Avalanche Forecast
Regions: Snoqualmie Pass.
Changes in the weather could rapidly affect the type, likelihood, and size of potential avalanches you encounter Sunday. Build options into your plan that allow you to minimize your exposure to avalanche terrain. If the sun comes out, expect natural loose wet avalanches to occur on steep sun-exposed slopes.
Discussion
Snow and Avalanche Discussion
The amount of new snow you find at Snoqualmie Pass will largely dependent on elevations. As of Saturday afternoon 4-6†of snow had accumulated at the top of Alpental with only a dusting at Pass level. Reports indicate that this new snow bonded well to the old wet snow surface. In some locations, this old wet snow has not refrozen yet.
The weather models continue to struggle with each round of precipitation. The latest information suggests that the bulk of the moisture will stay well south of Snoqualmie Pass and that the bulk of the warming will occur after the precipitation stops. At present, we’re even thinking the sun may make an appearance for part of the day. Will that be the case? You’ll have to pay attention to the weather and recognize if you are experiencing conditions that fall outside of the forecast. When you see observations that don’t line-up, seek lower angle and lower consequence terrain.
Even though the recent weather felt like winter, there are several springtime hazards in the mountains. Creeks, particularly at low elevations, opened wide during the recent warm weather. Many creek crossings are becoming difficult and potentially dangerous. Glide cracks continue to grow and a few glide avalanches have been found. Holes appeared near many trees and rock. Use caution when you travel near any of these spring hazards that could be hidden by a few inches of new snow.
Snow bridges over many creeks are beginning to sag, crack, and fail. Choose your creek crossings carefully. Photo: Dallas Glass
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
With warming temperatures and sunshine in the forecast, we’re worried about the new snow falling downhill. Expect the natural loose wet avalanches to occur from steep sunny slopes. The size of these avalanches will likely depend on the elevations where they start, and how much new snow is on the ground. Fortunately, loose wet avalanches often give us clues like new rollerballs and the snow shedding from trees. Don’t get surprised by this easy to predict and avoid avalanche problem. Limit your exposure to steep overhead slopes, and stay off of slopes greater than 35 degrees when the snow surface becomes wet and sticky.
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
Be on the lookout for places where the wind drifted the new snow into slightly deeper drifts. It's these wind affected areas where you are most likely to trigger a slab avalanche. How will these slabs handle the warmer temperatures and sunshine? Hard to say. Best to just give them a little time to heal. When you see uneven snow surfaces or find deeper drifts, steer away from nearby slopes greater than 35 degrees.
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