Avalanche Forecast
Regions: Cascades - South West.
We’re only a few days out from a massive winter storm and the snowpack still needs time to heal. If you travel to higher elevations, steer around steep wind drifted pockets, areas below cornices, and steep convex rollovers where you are more likely to trigger an avalanche. In the Crystal region, very large and destructive avalanches have been observed and require an added layer of caution.
Discussion
The snowpack and resulting avalanche hazard change by location and elevation around the West South region. You may experience locally lower avalanche danger in areas that did not receive the massive water numbers of Crystal and Paradise (7.45†and 6.57†respectively). At lower elevations there just isn’t enough soft snow to form an avalanche hazard. Instead, expect difficult travel conditions with refreezing snow, exposed objects, and open creeks.Â
Several very large (D3) natural and explosive triggered avalanches were observed in the Crystal region over the last few days. These events occurred on Friday and Saturday during the meat of the storm. Recent avalanches clearly indicate unstable snow is present.Â
A very large natural avalanche near the Crystal Mountain ski area which likely occurred during the storm. Lizard’s Back, N aspect, 6000ft. Photo: Robin Pendry.Â
Snowpack Discussion
December 19th, 2019Â (The regional synopsis is updated every Thursday @ 6 pm)
Let’s take a moment to recap what happened over the past week:
A potent winter storm arrived on the 12th, adding to the very shallow snowpack throughout the region. Anywhere from 6-36†of snow fell between the 11th and 15th. Places like Mt Baker, Paradise, Mt Hood, and White Pass were the winners regarding snowfall, with quite a bit less for areas further east of the crest like Mission Ridge, Blewett Pass, and Washington Pass.Â
The most notable avalanche activity occurred in the Mt. Baker backcountry where numerous human triggered storm slab avalanches occurred (with several big enough to injure, bury, or kill a person). The majority of these occurred on upper elevation, north and east facing terrain. Besides the danger posed by the size of the avalanche, many slides quickly revealed rocks and other obstacles barely hidden below the snow surface.Â
The storm layer took time bond with the underlying snow surface, and we saw the likelihood of triggering slab avalanches slowly decrease over a handful of days. The avalanche danger was at Considerable for many places on Saturday the 14th, then tapered to Moderate on Sunday, and eventually reached Low in many places by Tuesday and Wednesday.Â
A large, human triggered avalanche on the old snow interface. East aspect of Table Mountain (West-North Zone) at 5,500ft. December 15,2019. Photo by Brooks Broom.Â
Backcountry travelers have noted generally thin and shallow snowpacks, with a range of structures across the region. Many folks were skeptical of the layering they found. People were able to dig down and observe weak layers in many areas. These were buried in late November and early December. In some places they presented as weak snow over a crust, in others, a layer of feather-like surface hoar. Cold temperatures have likely preserved many of these layers, and will be worth considering as the snow piles up.
Below is a list of commonly visited locations and their snowpack depths in inches (as of December 19th at 4am). Check the weather station hourly data feed and watch as the subtropical javelin of moisture (also known as a strong atmospheric river) changes conditions dramatically over the next few days.Â
Total Snow Depth in Inches as of December 19th at 04:00 AM.
-Matt Primomo
Avalanche Problems
Storm Slabs
Storm slabs will be larger, more widespread, and easier to trigger as you ascend in elevation. When you find more than 6 inches dry snow, you’ve entered a different snowpack. Stop, regroup, and identify the features you want to avoid. You are most likely to trigger a storm slab in steep wind drifted snow, in locations below fresh cornices, and on convex rollovers. If you see natural avalanches or cracks shooting across the snow surface, dial down your terrain selection and avoid all slopes greater than 35 degrees.
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: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Persistent Slabs
Recent very large and destructive avalanches prove this persistent weak layer survived the storm loading in some areas. Avoid traveling on large steep slopes above 5800’ on W-N-E aspects. Snowpits and crown profiles identified a layer of weak sugary facets and/or feathery surface hoar associated with a thin crust about a foot above the ground. Persistent slabs are difficult to assess, but if you trigger an avalanche it could be 3-6ft deep, break on slopes as low as 30 degrees, and wrap around features in the terrain. While snowpack tests can confirm this layer’s presence, they can not prove its absence. Don’t rely on a snowpit to interpret stability. This layer and these events have only been confirmed in the Crystal region. We don’t know if this layer exists in other areas, please submit your observations to our website.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 2 - 2