Snoqualmie Pass

Moderate to heavy snowfall, strong winds, and known weak layers in the snowpack all point toward dangerous avalanche conditions developing on Tuesday. Overlapping avalanche problems and some uncertainty around the storm track could make this a complex day. Help yourself out by keeping things simple and staying off slopes over 35 degrees where triggering an avalanche is more likely.

Multiple waves of stormy weather have impacted the Pass since Sunday, bringing low-density snowfall and strong winds. The incoming wave on Monday night and Tuesday should be the most significant of this wave train, with higher precip totals and likely the strongest winds yet. With low-density snow already in place and Wind Slabs already formed on Sunday and Monday, this next round may be the one to start to tip the scales into more widespread avalanche activity. 

On Sunday, Alpental Pro Patrol reported a few small natural Wind Slab avalanches near the end of their day. There was also a skier-triggered Wind Slab in the Kendall Adventure Zone on Sunday. Snowmobilers in the Stampede Pass area reported wind drifts as large as 18” while other areas were scoured down to the previous surface. On Monday, more reports of wind effect came in from skiers in the Pineapple Pass area. These recent avalanches all occurred during the height of the storm on Sunday afternoon but showed lingering effects well after the storm had ended. On Tuesday, the bulk of the storm should be during the daylight hours and hazard will likely increase throughout the day, highlighting the need for you to evaluate terrain carefully. 

The other complexity of this forecast is our known buried weak layers. The most recent 12/18 interface is the one in the forefront of our minds for Tuesday, and it will receive its first true test. This is the interface you should be targeting when making your hazard analysis. Prior to the storm parade, weak snow in the form of surface hoar and near surface facets was found on most aspects. Some of this may have been wiped out by the winds, but in many areas, the light snowfall may have buried these weak grains intact. On slopes that are exposed to sunshine, a sun crust was observed prior to the storm and could make a great sliding surface for all of this new storm snow, whether that’s in the form of a loose or slab avalanche. Rimed precipitation particles/graupel were also identified near the 12/18 interface, adding another weak snow grain to the mix in some locations. If avalanches start to fail on these persistent grain types, they could act in surprising ways by breaking above you, wrapping terrain features, or spanning far distances. Again, your best mitigation tool is to avoid slopes over 35 degrees until this storm plays out.