Avalanche Forecast
Regions: Cascades - South West.
The forecast sunshine should be enough to get loose wet avalanches going on W-S-E aspects where wet surface snow or roller balls remind you to adjust your aspect or choose lower-angle slopes. These avalanches will be largest in areas that received the most recent snowfall, such as Paradise. You are most likely to trigger a large slab avalanche on a convex rollover greater than 35 degrees.
Discussion
Snow and Avalanche Discussion
The West South zone experienced a natural cycle of small-large loose wet avalanches on Friday. After several days of mostly cloudy or filtered sunshine, the sunshine was in full force on Friday and produced lots of loose wet activity throughout the region. NWAC professionals found these avalanches to be larger in the Paradise/Tatoosh region than in the Crystal backcountry.
Warm temperatures and sun following a storm on the 11-13th are the main drivers of our avalanche concerns. With increased daytime warming, high temperatures each day will be the warmest we've seen since January. Although the snowpack hasn't made the transition to spring, surface conditions are making that transition on W-S-E aspects. Consider wet snow, cornices, and timing of your travel through the terrain.
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 the 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.
The most notable recent slab avalanche was a natural wind slab on an E aspect of Crown Pt on Wednesday in the Crystal backcountry. There were also several small storm slabs on steep south-facing slopes and a few small loose wet avalanches and rollerballs reported from steep sunny slopes late in the day.
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
There’s still more cold snow to be affected by the strong March sunshine on Saturday, so expect large Loose Wet avalanches, particularly in the Paradise/Tatoosh areas where more recent snow fell during the storm. 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
We’ve kept the storm slabs around longer than we usually do because we have some interfaces that are taking longer to heal. Those interfaces can be found under storm slabs or wind-affected drifts and slabs. Don’t get caught up in the semantics of the slab avalanche problem, rather focus on terrain avoidance where you are most likely to trigger a slab. Be leery of slopes steeper than 35 degrees with convex rollovers on unsupported slopes particularly in locations where the wind drifted the snow. Wherever you travel, don’t try to outsmart these lingering slab avalanche problems. The colder snow on northerly aspects is likely to draw you to these locations, but your risk may be slightly greater on these aspects as well.
Details: Storm slabs may be largest in areas such as Paradise where the most snow (24”) accumulated during this storm. In some locations, particularly around Crystal, you may find several weak layers within the snowpack now ~12” inches below the snow surface. This layer has been lingering in the snowpack and NWAC observers confirmed propagation on this layer on Friday in snowpack tests, but a lack of avalanche activity is a sign that it will take unsupported terrain features to get these moving. The structure of the overlying slab is changing and another day of warm temperatures keeps those changes on our mind. Wind prone areas, such as White Pass and Paradise, may see only wind drifted slabs on northerly aspects or an increasingly dense layer of snow on southerly aspects. Any of these slabs may sit on a layer of fine-grained facets, with slightly less reactivity on the interface in the Tatoosh/Paradise area than in the Crystal region. Use observations as you travel to identify the slab structure in your area and test the weak interface 1-2’ down in the snow before you enter complex terrain.
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