Northwest Mountains

Several close encounters on the Aspen side (ex. Pearl Mtn, Marble), remote triggers, and a large triggered avalanche on Schuylkill Ridge (video) indicate sensitive conditions on our December dryspell weak layer.  This problem is most concerning near and above treeline on the north half of the compass, while recent drifts remain a concern in leeward terrain on other aspects.  Generally safe travel exists on wind-sheltered slopes with strong, thick crusts below the new snow—generally below about 10,700 feet on all aspects, or higher on the southern half of the compass.

Scroll through yesterday's avalanche observations for a clear picture of the types of slopes where you’re most likely to find trouble today.  With ongoing wind transport yesterday, the most transparent problem - wind slabs - remains a concern.  With the most recent wind loading pointed towards the southeast, many of these slabs formed over thicker crusts and non-persistent weak layers, making them less sensitive to triggers today. Even so, reduce your risk by watching for surface clues and avoiding obvious drifts, especially in steeper or more consequential terrain.

The more subtle but consequential instability is our newest persistent slab problem. This issue stems from this week’s snowfall landing on weak facets that formed during the three-week December drought, as illustrated by targeted fieldwork yesterday on Schuylkill Ridge. This problem requires two ingredients: a slab and a persistent weak layer.

• Slab: Since precipitation returned on Christmas Day, snowfall has favored higher elevations and the Northwest Mountains. 4-day SWE totals range from about 2 inches at Lake Irwin and Schofield, to 1 inch near Pearl Pass, and just 0.6 inches at CBMR. Slab formation is most common on wind-loaded slopes across the forecast area, and in some sheltered terrain in favored areas west and north of town. Slabs typically range from 1' to 2' thick. 
• Weak layer: This is where things get tricky. If you were chasing soft turns last week, you know they only existed in specific places—and those same slopes, or those with very thin crusts, now host the weakest snow below the recent snowfall. Southerly aspects were typically cooked into stout crusts, exposed slopes were sometimes hammered into wind board by extreme winds, and low elevations saw rain create thick melt-freeze crusts. Meanwhile, a subset of slopes remained shallow, cold, faceted, and protected throughout the December drought and subsequent bizarre weather. Where a slab now sits on top of those preserved facets, avalanches can be easy to trigger in steep terrain.

The current, somewhat unusual danger rating spread reflects this equally unusual combination of ingredients. The persistent slab problem is generally confined to the colder half of the compass near and above treeline. Near-treeline terrain in the Northwest Mountains may be the bullseye, where slab formation most consistently overlaps with preserved weak layers (noting that alpine observations following the extreme wind event were limited). Over the next few days, we expect slab sensitivity to gradually decline and the problem to become more localized as our young slabs decay. Given the number of people encountering or being surprised by avalanches yesterday, and because avalanche activity reached D2 in size, a conservative mindset is warranted in snow-favored parts of the forecast area today, just a day after the loading event ended.