Avalanche Forecast

Issued: Mar 18th, 2014 9:57AM

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

jlammers, Avalanche Canada

Purcells.

Summary

Confidence

Fair - Forecast snowfall amounts are uncertain on Wednesday

Weather Forecast

Wednesday: Light snowfall becoming moderate overnight (up to 15cm) / Strong southwest winds / Freezing level at 1500mThursday: Light flurries / Moderate westerly winds / Freezing level at surfaceFriday: A mix of sun and cloud / Light northwest winds / Freezing level at surface

Avalanche Summary

On Monday explosives control produced several slab avalanches to size 2.5 with a couple reaching the size 3 range. Explosives control also produced cornice fall to size 2. In a nearby region with a similar snowpack a size 3 natural persistent slab avalanche was triggered by solar warming. The avalanche occurred on a south facing slope at 2100m and destroyed timber in the run out. On Tuesday explosives control in the north of the region triggered a size 2.5 deep persistent avalanche which failed on basal facets in unsupported, rocky terrain.

Snowpack Summary

Up to 45cm of settling storm snow exists and has been blown by strong winds into much deeper deposits in lee terrain. These recent storm accumulations overlie crusts on previously sun-exposed slopes and surface hoar in shaded terrain. Rain at lower elevations have saturated the snowpack and lower elevation surfaces may now exist as a refrozen crust. Below the new snow, 60-100cm of settled snow has been bonding poorly to buried surface hoar and crusts. The deeper facet/crust persistent weakness buried at the beginning of February, now down 60 to 160cm, has been highly variable in terms of reactivity and is still very difficult to trust. Needless to say, any avalanche at these deeper, persistent interfaces would be large and destructive.Weak basal facets exist in many areas, but without a large load, triggering now is unlikely.

Problems

Persistent Slabs

There are a number of persistent layers in the mid to upper snowpack which have professional operators concerned. These layers, which include recently buried crusts and surface hoar, continue to produce large avalanches in many parts of the region.

Use conservative route selection, stick to moderate angled terrain with low consequence.>Dig down to find and test weak layers before committing to a line.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 4

Storm Slabs

Recent storm accumulations may take longer to bond due to the presence of buried surface hoar and crusts. Snow forecast for Wednesday afternoon will add to this existing storm slab.

Be cautious as you transition into wind affected terrain.>Choose conservative lines and watch for clues of instability.>Be alert to conditions that change throughout the day.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 4

Deep Persistent Slabs

The early February interface is variably reactive and has produced deep and destructive avalanches in a nearby region with recent solar warming. Possible triggers include cornice fall, thin spot triggering or solar warming.

Avoid convexities or areas with a thin or variable snowpack.>Be aware of the potential for large, deep avalanches.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

3 - 5

Valid until: Mar 19th, 2014 2:00PM