Avalanche Forecast

Issued: Apr 17th, 2018 4:45PM

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

cgarritty, Avalanche Canada

Purcells.

The severity of avalanche problems varies with new snow depths across the region. Expect a trend toward sunshine and warming to destabilize both loose snow and recently formed slabs over the course of each day.

Summary

Confidence

Moderate - Timing or intensity of solar radiation is uncertain

Weather Forecast

Wednesday: A mix of sun and cloud with isolated flurries and a trace of new snow. Light south winds. Freezing level rising to 1800 metres with alpine high temperatures around -5. Thursday: A mix of sun and cloud. Light southwest winds. Freezing level rising to 2200 metres with alpine high temperatures around 0. Weak overnight cooling.Friday: Mainly cloudy with flurries beginning in the evening and continuing overnight. Light southwest winds. Freezing level to 2300 metres with alpine high temperatures around +1.

Avalanche Summary

Poor visibility limited observations of avalanche activity on Monday.Sunday there was a report of a near miss for skiers who triggered a size 2 wind slab on a north aspect at treeline in the northern part of the region. While in the northwestern part of the region saw a warming-induced, natural avalanche cycle up to size 3 on large alpine features in the recent storm snow.Saturday, explosive control work in northern parts of the region produced several storm slab releases size 2-2.5 on predominantly treeline to alpine, west aspects with wide propagations up to 200 m. Ski cuts produced size 1-1.5 results on cross-loaded features.On Friday, several skier-triggered, size 1.5 wind slab releases were reported on north aspects at 2300 m. A few solar-triggered storm slab releases (size 1.5) were also reported on steep, unsupported. west-facing features at 2200 m.

Snowpack Summary

About 20 cm on new snow overlies a crust on all aspects to at least 2000 m (and possibly higher on south-facing slopes), while 30-40 cm can be found at higher elevations (and up to 60 cm in some areas in the northwest of the region). Below 2200 m this new snow has thin surface crust, breaking down to moist or wet snow in the afternoons.Within the upper snowpack there a now several buried crusts with only the mostly recently buried posing a concern as the uppermost layer of storm snow settles and bonds to it.There is some lingering concern around the mid-March persistent weak layer in the alpine where it is found 50 to 90 cm below the surface. This buried crust/surface hoar interface is likely trending towards dormancy, but we need to keep it in the back of our minds as it is capable of producing large avalanches should it be triggered (think step-down from a surface avalanche or cornice collapse)Deeper persistent weak layers from December and January are considered dormant at this time.

Problems

Loose Wet

Forecast warming temperatures and sunshine are likely to destabilize surface snow and initiate loose wet avalanche activity - especially on steep south facing features. The likelihood of cornice failures also increases with sun and warming.

Cornices become weak with daytime heating, give them a wide berth when traveling above or below.Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.

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

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2

Storm Slabs

Storm slabs and wind slabs from the last round of snowfall may remain reactive to human triggering on Tuesday. The greatest caution is needed around steep or convex terrain as well as sun-affected slopes where slabs formed over a recent crust.

Use extra caution around cross-loaded terrain features.Be especially careful around wind loaded pockets near ridge crests and roll-overs.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Valid until: Apr 18th, 2018 2:00PM