Avalanche Forecast
Issued: Apr 6th, 2019 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Wind Slabs.
NWAC,
Northwest Avalanche Center
Sunday will be a bit of a transition day in the West-North zone as a storm exits the area and the skies begin to break. You’ll need to balance wind slabs created during the storm and unstable wet snow caused by the sunshine. Either way, this is a good time to simply stay off of open slopes greater than 35 degrees and give the snowpack time to make this transition.
Summary
Discussion
NWAC staff observing several different areas in the West-North zone Saturday reported 6-10” of new snow in the near treeline area. More snow likely accumulated at higher elevations. On exposed slopes, the wind redistributed the snow and formed very stubborn slabs. In general, the new snow bonded strongly to the old snow surface, which in some areas had not yet refrozen. No significant avalanches were reported Saturday.
On Sunday, we’re expecting a change in the weather with decreasing precipitation and warming temperatures. As skies begin to clear, the strong April sun will quickly affect the surface snow. Expect snow conditions to change quickly, and give yourself plenty of options to limit your exposure to avalanche terrain. In general, the weather models keep the bulk of the moisture well south of the West-North zone but don’t rule out a passing shower.
Even though the recent weather felt like winter, there are still springtime hazards to consider. Cornices loom large, creeks are open, and glide cracks widen. Give all these hazards a wide berth and remember the recent snow may hide these and other obstacles.
Large sagging cornice and opening glide cracks in the Mt Baker area. The new snow hides some of these hazards. Photo: Lee Lazzara
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.
Problems
Loose Wet
It shouldn’t take long Sunday for the April sun to cause loose wet avalanches on steep sunny slopes. You’re likely to see these occur naturally during longer stretches of sunshine. Frequently we’ll see rollerballs and the trees shedding the new snow just prior to loose wet avalanche activity. 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
Expected Size
Wind Slabs
Wind slabs formed during stormy weather over the past several days. This slabs may grow a little deeper Saturday night as the last major round of precipitation impacts the area. It's tough to say how these slabs will react to the warmer and sunnier weather Sunday. Best to give them a bit of time to gain strength and heal. Use visual clues such as snow drifts, uneven snow surfaces, and firmer snow to help you identify and avoid wind loaded 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
Expected Size
Valid until: Apr 7th, 2019 11:00AM
