Surface hoar buried up to 18" deep may be triggered by skiers and riders above 6000' in the Seeley zone. Avalanches are more likely on the west side of the zone. Wind slab may be triggered on upper elevation slopes below ridgeline and along the edges of gullies. Avoid slopes over 35º, and wind loaded pockets.
Ridgetop winds out of the west have been strong enough to transport snow. Wind slab may be found today in upper elevations below ridgeline and along the edges of gullies on east slopes. Wind slab will be small but may sit on top of the buried surface hoar, making it both deeper and touchier. Avoid wind loaded pockets over 35º to manage this problem.
A layer of buried surface hoar can be found in both the Seeley zone at elevations above 6000. With recent fresh snow, it is up to 18" below the surface. Wind drifted areas below ridgeline have the potential to be deeper and denser. Slides were easily triggered on this layer on Saturday near Marshall lake, and several were triggered Monday in the Lake Dinah area. Given the nature of buried surface hoar, two days is not enough time for the problem to go away. However, we are not seeing obvious signs of instability like shooting cracks, so it feels stable despite the lurking weak layer.
On the west side of the zone, this layer can be found on all aspects, although southern aspects may be less touchy. In the southern Swans, it is somewhat patchy, and reports from yesterday indicate no movement on the layer.
Skiers and riders could trigger slides on this layer in the right terrain. Likelihood can be reduced by avoiding convex rolls over 35º and unsupported slopes. If you choose to ride in steep terrain, carefully assess the snowpack for a lower-density stripe of buried surface hoar. Step back to mellower slopes if it is found. Use small, safe, and representative convex rolls to test the reactivity of the layer. Avoid traveling in or above terrain traps. Whumpfing or cracking are obvious signs to stay in low angle terrain.
We are removing the basal facets from the problem list in the southern Swans. While facets can be found at the base of the snowpack in certain areas, they are well-rounded and increasing in strength. There hasn't been an avalanche on this layer since Christmas despite loading, warmth, and extensive testing by high markers. A slide is unlikely, however, it is always best to avoid shallow areas where sugary snow underlies a cohesive slab.
(1/14/2023)
D1.5 Avalanche that slid on buried surface hoar near Marshall Lake
Photo: Caleb Kesterke
A public observation yesterday noted sluffing on Murphy in the Rattlesnake that rapidly entrained more snow. Fresh snow in the Seeley zone is capable of similar loose dry avalanches. If you choose to enter very steep terrain, greater than about 40º, practice sluff management by skiing out of the sluff and allowing it to run. Plan your descent carefully and avoid terrain traps like tight chutes where loose snow can become funneled and powerful.
(1/18/2023)
The debris from a loose dry avalanche is visible on the apron below Mt. Murphy.
Photo: Kevin Bissett
Since the last forecast the Seeley zone has received up to 0.7" of snow water equivalent(SWE). Temperatures have been mild but below freezing, and winds have been light to moderate out of the west. These are good conditions for the snowpack that will help it gain strength. Basal facets that have been plaguing us for over a month have been removed from the problems in the Swans and Rattlesnake. They are gaining strength in the southern Bitterroots, but there is still too much uncertainty to remove them yet. New snow from Monday, up to 10" in the northern Bitterroot, is bonding well to old snow surfaces and melt-freeze crusts. However, a buried surface hoar layer is causing issues throughout the forecast area. This one will linger for a bit and may become more reactive as new snow consolidates into a cohesive slab.
Crusts extending to 7000' have been covered by new snow, and the riding has improved considerably since earlier in the week. The possibility of avalanches at mid-elevations remains low in the Bitterroot and Rattlesnake. However, exceptions exist, and avalanche conditions could be found. Stay alert to changing snow conditions and red flags as you travel.
The lower elevation snowpack has shrunk considerably, and many early-season hazards are re-exposed. In the central Bitterroot, slopes are almost bare to 6000' on the south aspects and 5300' on the north. Today temperatures and cloud cover should keep snow surfaces frozen. However, wet loose may be possible if temperatures warm enough in areas with fresh low-elevation snow. These would be small but could push a skier into hazards.
The high-quality observations that continue to be submitted greatly help the backcountry community and forecasters. Thank you for submitting them. You can submit further observations here.
Ski and ride safe.
