Avalanche Forecast
Regions: Cascades - North East.
Dangerous conditions can be expected for Sunday, with both dry snow, and wet avalanches. You may find a significant amount of new snow in the alpine, and temperatures may warm rapidly during the day. These are both red flags for avalanche danger. Plan a conservative tour that gives options to avoid avalanche terrain if conditions warrant.
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. We have no recent on the ground observations from the area, if you get out, let us know what you find by posting an observation. 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 the recent crust, or they may gouge into deeper layers. Rain on snow may cause snow to become wet and on all aspects up to 5,500ft or so. The snowpack may be punchy and unsupportable, especially in shallow areas near rocks, and previously shaded aspects such as northeast and northwest. The photo illustrates what can happen when a large slide is initiated, and weak snow is entrained on the way down. 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 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
Wind Slabs
Another round of light snow may arrive overnight into Sunday with snow levels initially quite low, but a rapid rise in temperature may aid in creating instability with previously drifted snow. Slab avalanches may be larger at higher elevations. 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 cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1