Archived Avalanche Advisory covering Berthoud Pass, CO for Monday, Dec 26, 2022

Avalanche Forecast

Issued on:Sat, Dec 24, 2022 at 4:30 PM

Moderate - Above, at and below treeline

You can trigger a large avalanche on any slope where you find a cohesive slab of dense wind-drifted snow resting on weak snow. Northerly and easterly-facing slopes at upper elevations are the most suspect, but any cross-loaded slope with the aforementioned structure should not be overlooked. New snow may prevent you from being able to see, and avoid, bulbous drifts of snow below ridgelines, in gullies, and just downhill of convex rollovers. If you find yourself traveling over hollow-sounding dense slabs beneath low-density new snow, then you should keep your slope angle below about 30 degrees. Low-angle slopes that are not connected to steeper slopes above offer safer travel options.

Avalanche Problems
(1)
Problem: Persistent Slab
Aspect/Elevation: NW through E all elevations, SE At and Above treeline
Likelihood: Possible
Size: Large to Small

Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.

Northern Mountains Regional Discussion

The avalanche danger in the Northern Mountains has dropped to MODERATE (Level 2 of 5) across the region for Sunday. A wind and snow event (mostly wind) on Wednesday and Thursday pushed the November drought layer to the brink yet again and we saw another cycle of natural and human-triggered avalanches. This weak layer was buried around Thanksgiving and has caused avalanches with every loading event since. Since the wind event, the snowpack has had a few days to adjust to the new load, and avalanche activity is decreasing. While the structure for these avalanches exists on many aspects, east and southeast-facing slopes have been the bullseye for recent activity. In the past three days, 19 of the 30 reported avalanches occurred on these slopes.

In the deeper areas of the Northern Mountains, like the Park Range, Flat Tops, and Gore Range, you are more likely to find the Persistent Slab problem on west and northwest-facing aspects. In areas further south and east, like the Front Range, Berthoud and Loveland Passes, and the Tenmile Range westerly aspects are more wind-battered and less problematic. That being said, wind events like earlier this week can create unusual loading patterns and any slope with a dense and cohesive slab resting on the November drought layer is guilty until proven otherwise.

Avalanche size is another differentiator between our current forecast areas. Areas with the most snow are the same places you can trigger the largest avalanches. Unfortunately, these same areas will likely be the most stubborn to trigger and give you the least feedback in the form of cracking and collapsing. If you find the right spot where the weak layer is closer to the surface, like near rocks or bushes, you can trigger an avalanche. Thin spots in the slab are nearly impossible to identify by looking at a slope. Keeping slope angles less than about 30 degrees and leaving wide margins for error around steeper slopes are better management tools than trying to stay on the thickest parts of the slab.

Low-density snow from Friday night and Saturday may obscure surface clues like bulbous pillows of wind-drifted snow beneath ridgelines that should be avoided. Backcountry travelers should continue to dig and investigate any steep slopes they intend to ride on Sunday to make sure they are not on a dangerous slope. Incremental loading will continue through the weekend for the Park Range, but this likely will not add enough weight to increase the danger. With incremental loading events, it's difficult to pinpoint exactly when a slope might reach its tipping point. Again, dig and investigate to be sure that you're making conservative decisions.