Triggering a wind slab along ridgelines is likely today. Persistent slab avalanches with high consequences remain possible on upper elevation northern terrain.
If you find poor structure or notice wind loading, avoid slopes over 35 degrees.
10 inches of new snow overnight combined with shifting wind has created sensitive wind slabs. These slabs are expected on most aspects along ridgelines and in cross loaded gullies.
Avoid any pillows or loaded features and look for cracking.
These avalanches are more likely to slide on slopes steeper than 33 degrees. Terrain management is the best avoidance strategy.
Triggering a persistent slab up to 5 feet deep is a possibility on north aspects at upper elevations. While unlikely, the consequences for an avalanche this size are high.
These avalanches are likely to fail on basal facets, especially in the comparatively shallow snowpack of the southern Swans.
Terrain management is the only way to avoid this problem. Dig in areas you are hoping to ski or ride and if you find sugary snow underneath a slab, stick to terrain below 35 degrees. Avoid convex rollovers or shallow areas of the slab, as these are likely trigger points.
Over the last 24 hours, a quick system brought new snow and shifting winds. Snow totals were higher in the Seeley Lake zone, with the North Fork Jocko SNOTEL reporting 10 inches of snow at .7 inches SWE (snow water equivalent). Moving south, the Rattlesnake received 4 inches of snow at .6 SWE, and the central Bitterroot received 2” new snow at .2” SWE. Temperatures for this storm were warm, especially in the southern half of the forecast zone resulting in high snow density. Winds during the storm shifted from east to south to northwest, complicating the distribution of wind loading. Weather previous to this storm was generally calm, with mild temperatures, light winds, and minimal new snow.
Today, expect to find pockets of wind slab on all aspects at upper elevation. Dense snow combined with shifting winds makes the exact distribution of these slabs problematic to forecast. Observations show evidence of previous wind loading onto east slopes and associated avalanche activity. Two natural wind slabs were seen in the southern Bitterroot, likely failing in the past 48 hours. In the gash, large cornice growth and failure were seen as well. The older wind slab is stubborn. Expect any wind slab formed in the past 24 hours to be sensitive and reactive.
The persistent slab problem is improving, and the snowpack is gaining strength. The poor structure remains with facets on north slopes with a slab above. While the likelihood of triggering one of these avalanches is low, the consequences are still very high. Avoid steep northern terrain and give this problem more time to heal. At middle and lower elevations, where the Christmas crust is thicker, it is unlikely to affect the snow below the crust. At upper elevations with a thinner crust, it is possible to trigger weak layers below the crust.
Dig in the snow, and if you find weak snow and a slab above, avoidance is the best strategy. When in doubt, manage terrain and stick to slopes below 35 degrees.
Finally, there have been multiple avalanche fatalities this year in the United States where individuals involved did not have all their equipment. Remember, always carry a beacon, probe, and shovel. Practice safe travel and only expose one person at a time to hazard.
Please help us by submitting an observation of what you are seeing out there. Even an observation as simple as a few words and a photo helps us produce the best forecasts possible.
Ski and ride safe!
