Avalanche Forecast

Issued: Mar 19th, 2015 9:33AM

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

swerner, Avalanche Canada

Purcells.

Snow, rain and wind combined with buried weak layers is the perfect recipe for avalanches. The upper snowpack is complex, keep it simple and use a conservative approach to terrain selection.

Summary

Confidence

Fair - Timing, track, or intensity of incoming weather system is uncertain on Friday

Weather Forecast

A series of pacific frontal systems will bring waves of precipitation to the Interior Ranges. Friday brings overcast skies, precipitation amounts 5-15 mm, ridgetop winds moderate from the SW and freezing levels 1900 m. Continued precipitation on Saturday with amounts 5-10 mm , ridgetop winds light-moderate from the SW and freezing levels near 2000 m. Later Saturday the front will move east setting up for a clearer, drying trend on Sunday. Freezing levels will initially drop to valley bottom overnight Saturday then rise steadily during the day with treeline temperatures near -2 degrees. The way this system is tracking, southern parts of this region will see less precipitation amounts then what's posted above.

Avalanche Summary

On Wednesday, a couple of natural slab avalanches up to size 2 were reported from northerly and westerly aspects above 2300 m. Explosive controlled slab avalanches up to size 2 were also initiated in steeper terrain features on north-easterly aspects. There is a still a concern for avalanches to step down to deeply buried weak layers resulting in larger slab avalanches, especially in shallower snowpack areas. With new snow, rain and strong winds natural avalanche activity will likely persist through the weekend.

Snowpack Summary

At higher elevations, 10-30 cm snow sits over a plethora of old surfaces including wind affected surfaces, and/or old wind slabs and crusts which were buried mid-March. Previous strong winds have redistributed new snow into wind slabs on leeward terrain features and lower elevations (below 2000 m) are sporting spring-like, melt-freeze conditions. Digging deeper (20-50 cm below the surface) sits the mid-February facet/ crust interface. This interface has not been reactive in the Purcell's unlike regions to the North. However, it is alive and well in test profiles and may just require additional load and/ or a change in slab properties before it reaches threshold and becomes reactive. The late-Jan crust/surface hoar layer (around 1m deep) and the mid-January surface hoar (around 1.5m deep) have been dormant for several weeks.

Problems

Wind Slabs

Wind slabs found in lee terrain continue to be reactive to human triggering. The mid-February interface is buried 20-50 cm below the surface and seems to be stubborn. However; wind slabs failing on this layer will have larger destructive potential.

Caution around convexities, whumphing and cracking are indicators of a buried weak layer. >Use caution on lee slopes and terrain features. Recent storm snow has been redistributed forming touchy wind slabs.>

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

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 4

Loose Wet

Steady rain and warming at lower elevations will saturate and weaken the upper snowpack.

Watch for clues, like natural avalanche activity, sluffing off of cliffs, and snowballing.>Avoid exposure to terrain traps where the consequences of a small, wet avalanche could be serious.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely - Very Likely

Expected Size

1 - 3

Valid until: Mar 20th, 2015 2:00PM