Avalanche Forecast
Regions: Stevens Pass.
You can trigger avalanches in the recent snow that fell since Friday. Expect predictable, loose wet avalanches on steep slopes and some isolated wind slabs near ridge tops. Light precipitation with warm temperatures will continue for Monday.
Discussion
Snow and Avalanche Discussion:
About 1 foot of snow accumulated above 4,000ft on Stevens Pass since Friday. Westerly wind formed drifts up to 2 feet deep. On Sunday, observers reported a widespread cycle of natural and triggered loose wet avalanches in the recent snow on steep slopes above 4,000ft and on all aspects. One slab avalanche occurred on an east aspect, 5,000ft near the ridge at Gemini Pass. The recent snow is sitting on a thin crust over weak, wet snow.
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.†When surfaces freeze and become firm, bring the right equipment to mitigate the hazard of sliding down icy slopes.
Debris from a natural loose wet avalanche (D2), Lichtenberg Mtn, SW, 5,000ft. 4/7/2019. Photo: Josh Hirshberg
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.
Â
A natural loose wet avalanche (D1), Lichtenberg Mtn, N, 4,850ft. 4/7/2019. Photo: Will Govus
Â
Spring avalanche considerations
As you head into the mountains there are a few questions to ask yourself common to spring avalanche conditions:
-
Is there any recent snow accumulation that could cause avalanches? If so, what kind of avalanches could you trigger? And where?
-
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?
-
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?
Debris from a natural loose wet avalanche (D2), Lichtenberg Mtn, SW, 5,000ft. 4/7/2019. Photo: Josh Hirshberg
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.
Glide avalanches and holes opening up in rocky terrain near Mount Herman. Photo: Andrew Kiefer
Avalanche Problems
Loose Wet
You may be able to easily trigger loose wet avalanches in the recent snow on steep slopes (40 degrees and steeper). If the sun comes out, you may see natural avalanches on sun-exposed slopes. Most loose wet avalanches will be small and predictable, involving the top 1 foot of the snowpack. It’s important to respect these avalanches as they can be heavy, forceful, and could be dangerous if they catch you off guard.
If you don’t have experience with these avalanches, consider staying off of slopes steeper than 35 degrees. Use small test slope to check the bond of the recent snow. Make sure other backcountry travelers don’t trigger avalanches on top of you. Stop and regroup in safer areas, out from under slopes that are being traveled on.
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: Possible
Expected Size: 1 - 1
Wind Slabs
Wind slabs will be very localized to leeward slopes near the top of ridges. You can find fresh cornices and drifts up to 2 feet deep. Be especially aware of recently drifted snow if you plan on traveling to peaks above 5,500 feet or well into the above treeline band. Wind slab avalanches are becoming harder to trigger. Watch for slopes over 35 degrees where you see cracks in the snow or areas of snow that feel stiffer underfoot. Steer around obvious drifts and avoid being on or under cornices.
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