Avalanche Forecast
Regions: Cascades - South West.
Update: Avalanche danger decreased to reflect lower snow totals than expected. You are still likely to encounter the highest avalanche danger around Mt Rainier and the other Cascade Volcanoes.
Be cautious Tuesday as dangerous avalanche conditions develop across the West South zone. Stay away from open slopes greater than 35 degrees and don’t linger in locations where avalanches can run and stop. You’ll find the highest avalanche danger concentrated on the volcanoes where more precipitation creates larger and more widespread avalanches.
Discussion
Snow and Avalanche Discussion
A significant winter storm will bring rapidly rising avalanche danger to the West-South zone. Unfortunately, this system may not impact all areas of the zone equally. Expect the highest snowfall totals and avalanche danger on and near the volcanoes with slightly lower avalanche danger in other locations such as Crystal Mt and White Pass. No matter where you are, this storm will bring a big change from recent conditions and should cause you to step back.
There is some uncertainty in the weather and avalanche forecasts Tuesday afternoon and into Tuesday evening. The weather models are in good agreement that a Puget Sound convergence zone will form and impact the mountains. The exact positioning of the precipitation band is tough to nail down, but it may impact areas near Crystal Mt. Any area impacted by a convergence zone can expect heavy snowfall rates and increasing avalanche danger.
This new round of snow will bury yet another potentially weak interface. Firm crusts, facets, and surface hoar have been reported over the last few days. Will these layers be buried intact and cause a problem? That’s hard to say. However, now we have potentially two weak layers in the upper snowpack. This may mean we see larger avalanches than the storm totals might suggest. This is a good time to be patient and wait and see how these layers respond to the new snow load.
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
Significant snowfall amounts, high precipitation rates, and moderate to strong winds all indicate dangerous avalanche conditions may develop Tuesday. You may see avalanches occurring naturally, especially at higher elevations and in areas with intense wind loading. This isn’t a day to try and outsmart mother nature. Stay off any open slope greater than 35 degrees and don’t linger in the tracks and runouts of large avalanche paths. You can use simple observations to confirm storm slabs as you travel. Do small slopes avalanche as you travel on them? Do you find strong snow over weak snow in the upper snowpack? Do you see cracking in the storm snow as you travel? If you answer yes, you are dealing with storm slabs.
Be leery of areas where the wind affects the snow. Even in locations that receive lower snowfall amounts, winds will form thicker and firmer slabs. If you see signs of wind transported snow, steer away from these areas where avalanches may be larger and break above you.
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