Avalanche Forecast
Regions: Snoqualmie Pass.
UPDATE: Avalanche danger lowered to reflect less than anticipated snowfall totals. Uncertainty still remains about a Puget Sound convergence zone Tuesday afternoon. If this impacts Snoqualmie Pass, avalanche danger could rise rapidly.
A winter storm will bring rapidly changing avalanche conditions to Snoqualmie Pass. Expect dangerous avalanche conditions as new and reactive storm slabs grow larger during the day Tuesday. When storms like this impact the area, it’s a good time to dial it back and avoid any open slope greater than 35 degrees, and do not linger in areas where avalanches can run or stop.
Discussion
Snow and Avalanche Discussion
The largest winter storm in several weeks will impact the Snoqualmie Pass area Tuesday and cause the avalanche danger to rise rapidly. A natural avalanche cycle may occur as accumulating snow and high snowfall rates overload the snowpack. You may see some natural avalanches at any elevation, but avalanches will be larger and more widespread in the highest terrain. Continued snowfall and moderate ridgetop winds will keep the mountains dangerous throughout the day.
There is a bit of uncertainty heading into the late afternoon hours Tuesday. A Puget Sound convergence zone is expected to form. The exact positioning of this precipitation band is hard to nail down, but it may bring locally heavy snowfall and increasing avalanche danger to areas near Snoqualmie Pass.
The new snow is falling on a variety of snow surfaces. It’s tough to say how this will impact the avalanche danger Tuesday, but they are worth keeping on your mind as you head into the mountains. Observations from Snoqualmie Pass over the last several days found firm melt-freeze crusts, old wind-packed snow, small facets, and feathery surface hoar. You can use small hand pits, and shovel tilt tests to look for this new weak interface. If the facets or surface hoar are buried intact, avalanches on these slopes could break widely, fail remotely, or occur on slopes near 30 degrees.
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:
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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.
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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
A significant amount of snow, high precipitation rates, and winds strong enough to transport snow all indicate that natural and human triggered avalanches can occur Tuesday. This isn’t a day to try to out-think mother nature. Stay off open slopes greater than 35 degrees and don’t linger in the tracks and runouts of large avalanche paths. Use small slope tests, hand-pits, and visual observations to verify storm slabs as you travel. Do you feel strong snow over weak snow? Do you see cracking in the snow surface? Do small slopes fail? If you answer yes, storm slabs are present.
Be leery of areas where the wind drifts the snow. Moderate to strong winds during this system may form deeper and firmer slabs at higher elevations. In these locations, avalanches may be larger and break above your head.
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