Avalanche Forecast

Issued: Apr 15th, 2015 9:14AM

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

Avalanche Canada rbuhler, Avalanche Canada

Strong solar inputs will destabilize the snowpack in the afternoon. Loose wet sluffing, cornice failures, and solar triggered slab avalanches are all possible and are most likely on steep sun-exposed slopes.

Summary

Confidence

Fair - Due to the number of field observations

Weather Forecast

A ridge of high pressure should keep the region dry and mostly sunny until Friday afternoon. On Thursday, expect sunny conditions with light alpine winds and freezing levels reaching over 2000m in the afternoon. On Friday, increasing cloudiness is expected during the day with alpine winds progressively increasing to moderate from the SW. Freezing levels are forecast to be around 1500m in the morning and around 2500m in the afternoon. On Friday afternoon or evening, a weak frontal system is forecast to reach the region. Models are currently showing 2-4mm of precipitation. By Saturday morning, the system should have passed and mostly sunny conditions are expected by Saturday afternoon. Freezing levels are expected to stay high for the weekend.

Avalanche Summary

On Tuesday, a natural cornice failure triggered a size 2 slab avalanche on the slope below. Three size 1-1.5 skier triggered avalanches were also reported. Two of these were triggered remotely with the furthest being triggered from 100m away. Several remotely triggered avalanches have been reported recently as well as whumphing and wide propagations. These indicate that the weak layer below the storm snow has been very reactive in some areas. Over the weekend, natural, human-triggered, and explosive-triggered storm slab avalanches up to size 2.5 were ongoing. Towards the southern parts of the region that received less recent snowfall, wind slabs in specific areas have been the primary concern. Solar triggered slab avalanches, cornice releases, and loose wet sluffing are all expected to have occurred on Wednesday. On Thursday, the same types of solar related activity are expected. If a supportive crust forms Wednesday night, stability will begin to improve. Human-triggering remains a major concern at higher elevations, especially north facing terrain where the storm slab is unaffected by the sun on Wednesday.

Snowpack Summary

The sun on Wednesday is expected to have melted the snow surface at lower elevations and on sun-exposed slopes into the alpine. A new surface crust is expected form as temperatures drop overnight. 20-60cm of recent snowfall overlies a weak layer that was buried on Friday. This weak layer typically consists of surface hoar and facets overlying a melt-freeze crust that exists everywhere except high elevation north-facing terrain. Recent snowfall amounts are highest in the north of the region and taper off to south. In exposed alpine terrain, strong SW winds have redistributed the recent storm snow forming thicker wind slabs in leeward features. Large cornices exist in the alpine and may become weak with daytime warming. There are three dormant persistent weak layers that we are continuing to track. The late-March crust is down 50-70cm and was reactive last week during the warm period. The mid-March and mid-February layers are typically down between 70 and 100cm and have been dormant for several weeks. These layers have the potential to wake up with sustained warming, a significant rain event, and/or a big cornice fall.

Problems

Storm Slabs

An icon showing Storm Slabs
Recent storm slabs sit over a weak layer and remain reactive to human-triggering. Strong SW winds have loaded leeward terrain features in the alpine. These slabs have been reactive for longer than normal due to the presence of buried surface hoar.
Avoid freshly wind loaded features.>Be cautious around steep unsupported slopes and convexities.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 4

Loose Wet

An icon showing Loose Wet
Afternoon sun is expected to trigger sluffing on steep sun-exposed slopes. Sluffing has the potential to trigger larger slab avalanches.
Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.>Sluffs may trigger deeper instabilities.>

Aspects: North, North East, East, South East.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2

Cornices

An icon showing Cornices
Recent cornice growth has been observed and large cornices are expected to become weak with daytime warming, especially when exposed to the sun. Cornices have the potential to trigger large slab avalanches.
Cornices become weak with daytime heating, so travel early on exposed slopes.>Give cornices a wide berth when travelling on or below ridges.>

Aspects: North, North East, East, South East, South.

Elevations: Alpine.

Likelihood

Possible - Likely

Expected Size

2 - 5

Valid until: Apr 16th, 2015 2:00PM