Avalanche Forecast

Issued: Mar 9th, 2017 5:38PM

The alpine rating is high, the treeline rating is high, and the below treeline rating is considerable. Known problems include Storm Slabs, Persistent Slabs and Loose Wet.

cgarritty, Avalanche Canada

South Coast.

Persistent weak layers are not typical on the South Coast. Backcountry users in the region need to keep in mind the potential for storm slab avalanches to 'step down' well beyond the depth of new snow.

Summary

Confidence

Moderate - Forecast snowfall amounts are uncertain on Friday

Weather Forecast

Thursday night: Periods of snow and wet snow bringing 20-40 cm of new snow to higher elevations. Strong to extreme southwest winds. Freezing level around 1500 metres with alpine temperatures around 0. Friday: Continuing snowfall and wet snow bringing 10-20 cm of new snow to higher elevations. Moderate to strong southwest winds. Freezing level to 1400 metres with alpine temperatures around 0. Saturday: Another pulse of wet snow and rain bringing up to 20 cm of new snow to higher elevations. Strong to extreme south winds. Freezing level to 1400 metres with alpine temperatures around 0. Sunday: Continuing wet flurries and rain bringing up to 20 cm of new snow to higher elevations. Moderate to strong southwest winds. Freezing level to 1500 metres and alpine temperatures around +1. Forecast snowfall amounts are subject to freezing levels. Wet snow can generally be expected to accumulate 300-400 metres below the freezing level.

Avalanche Summary

On Wednesday, explosives triggered two size 1 storm slabs which released down 10-25 cm. These were described as having minimal propagation and minimal entrainment. On Monday, a report includes whumphing and a ski cut triggering a size 2 slab which released on the crust layer. Explosives also triggered numerous storm slabs 20-30 cm thick and one stepped down to the crust layer down 40-60 cm. On Thursday, recently formed storm slabs may still be reactive to human triggering, especially in wind loaded terrain. Large persistent slab avalanches have generally become unlikely but still may be possible in isolated areas. Natural sluffing is possible from steep sun exposed slopes.

Snowpack Summary

Another 15-30 cm of snow fell in the North Shore mountains on Wednesday, bringing the typical storm accumulation in the past week to 115-230 cm. Recent winds have been from a variety of directions and wind slabs should be expected on all aspects; some may exist below less wind affected new snow accumulations. This recent snow overlies the late-February interface that is composed of a sun crust on southerly aspects and surface hoar on shaded aspects. An observation on Wednesday suggests this layer has gained a lot of strength since the weekend. However, conservative choices are still recommended until we are sure this layer has become unreactive. We could be in a low probability, high consequence scenario where very large avalanche are still possible in isolated areas. Substantial snowfall and warming in the forecast are two factors that may promote increasing reactivity at this interface over the weekend. the Below this interface, the mid and lower snowpack appear to be well settled and strong.

Problems

Storm Slabs

Another 30-60 cm of new snow is likely to accumulate in the South Coast mountains by Friday afternoon. Warm temperatures and strong to extreme winds will promote the formation of touchy storm slabs that will be primed for human triggering on Friday.

Use conservative route selection, choose moderate angled and supported terrain with low consequence.Minimize overhead exposure during periods of heavy loading from new snow, wind.Use extra caution on slopes if the snow is moist or wet.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1 - 2

Persistent Slabs

Storm snow from the past week sits over a buried persistent weak layer. While this layer has gained a lot of strength, very large avalanches may still be possible in isolated terrain. This likelihood increases while new snow adds load to the snowpack

Dig down to find and test weak layers before committing to a line.If triggered, storm slabs may step down to deeper layers resulting in very large avalanches.Be aware of the potential for very large avalanches due to deeply buried weak layers.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 4

Loose Wet

Wet snow and rain at lower elevations will allow surface snow to sluff easily from steep terrain. These sluffs may entrain significant mass and become a serious hazard around terrain traps like cliffs or above terrain depressions.

Be cautious of sluffing in steep terrain.Use extra caution on slopes if the snow is moist or wet.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 2

Valid until: Mar 10th, 2017 2:00PM