Northwest Mountains

Skiers and riders yesterday easily triggered slabs breaking in the drifted snow.  More snow and wind transport today will continue to fuel dangerous avalanche conditions on wind-loaded slopes, where the threat of large avalanches breaking in the new snow is most severe.  In wind-sheltered terrain, monitor for shallow storm instabilities and assess the snowpack for the isolated chance of a large avalanche breaking deeper into the snowpack on buried weak layers.   

Today's storm is tracking a bit further south than yesterday's forecast, and storm totals have been downgraded.  We're keeping the danger at Moderate below treeline, but if storm totals exceed the forecast, expect a higher danger.  Avalanche conditions are most dangerous near and above treeline, where intense wind loading occurred throughout yesterday.  Today's pulse will reignite snow and wind transport, thickening yesterday's wind-drifted slabs and forming a fresh generation of drifted slabs.  Stay away from leeward terrain to avoid the most dangerous slopes.  

In wind-sheltered terrain, there is potential to trigger sluffs or soft slabs in the new snow.  Irwin reported storm slabs starting to develop yesterday afternoon.  Periods of heavy snowfall this afternoon could form thin and reactive slabs in today's new snow, or could spur instabilities at the storm interface, which is about 6" to 12" deep this morning.   The hazard posed by these relatively shallow instabilities will depend on what type of terrain you are riding in: the more consequential, the more hazardous.  You can monitor for storm slab formation on small, steep test slopes and by keeping an eye on new snow accumulations. 

Unfortunately, we have the nagging threat of avalanches breaking deeper than the storm snow.   Persistent weak layers in the upper and lower snowpack have been getting stressed during the loading events of the past few weeks.  The persistent slab problem has recently produced avalanches and obvious signs of instability at all elevations in the Southeast Mountains.  One layer of concern is small-grained facets or faceted crusts up to a few feet deep, which has produced collapses and avalanche activity on Mount Emmons (Example A, Example B, Example C).  We have yet to observe collapses or avalanches on these layers in the deeper snowpack of the Northwest Mountains.  However, we have found a few concerning pit results in recent weeks, such as this propagating test this week in the Anthracite Range.  Given the significant loading this week, we recommend carefully assessing the snowpack and/or choosing more conservative terrain to manage for potential surprises of avalanches breaking deeper than expected.  Most concerns are in the upper few feet of the snowpack, and a well-placed extended column test can help in identifying potential issues.  In the shallower parts of the Southeast Mountains, there is also the isolated threat of avalanches breaking on weak layers near the ground, exemplified by this unstable test result near Mt. Whetstone.   Wind-loaded slopes are most worrisome for these deeper persistent slab issues.