Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1–2.5
Recent avalanches: There were a few reports of shallow natural avalanches in the Girdwood Valley yesterday, large enough to bury a person. These likley failed toward the tail end of Tuesday's storm. There have also been several observations of large collapses in the treeline elevation band from 1000' to 2500' across the advisory area. In the Snug Harbor zone (outside of this advisory area), we observed a snowmachine-triggered wind slab yesterday afternoon.
The liklihood of triggering avalanches today will depend on what areas have seen the most wind and snow over the past 48 hrs. Based soley on Tuesday storm totals, Girdwood, and Portage Valley recieved notably more snow than Turnagain Pass; 13" at Alyeska mid station (1,700') versus 7" at Center Ridge (1,880'). A skier or snowmachiner will be more likely to trigger an avalanche where a deeper slab (more snow/ wind) has formed over weak facets, buried 1-2' deep. In areas that recieved less snow and wind since Tuesday's storm, such as the core Turnagain Pass area, human-triggered avalanches will be less likely and smaller.
It'll be important today to evaluate snow and terrain carefully and identify any features of concern, such as wind-loaded convexities/ terrain traps. Keep your consequences in mind if you're committing to travel on a particular slope. You can always stick to low angle, simple terrain to completely avoid this avalanche problem. Even a small avalanche involvement could become a much bigger deal given the extreme cold temperatures today.
Given the cold temps and relatively shallow seasonal snowfall, the majority of our snowpack is made up of weak, faceted snow. Pictured here are 1 to 2 mm facets sitting just below a crust in a snowpit at 2,200' on Tincan yesterday. This setup is widespread throughout the forecast area.
Photo: Mary Gianotti
