Turnagain Pass Avalanche Forecast

Once again, our primary concern deals with deep slab avalanches. These are failing in the weak layers at the bottom of the snowpack and can bring down large, dangerous and far running slides. It has been 4 days since the last deep slab was triggered but today the likelihood will go up with the rapid rise in temperature and the addition of rain and/or wet snow.

The precipitation amounts for this storm are not exceptional for this year around Turnagain Pass but the warm temperatures are. Most of our start zones lie in the 2,000 – 4,000’ elevation band and it is exactly this band which has harbored colder, dry snow so far this year and will get a shock as it warms up today. The slab that overlies our weak October and November snow is essentially one cohesive layer 4-8′ thick. It is a complex phenomenon as to why rapid warming destabilizes a dry snowpack. Essentially, the properties of the slab (our 4-8′ cohesive layer) change which increases the stress on the underlying weak layers (our October and November facets). There is a good chance that there are several slopes out there teetering on the balance and that balance may be tipped today.

Rain on snow near 3000ft and below will give our mid-elevation snowpack a jolt and decrease stability rapidly. Though the lower elevations (below 1,000ft) have seen their fair share of rain it is the next 2,000ft above this that is the biggest problem. As many old-timers say “rain on dry snow is never a good thing”. These wet avalanches can initiate in the top foot or two of the pack where the rain is being absorbed and become quite large on their descent by entraining additional snow in its path. They also have the ability to “step down” and trigger a deep slab avalanche, in which case will mostly likely be quite large. Wet snow avalanches contain very dense snow that is typically slow moving but they also mow down and destroy most things in their path.