Avalanche Forecast
Regions: Stevens Pass.
UPDATE: Avalanche danger changed to reflect lower than anticipated snowfall amounts.
A potent storm will increase the avalanche danger for Tuesday. Over a foot of new snow is expected, at slightly warmer temperatures and with winds. The new snow may not bond well to the old surface. Stay off of, and out from underneath steep slopes.
Discussion
Snow and Avalanche Discussion:
A major change is on the way. The storm headed our way will be one of the strongest in a few weeks. The temperatures have been quite cold recently, and current snow surfaces are a mix of sun crusts on southerly aspects to weak, faceted snow on northerlies. Recently buried layers were also weak and faceted.  The slightly warmer temperatures, and winds with this storm are ideal for slab formation. If a convergence zone sets up, Stevens Pass may get continued snowfall into the night on Tuesday, further increasing the danger. Now is as good a time as any to remember that most avalanches occur during, or directly after a snowfall event.Â
Over the weekend, the avalanche danger eased, and folks were able to get out and enjoy a couple of very nice days with good stability and great snow in the mountains. This storm will increase the danger from Low to High, a major bump up!Â
Snowpack Discussion
March 10, 2019
February started stormy and cold with very active avalanche conditions on persistent weak layers. The month finished with continued cold temperatures but drier weather allowed avalanche activity to taper off. Persistent weak layers that were buried early in the month (Feb 8th) are now unreactive, though you can still find the grains.
Aside from the series of storms in early to mid-February, most regions have measured relatively light snow accumulation in the past 3 weeks. Snow has stayed soft especially on shaded slopes and faceting and surface hoar have been plentiful. While there has been plenty of sunshine since mid-February, very cold temperatures have kept melt-freeze crust to a minimum on sunny slopes.
A natural loose wet avalanche (D1.5) on a southeast aspect of Lichtenberg Mtn at 5,400ft. 3/10/2019. Photo: Josh Hirshberg
Recent Avalanches
Our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow and weak snow surfaces on both dry, shaded slopes and sun-exposed aspects. In some locations, a weak layer of surface hoar and facets was buried on March 6th. We’ll keep an eye on this interface as we move into the future.
A skier-triggered storm slab avalanche (D1.5) on Shuksan Arm, north aspect, 5200ft. 03/09/19 Adam U Photo.Â
Moving Forward
As we move further into March, there are two points to consider:
-
The strength of the March sun: As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.
-
Weak surfaces: facets and surface hoar have blanketed snow surfaces in many zones. Any significant snowfall will bury well-developed and widespread persistent weak layers. Recently, most storms have not delivered enough precipitation to cause a problem.
Avalanche Problems
Storm Slabs
Heavy snow and winds will rapidly increase the danger, creating storm slab instabilities by early morning on Tuesday. The new snow will be landing on a variety of surfaces from surface facets and surface hoar on shaded aspects to melt freeze crusts on southerly aspects. The slightly warmer temperatures associated with the storm, along with wind should be ideal for slab formation. Slab avalanches may run naturally during the day at upper elevations, where heavy wind loading will transport snow-both new and old, onto leeward slopes. The snow may fracture quite wide on shaded aspects where the recently buried layer is weak and faceted. Any avalanches on the old surface may step down a foot or so to layers from early March. Slides may run quite a distance, encompassing much of a terrain feature at upper elevations. If you see obvious signs of instability such as shooting cracks and recent slab avalanches on small steep slopes, it is time to stay off of, and out from underneath steep open slopes.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Very Likely
Expected Size: 1 - 1