In the SE Mountain forecast, dangerous avalanche conditions are primarily driven by the threat of a persistent slab avalanche releasing on weak layers near the bottom of the snowpack. This avalanche problem is expected to be the most sensitive to triggering at near and above treeline elevations. At lower elevations, the avalanche problem is slightly less reactive. Either way, a triggered avalanche would be large in size and potentially deadly. Cautious travel and conservative terrain selection are recommended.
Fresh snow available for transport has combined with southwest winds and is forming a new round of wind slabs. These slabs will grow in both size and distribution throughout the day. Wind slabs most commonly form below the lee sides of ridgelines and in cross-loaded terrain features at near and above treeline elevations.
Example of an old wind slab that failed on a cross-loaded terrain feature.
Slabs ranging from 3 to 6 feet thick have formed around much of the compass and at all elevations over faceted weak layers in the lower snowpack. These weak layers have adjusted to their current load and were slowly becoming more difficult to trigger. However, the variability in the slab depth, especially at near and above treeline elevations, is creating more locations for lighter loads, such as the weight of a human, to affect those weak layers and release the avalanche problem. These persistent slab avalanches can propagate widely through the terrain, or be remotely triggered from low-angle slopes connected to steeper slopes. You can reduce your risk by traveling on slopes with a deep, uniform snowpack. While you can further avoid the problem by traveling on slope angles that are consistently less than 30 degrees and not connected to steeper terrain above.
1/4: A destructive natural avalanche in Red Lady Bowl broke full-width last week. This avalanche would be hard to survive.
5am snow totals are fairly minimal. The NW Mountains are looking at about 2 to 4" while the SE Mountains are in the 1 to 2" range. Snowfall rates look to pick up by mid-day and into the evening. The wind forecast isn't particularly bad, but southwest-to-west winds will still be strong enough to transport the new snow into fresh wind slabs.
This new snow avalanche problem will be the driver for rising avalanche danger in the NW Mountains for today, and into the start of next week. Above treeline, elevations will be the first to see a rise to CONSIDERABLE avalanche danger later this afternoon. We will eventually see that danger rating expands to more elevation bands in the coming days.
The avalanche danger ratings in the SE Mountains are being driven by the persistent slab avalanche problem. In general, this part of the forecast area has thinner slabs where there are more locations for light loads such as a human to affect the weak layers below. At below treeline elevations, this avalanche problem has had some time to adjust to past loading events and become slightly less reactive. This is where we get the MODERATE avalanche danger rating. At near and above treeline elevations the danger rating is CONSIDERABLE, as previous wind loading earlier in the week continued to load this avalanche problem and keep it more sensitive to triggering. At near and above treeline elevations there are also more variations in the snow depth, creating more trigger points and opportunities to trigger the avalanche problem from a thin snowpack area, which could then propagate to and deeper area.
Soft slabs or storm slabs are not yet expected to become very problematic in wind-sheltered areas for either forecast area. However, there were a few observations of surface hoar on Friday and Saturday, that will now be getting buried in some locations. The distribution of this potentially weak layer is somewhat sporadic or isolated. It would most commonly be found in areas that are both protected from the sun and the wind. So the take-home point is if you are riding in areas where surface hoar may be under the new snow, keep an eye out for a developing storm slab avalanche problem.
We will of course continue to monitor the accumulating snow throughout the week and adjust the avalanche danger ratings and avalanche problems as necessary. For now, enjoy the new snow and stay alert to the changing avalanche conditions as things develop.
