Avalanche Canada swerner, Avalanche Canada

Sea to Sky Avalanche Forecast

Dec 2nd, 2012 9:50AM

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

Summary

Confidence

Fair - Due to limited field observations

Weather Forecast

A broad upper trough remains just offshore while a cooler, unstable onshore flow will persist and strengthen through Monday. On Tuesday an embedded frontal system will bring heavy amounts of precipitation, rising freezing levels and strong SW winds hammering the region. Wednesday the flow will switch from SW to NW, and precipitation amounts will ease up.Monday: Fzlvl’s 1000 m, snow amounts 5-10 cm, ridgetop winds S 30, alpine temps -4.Tuesday: Fzlvl’s 1700 m in am falling to 700 m, snow amounts 15-30 cm, ridgetop winds SW 30-gusting to 70 km/hr, alpine temps -3.Wednesday: Fzlv l’s 700 m, snow amounts 5-10 cm, ridgetop winds SW 20 km/hr, alpine temps -8.

Avalanche Summary

On Sunday widespread natural avalanche activity was reported size 1.0-2.0. On Saturday a size 1.5 skier triggered avalanche occurred below treeline, running on a crust in a specific terrain feature. Explosive testing done produced several size 1-2 slab avalanches, initiating within the storm slab instabilities. Wider propagations have been seen from convex terrain, remote triggering is likely.

Snowpack Summary

The Sea to Sky Region has seen anywhere from 90-140 cm of storm snow over the past 4 days. Storm slabs and fresh wind slabs continue to build, and natural avalanche activity has been widespread. I suspect the old weak snow surfaces (thin sun crusts, SH, surface facets) that developed early last week have been cleaned out in most places.  Below this, the mid-pack seems to be fairly settled, strong and possibly bridging instabilities that may exist deeper. Under the mid-pack (90-150 cm down) sits the early November facet/ crust persistent weakness. Recent snowpack testing done on an East aspect @ 2000 m have shown hard results with sudden collapse characteristics. Because this weakness is so close to the ground in most areas, any associated avalanche activity will likely be limited to slopes with smooth ground cover (e.g. scree slopes, rock slabs, summer firn, glaciers, etc.). Areas that are most susceptible to trigger this deep persistent weakness are shallow, thin and variable snowpack deposition spots. The continuous loading pattern should provide direct testing on the early November facet/crust weakness.

Problems

Storm Slabs

An icon showing Storm Slabs

Likelihood

Very Likely - Likely

Expected Size

1 - 5
New storm slabs and wind slabs have formed and have been reactive. Natural avalanches are likely, and rider triggered avalanches are very likely; especially on lee slopes, gullies, and behind terrain features that have been wind loaded.
Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.>Whumpfing, shooting cracks and recent avalanches are all strong indicators of an unstable snowpack.>

Aspects:

All aspects.

Elevations:

All elevations.

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

Likelihood

Possible - Unlikely

Expected Size

2 - 6
Slopes with smooth ground cover and reloaded bed surfaces may be prone to remote triggering and step down avalanches. Typical trigger points include thin rocky areas. May be difficult to trigger, but often very large with serious consequences.

Aspects:

All aspects.

Elevations:

Alpine, Treeline.

Valid until: Dec 3rd, 2012 2:00PM