Avalanche Forecast

Issued: Feb 19th, 2012 9:00AM

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

pmarshall, Avalanche Canada

South Columbia.

It is likely that avalanche danger will start to ramp up this week as new snow settles into a cohesive slab and overloads several weaknesses lurking below.

Summary

Confidence

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

Weather Forecast

In general we should see unsettled conditions as a series of disturbances move across the province this week. Monday: Light to locally moderate snowfall - 5-10cm. Freezing level (FL) 800-1000m. Winds light to moderate from the southwest. Tuesday: Light to locally moderate snowfall - 10-15cm. FL rising to 1500m. Winds increasing to moderate from the W-SW. Wednesday: The strongest pulse should arrive late on Tuesday and bring moderate to locally heavy precipitation into Wednesday. 15-30cm is possible. FL lowering to 1000m. Winds moderate from the SW.

Avalanche Summary

Many areas have reported natural and easily triggered soft slab avalanches up to size 2. Most of these avalanches were 20-40cm deep and released on the Feb. 16 surface hoar layers, with some releasing on the Feb. 8 surface hoar/ facet/ sun crust interface. The size and likelihood of avalanches should continue to increase with continued incremental loading and particularly with any increase in winds.

Snowpack Summary

The upper snowpack is quite complex and we could see enough precipitation and wind early this week to tip the scales to more widespread and larger avalanches. Up to 40cm of low density new snow now sits on the February 16th Surface Hoar layer. The February 8th weakness is now down 50-60cm. Feb. 8th is used to describe a combination of weak layers: In most locations it marks a widespread surface hoar layer. On steep south & west facing aspects a sun crust that formed during the drought was also buried on Feb. 8th. This crust likely has a bit of faceting below it too, meaning that most terrain in the region has a weak layer 40-60cm under the snow surface. Below this problematic interface a well settled mid pack is in place. The exception is the eastern and southern parts of the region where deeper weaknesses and basal facets may still be a factor. In these areas, a low-probability/high-consequence condition exists. It should be noted that large cornices remain widespread throughout the region.

Problems

Wind Slabs

Wind slabs are starting to build in exposed leeward terrain and may be susceptible to human triggering.

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

Elevations: Alpine, Treeline.

Likelihood

Likely - Very Likely

Expected Size

1 - 4

Persistent Slabs

Up to 60cm of storm snow has started to settle into a soft slab and sits on several weaknesses including surface hoar. The size and likelihood of natural and human triggered avalanches will increase throughout the forecast period.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 4

Valid until: Feb 20th, 2012 9:00AM