Avalanche Forecast
Regions: Cascades - South West.
We’re in a time of transition from winter to spring and things aren’t simple: you can trigger a large loose wet or slab avalanche on Friday. All areas have the potential to produce wet loose when if the sun comes out in full force. Slab avalanche problems may differ, so you will need to identify the problem and then adapt where and how you travel, noting recent avalanche activity, roller balls, or changing snow conditions as signs to consider adjusting your terrain selection.
Discussion
Snow and Avalanche Discussion
The West-South zone is demonstrating its variability. This makes it difficult to summarize conditions and point to specific avalanche problems. Situations like this rely on observations to verify the avalanche forecast. When your observations line-up with information you find here, then the forecast may be valid. If you see things that don’t line-up, stop, reevaluate, and dial back your terrain travel.
After several days of mostly cloudy or filtered sunshine, Friday is expected to feature the full force of the march sunshine. If this occurs, the snow will change rapidly. Loose wet avalanches may occur on any aspect receiving direct sunshine.
The most notable recent avalanche was a natural wind slab on an E aspect of Crown Pt on Wednesday in the Crystal backcountry. A few small loose wet avalanches and rollerballs were reported from steep sunny slopes late in the day.
A natural wind slab avalanche on the west face of Crown Pt in the Crystal backcountry. Photo: Ian Nicholson
Snowpack Discussion
March 14, 2019
It’s starting to feel a lot like spring. The sun is stronger. Temperatures are warmer. The days are longer. Each year March brings a period of transition for the snowpack. Despite the feel of spring, the transition to a spring snowpack could take quite a while. Right now, it’s still winter in the mountains.
A northeast through southeast facing bowl in Tunnel Creek, Stevens Pass. The red line shows the transition from northeast to east aspects. Crusts were on the surface of east aspects (right of the line). Surface snow on northeast was soft and dry. March 11th, 2019. Photo: Josh Hirshberg
Aspect dependent snowpacks
A common trend this time of year is the distinct difference between snowpacks on shaded slopes and those on sunny slopes. We might refer to this as aspect dependent snowpacks. The strong March sun can create crusts and send melt-water through the snowpack on sunny slopes.
That process is beginning, right now. As of the 14th, sun-exposed aspects have a crust at the surface. Otherwise, these slopes hold mostly dry layers in the upper 3 feet of snow. The sun now affects more aspects than in the earlier half of the winter. You may find sun affected and melt-freeze crust on any slope that received direct or indirect sunshine. This could include some northwest or even northeast aspects.
Shaded slopes are still holding onto cold winter like snowpacks. That doesn’t mean you won’t find any melt-forms in these areas, but the snowpack looks, feels, and acts more like mid-winter. Avalanche problems can follow a similar trend by aspect. You may see wet avalanche problems on sunny aspects and lingering dry slab problems on shaded slopes.
Two photos taken on March 10th in the Snow Lake area. The left is N facing terrain on Chair Peak with cold, dry surface snow. The right photo shows loose wet avalanches on E facing slopes of Mount Roosevelt. Photo: Ian Nicholson
Balancing problems
So, how do we balance these sometimes opposing snowpacks? In a word, observations. Transitional periods like this require diligence and planning. It seems obvious, but observations you make on a south-facing slope aren’t applicable to N facing terrain. Consider this as you plan your day. Secondly, you’ll want to make observations to verify the avalanche forecast. As we’ve discussed, snowpack structures can diverge during these transitional periods. When your observations line-up with the information you find in the daily avalanche forecast, the forecast is likely valid. If you see observations that don’t match, it’s time to stop, reevaluate, and possibly choose more conservative terrain options.
Making snowpack observations to verify the forecast. Photo: Pete Durr
Less elevated danger and changing trends
You may not find low avalanche danger on every avalanche forecast, but spring is a time of year when less significant hazards can allow backcountry travelers to explore deeper into the mountains. The longer days and often lower avalanche hazard can support stepping into bigger terrain. Remember that low danger doesn’t mean "no danger." Additionally, avalanche conditions can change quickly during spring due to rapid warming over the course of a day or snowfall from potent storms.
We forecast for very large regions. You travel on individual slopes. You may encounter anomalies in the landscape where conditions don’t match the forecast. As with any time you travel in snowy mountains, you can stay safe by using standard travel protocols to minimize your exposure to avalanche terrain and the hazards of the day. Keep your eyes open and be ready to adapt to changing conditions.
What's next?
Winter is far from over. Keep checking the forecast, tracking conditions, making observations, and telling us what you find.
Avalanche Problems
Loose Wet
The loose wet avalanche cycle didn’t get a chance to run its course in many areas Wednesday and Thursday with filtered sunshine. That means there’s still plenty of cold snow to be affected by the full March sunshine when it comes out on Friday. Navigate the slopes to avoid direct sunshine or exposure to slopes receiving direct sunshine above you. New roller balls, fan-shaped avalanche debris, or moist surface snow all indicate loose wet avalanches can occur; you should change your aspect and avoid similar slopes.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Storm Slabs
Not much has changed from Thursday to Friday. Don’t get caught up in the semantics of the slab avalanche problem. Locations across the West-South seem to be dealing with different types of slab avalanches. Here are the common themes. Slabs in every area formed by Wednesday, and are becoming harder to trigger. Slabs can be difficult to assess and predict. Be leery of any feature where you are more likely to trigger an avalanche such as near convex rollovers, on unsupported slopes, in locations where the wind drifted the snow, and in areas where the new snow is shallow. Where ever you travel, don’t try to outsmart these lingering slab avalanche problems,
Details: Storm slabs may be largest in areas such as Paradise where the most snow (18-20”) accumulated during this storm. In some locations, particularly around Crystal, you may find a weak layer of snow 12” inches below the snow surface. This layer has been lingering in the snowpack and Crystal Mountain Pro Patrol and NWAC observers recently confirmed reactivity on this layer and it may become more reactive if warming temperatures and sunshine help the slab to become more cohesive. Wind prone areas, such as White Pass, may see only wind drifted slabs. Use observations as you travel to identify which of these scenarios you are most likely experiencing.
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: Possible
Expected Size: 1 - 1