Avalanche Forecast
Issued: Apr 4th, 2019 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
A significant storm will begin to impact the West-North region during the day Friday developing new avalanche problems and increasing the avalanche danger. Be leery of any slope greater than 35 degrees where you see evidence of wind affected snow. At lower elevations, you’ll find the warm temperatures and rain cause the snowpack to melt and fall apart especially around creeks, rocks, and cliffs.
Summary
Discussion
A change in the weather will begin Friday as a series of storm systems begin to impact the West-North zone. This will cause the avalanche danger to increase throughout the day. Weather models are in poor agreement about the timing and strength of each wave of precipitation. As a result, you’ll need to monitor conditions and recognize when the weather you’re experiencing diverges from the forecast. If you find your observations don’t line-up with what you expected, dial back your terrain use to lower angle and lower consequence slopes.
More water at lower elevations will only serve to fuel the ongoing spring transition of the snowpack. This week NWAC staff observed disappearing low elevation snow, opening creeks, holes around rocks and trees, and snow shedding from cliffs. In some ways, the low elevation snowpack is falling apart. This brings with it a unique set of non-avalanche related hazards to backcountry travelers. Give these springtime issues a wide berth especially during periods of rain and warm temperatures.
Glide avalanches and holes opening up on an east aspect of Mt Herman at 4300ft. 04/03/19 Photo: Andrew Kiefer
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
Wind Slabs
We don’t have much information about the state of the snowpack at upper elevations from the past few days, but more snow on Friday will only add to the snow received earlier in the week. This should allow for new wind slabs to form during the day as increasing winds redistribute the snow. In some areas, older more stubborn winds slabs could still linger. Use caution around any slope greater than 35 degrees where you see evidence of wind affected snow. Blowing snow, uneven snow surfaces, and deeper snow drifts all indicate that the wind may have formed wind slabs on nearby slopes.
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
Loose Wet
We can’t ignore the addition of more water into the snowpack at lower elevations. This snowpack has been through a lot, but when it gets this wet and slushy, unconsolidated snow can move downhill. Be leery of steep slopes where you find punchy wet snow. Use extra caution around creeks, gullies, and cliffs where even small loose wet avalanches could harm you.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Apr 5th, 2019 11:00AM
