At upper elevations, the new snow and the old snow are not expected to bond well. This tricky, reactive storm interface warrants careful assessment and conservative terrain choices.
Wednesday's storm brought 15-20 cm of new snow and strong southwest winds to the mountains as the freezing level climbed to 1500 m. Where weak surface hoar at the interface was preserved by cold temperatures and where the new snow is deep or drifted, we expect that storm slab avalanches will remain sensitive to human triggering.
Wednesday night: Decreasing cloudiness, scattered flurries with up to 5 cm of snow above 1000 m, winds decreasing to moderate and becoming west, freezing level dropping to 500 m.
Thursday: Increasing cloudiness, no precipitation expected, moderate to strong northwest winds, treeline high temperatures near -6 C, freezing level around 300 m.
Friday: Mostly cloudy, 5-15 cm of new snow, moderate south winds, treeline high temperatures near -5 C, freezing level just above sea level.
Saturday: Mainly cloudy, scattered flurries with 2-10 cm of snow, moderate southwest winds, treeline high temperatures near -7 C, freezing level dropping to sea level.
Observers on Saturday reported roller balls and very small (0.5-1) wet loose avalanches releasing naturally in saturated snow on steep features below treeline.
It may be likely to trigger avalanches at upper elevations on Thursday, particularly where new snow has been transported by wind into deeper drifts and where surface hoar has been preserved at the interface as a result of colder temperatures.
Wednesday's storm brought 15-20 cm of new snow and strong southwest winds to the mountains over a widespread layer of large, feathery surface hoar crystals. You can see photos of the surface hoar in our field team's MIN report from Elk Mountain on Tuesday. Freezing levels peaked at around 1500 m during the storm. As temperatures cool and the freezing level drops on Thursday, rain-saturated snow below treeline is expected to stabilize. At upper elevations, expect reactive storm slab conditions to persist where the surface hoar is preserved and where the new snow has formed a slab.
Following the strong southwest winds during the storm, winds are forecast to become northwest on Thursday. As winds pick-up, stay alert to the potential for tricky cross-loading or reverse-loading patterns. Monitor for snow that feels more dense, deep, or "slabby" from exposure to wind, and gather information about the bond between the new and old snow before stepping out. Start simple and seek out softer conditions in wind-sheltered areas.
Below a meter of snow from mid-December storms, two crust layers can be found that formed in early December. Small facets (sugary snow) had been reported near these crusts that are likely trending unreactive. Below these crust layers, the snowpack is generally well-settled. Early season hazards such as rocks, stumps, and open creeks remain at lower elevations.
