Archived

Avalanche Forecast

Apr 20th, 2017–Apr 21st, 2017

Alpine

3: Considerable

The avalanche danger rating in the alpine will be considerable

Treeline

2: Moderate

The avalanche danger rating at treeline will be moderate

Below Treeline

2: Moderate

The avalanche danger rating below treeline will be moderate

Alpine

3: Considerable

The avalanche danger rating in the alpine will be considerable

Treeline

2: Moderate

The avalanche danger rating at treeline will be moderate

Below Treeline

2: Moderate

The avalanche danger rating below treeline will be moderate

Alpine

2: Moderate

The avalanche danger rating in the alpine will be moderate

Treeline

2: Moderate

The avalanche danger rating at treeline will be moderate

Below Treeline

1: Low

The avalanche danger rating below treeline will be low

Regions: Purcells.

Sunshine and rising freezing levels will elevate the potential for cornice failures resulting in large avalanches on Friday.

Confidence

Low - Due to the number of field observations

Weather Forecast

FRIDAY: Mix of sun and cloud / Light northeast wind / Freezing level 2200mSATURDAY: Mix of sun and cloud / Light southeast wind / Freezing level 2200mSUNDAY: Cloudy with flurries / Light southwest wind / Freezing level 2100mNOTE: Convective flurries, which are common this time of year, can result in widely varying snowfall amounts. These spring squalls routinely drop 20+ cm of snow in one valley while the adjacent drainage remains dry. For this reason, the distribution of avalanche problems associated with new snow vary greatly within a region.

Avalanche Summary

Numerous explosive cornice triggered storm slab avalanches on northerly aspects at treeline and in the alpine were reported on Wednesday. One natural cornice triggered size 3 that stepped down to facets near the bottom of the snowpack was also reported. Exposure to large overhanging cornices remains a significant concern.

Snowpack Summary

Recent new snow tree line and above has been redistributed by west and south winds. Cornices remain large and continue to become more overhanging. The middle of the snowpack is generally well settled and continues to transition into a spring melt/freeze regime except for high elevation northerly aspects. Expect the entire thickness of the snowpack to be moist or wet at lower elevations. At upper elevations a deep persistent layer consisting of facets sitting on a crust that was buried back in November has been sporadically reactive to heavy loads i.e. a cornice fall or the weight of a smaller avalanche running.

Avalanche Problems

Cornices

The likelihood of cornices triggering increases with intense sunshine and/or during the warmest parts of the day.

Cornices often break further back than expected; give them a wide berth when traveling on ridgetops.Cornice failures could trigger large avalanches on sugary facets near the bottom of the snowpack.

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

Elevations: Alpine.

Likelihood: Possible

Expected Size: 2 - 4

Wind Slabs

Convective flurries and moderate southwesterly winds may create wind slabs in lee features below alpine ridgetops.

Use ridges or ribs to avoid pockets of wind loaded snow.Be cautious as you transition into wind affected terrain.

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

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 2

Loose Wet

Loose wet avalanches may trigger deeply buried weak layers, resulting in large avalanches.

Avoid exposure to terrain traps where the consequences of a small avalanche could be serious.Solar radiation is a common trigger, especially where new snow is sitting on a crust.Pinwheeling snow below cliffs is a common sign that loose wet avalanches are becoming more likely.

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

Elevations: All elevations.

Likelihood: Possible - Likely

Expected Size: 1 - 3