Avalanche Forecast
Regions: Stevens Pass.
Dangerous conditions can be expected for Sunday, with both dry snow, and wet avalanches. Significant recent accumulations may be found up high, and temperatures may warm rapidly during the day. These are both red flags for avalanche danger. Sunday would be a good day to travel with a conservative mindset, and avoid avalanche terrain.
Discussion
Snow and Avalanche Discussion:
The mountains are caught between the changing seasons. Significantly more snow may be found as one goes up in elevation. On Saturay Stevens Pass Mountain Resort reported 4-5" of new snow. Small loose wet avalanches became easy to initiate, as temperatures and freezing levels crept up in the afternoon. The stormy weekend continues, and so will the avalanche danger. Dangerous conditions may develop by Sunday morning with another bout of snow and rain with warming temperatures. There is a wide range of precipitation in the weather models, as such, avalanche danger may vary widely. You’ll need to pay attention to the conditions around you and determine if the weather you’re experiencing lies outside of the forecast. When you see observations that don’t line-up with what you expected, dial back your travel to lower angled slopes.
Be sure to consider all the hazards that come with spring in the mountains. Factor in a good margin for error as hard to predict events like cornice fall, glide avalanches, icefall, rockfall, and a general “shed cycle†in the mountains has been the theme, but maybe more so after this latest round of storms.Â
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Snowpack Discussion
April 3rd, 2019
Spring snowmelt
The snowpack in much of the Cascades has changed dramatically in the past two weeks. The weather has shifted solidly to spring-like patterns. The spring warm-up started in mid-March with a prolonged period of relatively clear skies and warm temperatures. Moving into April, we’re seeing periods of unsettled spring weather bringing rain to many low and mid-elevation slopes and snow to upper elevations.
Very bare southeast aspects of Rock Mtn/Nason Ridge. April 2nd. Photo: Josh Hirshberg
Since the peak height of snow in mid to late February, mountain weather stations in the 4,000-5,000ft range show an average of 27% decrease in height of snow. Looking at weather stations in nearly every zone, the percentage decrease ranged from 22-29%. This year's spring snowmelt is much earlier than normal. Traveling in the mountains the loss of snow coverage is most noticeable on southerly, sun-exposed slopes and below 4,000ft. On northerly aspects and slopes above 5,500ft, the snowpack has seen less dramatic changes and has even maintained some dry layers.
NWAC climatological snow depth data from April 1st. You can view it on our website here.
Spring avalanche considerations
As you head into the mountains there are a few questions to ask yourself common to spring avalanche conditions:
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Is there any recent snow accumulation that could cause avalanches? If so, what kind of avalanches could you trigger? And where?
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What are the high and low temperatures of the past 24 hours as well as the forecasted temperatures during the time you’ll be in the mountains? Could these create weak, wet snow surfaces?
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How is the cloud cover contributing to the melting or freezing of surface snow? Did clear skies allow for a sufficient overnight freeze? Will the sun be strong enough to weaken surface layers?
Glide avalanches and holes opening up in rocky terrain near Mount Herman. Photo: Andrew Kiefer
Other considerations
In addition to daily avalanche hazard, the early snowmelt is creating some other travel considerations. Some roads and lower elevation slopes may not have enough continuous snow coverage for travel on snow machines. Holes melted around rocks, trees, and creeks could create a fall hazard. When nighttime temperatures and cloud cover allow for surface freezes, bring appropriate equipment to mitigate slip and fall hazard on steep slopes.
We are approaching the end of our daily avalanche forecasting season. The mountain weather forecast will continue into the spring, and the weather station data is available year round. Keep checking the forecast for conditions updates on the end of season information.
Avalanche Problems
Loose Wet
New snow sitting on a crust is a great recipe for wet loose avalanches, especially with a rise in temperatures and some strong sun! We may get all of these ingredients on Sunday. Wet loose slides may run on a recent crust or wet snow, and they may entrain significant amounts of snow on their way downhill. A rapid rise in temperatures and rain may cause snow to become wet on all aspects up to 6,000ft or so. Watch out for these on slopes that face the sun if the sun breaks out during the day as well. Signs that may clue you into wet loose instabilities are rollerballs, tree bombs, moist snow over dry snow, and other fan shaped avalanches. When you see these things happening, avoid the runout zones of steep slopes, and avoid going onto them.
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: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
Storm Slabs
Another round of snow may arrive overnight into Sunday with snow levels initially quite low, but a rapid rise in temperature may create instabilities. Previously drifted snow may get covered up and disguised at higher elevations, making conditions tricky to read. The bottom line is that lots of new snow can be expected in the high mountains, a few potential weak layers exist, and slab avalanches may become large. If you find signs of instability such as cracking underfoot, and recently formed drifts, avoid steep slopes. The warming temperatures and strong sun by afternoon may increase your chances of triggering slides, and wet slabs are also possible.
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