Avalanche Forecast
Regions: Cascades - North West.
Heavy snow and wind during the day will develop dangerous conditions as the day progresses, with slabs rapidly increasing in size, extent, and reactivity. Anywhere you find more than 8” of snow over the crust, evaluate the new snow layering and bond to the crust using frequent hand shears or test slopes. As conditions deteriorate, steer clear of slopes steeper than 35 degrees.
Discussion
We’re not sure whether we’ll get to considerable danger at all elevations on Tuesday and it largely depends on how quickly the snow piles up. A period of very active weather and heavy snowfall starting Tuesday will continue through Wednesday resulting in further increased avalanche danger on Wednesday.
Looking backward, it’s been a wild week in the West North zone with cold deep snow conditions rapidly transitioning to wet surface snow with 10 inches of snowpack settlement at Heather Meadows since snow changed to rain on Sunday morning. Mt. Baker picked up unexpectedly significant rainfall totals on Monday (0.66†during the day) with snow mixing with rain at 5000 ft as temperatures cooled, dropping an inch of snow at Pan Dome. The inch or more of water since Saturday has wet the upper foot of the snowpack and triggered a frequent-flying glide slope:
Large glide avalanche on the Boulderfield, SSW aspect, Mt. Herman. Photo credit: Zack McGill, Baker Mountain Guides.
While we expect this is an isolated incident, it’s easier for us to anticipate where they might occur (steep, smooth rock slopes below and near treeline) than exactly when they might occur. The threat may persist up to several days after today's rain event.
Snowpack Discussion
New Regional Synopsis coming soon. We update the Regional Synopsis every Thursday at 6 pm.
Avalanche Problems
Storm Slabs
Heavy snowfall will build small storm slabs rapidly that may become large and touchy by the end of the day on Tuesday. You may start the day with firm snow surfaces and gradually building slab danger on wind-loaded slopes, but expect avalanche conditions to deteriorate and become more widespread as the riding conditions improve. Plan routes with return options that don’t expose you to large avalanche slopes. The old snow may or may not refreeze Monday night prior to the arrival of the new snow. A fully refrozen and smooth bed surface would make the new snow more reactive. Heavy snowfall also creates subtle weak layers within the new snow. Test these layers using hand shears, isolating columns or cutting across steep, small-scale terrain features. When in doubt, dial back your plans, and choose supported slopes less than 35 degrees.
Keep in mind that recently sagging cornices are weakened, but may break farther back than you expect and may trigger a large storm slab on the slopes 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: Likely
Expected Size: 1 - 1