South Rockies

If areas get more than 20mm of precipitation, treat the local danger as HIGH. Deep persistent slab avalanches are a major concern for the weekend and conservative decision-making is essential.

Poor - Timing, track, or intensity of incoming weather system is uncertain

An atmospheric river is bringing warm air and heavy precipitation to the Columbia regions Friday night and Saturday but a ridge of high pressure sitting over Idaho is forecast to deflect most of the precipitation from the South Rockies. However, there is currently weather model uncertainty with some models showing some of the precipitation making it through and others keeping it mostly dry. Amounts are ranging from 0-20mm but the most consistent amounts look like 5-10mm. Alpine winds are forecast to be strong from the SW to W. Freezing levels look to start at around 1500m at the beginning of the storm system but will climb to around 2500m by Saturday afternoon. Freezing levels are forecast to be around 3000m on Sunday and Monday. Both days are showing mostly sunny conditions with moderate-to-strong alpine winds from the west.

There has been limited observations from the South Rockies lately but there are some concerning trends. Explosives have been triggering large avalanches on the mid-Dec layer and tests are suggesting that the Nov layer near the ground may still be potentially reactive if triggered. In the Lizard region, there have been several human-triggered avalanches on the mid-Dec layer and two very large avalanches on the Nov layer. There are three major concerns for the weekend and into next week: (1) Models are currently not showing much precipitation but there is some chance that the region might see enough to form new storm or wind slabs. These smaller slab avalanches would have the potential to step-down to a deeper layer. (2) Prolonged warming at higher elevations will potentially make it easier to trigger one of the deep persistent layers. (3) When sun is coupled with the warm air on Sunday, natural avalanches may occur on south-facing slopes and may step down to deeper layers. Until we have more information, assume that deep persistent slab avalanches are possible.

Moderate-to-strong SW winds have and will continue to redistribute snow in the alpine and will continue to load leeward features. In sheltered areas, 10-20cm of snow overlies a rain crust to 1900m, facets, and/or surface hoar. The mid-December crust/facet layer is down 40-80cm. Tests indicate that this layer is still reactive and isolated large avalanches continue to release on this interface. Closer to the ground a crust/facet interface that formed in November is generally dormant but may still be reactive in isolated areas.