Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2–3
Recent Avalanches: Several human-triggered avalanches occurred on Sunday across the Turnagain forecast area. There was a skier triggered large slab on Eddies (pictured here) that was up to 3 feet deep. A snowmachiner was caught and carried in a slab avalanche in Lynx Creek. And, on the backside of Seattle Ridge, a skier triggered an avalanche, was caught, carried around 1,000' with an airbag deployment, and reportably is alright. The most recent avalanches we know of were on Monday when multiple reports of small wind slab avalanches were triggered that were around a foot deep.
Avalanche conditions remain dangerous despite the quiet weather. After 2-3" of light snow fell to sea level overnight, partly cloudy skies are expected today with light winds. Combine this with more daylight hours and mountain travel should be easier. This is worrisome as an avalanche forecaster as we know there is a buried weak layer capable of producing large and dangerous avalanches. The most recent big avalanche we know of that released on this scary layer (Valentine's Day buried surface hoar) was on Sunday on Eddies. Mary went up to Eddies yesterday to look at this slab avalanche. It was triggered by a skier who was not caught. This was a much larger avalanche than originally thought with many rollovers releasing and debris running into the thick alders below. Although it was up to 3 feet deep, it appeared to be triggered from a thin scoured spot near the top of a rollover. See more photos and write up HERE.
The main point is this type of avalanche (persistent slab avalanche) can still be triggered. This Valentine's Day weak layer is buried more deeply now (2-3' deep) after Sunday's foot-ish of snowfall, which means the potential size of avalanches are larger. It can also mean it's tougher to trigger and so those hidden thin spots end up being the places we do trigger these slabs. The weak layer has been found on all aspects above 1,500' and is the perfect setup for remote triggered avalanches. For example, a thin flat spot could be the trigger point for a slope nearby. This is a case where no warning sigs may be present, meaning no whumpfing or cracking in the snow around us; a tricky situation as all can seem just fine until it is not. Because this layer is so challenging to assess, the safest option is to make conservative decisions and stick to less consequential terrain away from steep overhead slopes.
(3/12/2025)
Skier triggered avalanche that occurred on Sunday, March 9th. This was a large slide with debris making it into the alders far below the crown. Skier was not caught.
Photo: Mary Gianotti
