrain and Travel

  • Avoid the runout zones of avalanche paths. Very large avalanches have been running full path.
  • Make conservative terrain choices and avoid overhead hazard.
  • Be alert to conditions that change with elevation.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.

Problems

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.

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.