Northwest Mountains

The softest snow and safest riding is in wind-sheltered terrain, where your primary concerns are shallow sluffing in very steep terrain.   Follow the direction of recent wind-loading to find the most dangerous slopes.  On terrain features loaded by recent northerly winds, you can trigger hard slabs up to several feet thick.   Slabs could break unexpectedly above you as you cross from the thick to thin parts of the slab.   

In Upper Cement Creek yesterday, we observed more significant snow transport from the Christmas wind event than expected and compared to what we observed closer to town.  The northerly winds redistributed far more than the few inches of recent snow; they transported most of the season's snowpack (even some brown pow!) in exposed areas and gathered more fuel as they shifted to atypical northeast winds.  We estimate that some of the most wind-loaded features were stressed by water weights greater than our major winter storms (3" to 8+" of SWE).   It's no surprise, then, that a handful of larger persistent slabs released with that event, breaking on early season weak layers.   With this new evidence of significant activity and frequent signs of instability yesterday, the danger will be slower to fall, particularly in the Southeast Mountains.  

Now that we are a few days out from the major wind-loading event, wind slabs are expected to be healing except where they formed over persistent weak layers.  We've made some notable changes to the persistent slab problem by merging our recent wind slab and old persistent slab problems.  The distribution rose now highlights slopes where you are most likely to encounter issues with our early season weak layers (11/16 and 11/30 interfaces, several feet deep), and more commonly, slopes where the recently drifted slabs formed over facets or faceted crusts (12/23 interface,  up to 1 or 2 feet deep on drifted slopes).    We have fewer data points for the 12/23 interface, so we may be adjusting the rose in the upcoming days as we get more data.  A few of the key points: 1) the north and northwest aspects have seen more erosion than loading in recent weeks, and activity on early-season weak layers there has been quiet since December 10th, so we've trimmed those aspects.  2) High-elevation terrain on the south half of the compass has some concerning faceted crusts underneath the most recent drifts, and in some cases, underneath older generations of wind-drifted slabs.  3). Recently buried facet layers or faceted crusts (12/23 interface) are weaker on slopes with previously shallow snowpacks, either from previous avalanches, seasonal drought east of town, or previous wind erosion.  4) For lack of major snowfall events in the past few weeks, the extent of recent drifting is the major driver for our concerns right now; slopes with productive fetches loading onto leeward slopes are most worrisome.  You can visually identify the most loaded slopes by reading snow texture and considering terrain exposure to recent wind events.  5) After a few days of cold, clear weather and slab faceting below treeline, our concerns for triggering a shallow soft slab on buried surface hoar are diminishing, so we've removed below treeline terrain from the problem list.