Avalanche Forecast

Issued: Dec 10th, 2015 9:01AM

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

ghelgeson, Avalanche Canada

Purcells.

We've arrived at the tail end of the natural avalanche cycle and now is when it gets tricky. Be particularly conservative between 1500 and 1800m where the storm slab overlies a persistent weak layer that remains sensitive to human triggering.

Summary

Confidence

Low - Timing, track, or intensity of incoming weather system is uncertain on Sunday

Weather Forecast

The region has been bombarded by the storm chain since December 2nd, but the caboose is in sight. No significant weather is forecast for Friday and Saturday. A rather mundane storm Saturday night will likely be the last pulse before the region enters into a period of high pressure Monday. FRIDAY: No significant precipitation. Freezing level around 800m, light winds generally out of the west. SATURDAY: Winds ramping up to strong SW during the day. 1 to 5cm expected Saturday night with a freezing level around 500m. SUNDAY: 1 to 6cm of snow, light SW winds, freezing level around 1000m.

Avalanche Summary

Observations have been limited during the storm, especially from alpine elevations. On Wednesday natural avalanches ran to size 3 on various aspects. Riders triggered avalanches to size 1.5 on N and NE facing features between 1600 and 2000m. Interesting to note that one rider triggered size 1.5 avalanche was actually remote triggered from 100m away on a steep north facing slope in a cutblock. The resulting avalanche had a 50cm deep crown.

Snowpack Summary

Over the past week, 50-80cm of storm snow has typically accumulated in the region. On Tuesday, rain affected the snow surface as high as 2000m but new snow has subsequently fallen as low as 1500m. The storm slab sits over the early-Dec interface which consists of large surface hoar below 1800m, sun crusts on south-facing slopes, old wind affected surfaces above treeline, and possibly faceted surfaces in some areas. This interface appears to be quite variable throughout the region and information on this layer has been limited, so treat the layer with extra respect until more info is available and give the storm snow extra time to stabilize. Recent strong SW winds have built thick wind slabs in leeward features at alpine and exposed areas at treeline. Deeper in the snowpack, the surface hoar interface from early November has been dormant but still may wake up with heavy loading or smaller avalanches stepping down. On very high northern aspects, a crust/facet interface may be found near the ground.

Problems

Storm Slabs

A thick storm slab overlies a highly variable interface which is still reactive in many areas. Strong SW winds have created thicker wind slabs in leeward features.

Avoid open slopes and convex rolls below treeline where buried surface hoar is highly reactive. >The areas we usually consider safe below treeline may be some of the most volatile right now, as this is where the surface hoar is best preserved.>Start with simple terrain options and take a curious approach to the snowpack. You may be able to step out into some challenging terrain at and above treeline after learning more about the snowpack.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

2 - 4

Valid until: Dec 11th, 2015 2:00PM