Avalanche Forecast
Regions: Stevens Pass.
Expect wind slabs to be more stubborn on Thursday, but still quite possible. As the temperatures warm up, loose wet avalanches may become easy to initiate, even with cloud cover. Avoid recent, firm wind slabs on steep slopes and be alert to changing snow surfaces.
Discussion
Snow and Avalanche Discussion:
The recent storm buried a mix of sun crusts on southerly aspects and weak, faceted snow on northerlies. Stevens Pass received about 8" of snow with less than 0.5" of water equivalent out of Monday/Tuesday's storm. A few new, small loose wet avalanches were observed by mid morning on the 13th. An observer was able to easily initiate small (D1) loose dry avalanches on steep west to north to northeast aspects around 5,000ft. On the 12th, Professional control teams reported numerous dry loose avalanches (up to D1.5) in the morning, and WSDOT triggered one large (D2) wind slab on a northwest aspect in wind exposed terrain above the highway. This highlights where some of the most dangerous conditions may be found, places where thick wind slabs overlie weak snow.
For Thursday, expect warming temperatures to begin changing the snow surfaces.Â
Snowpack Discussion
March 10, 2019
February started stormy and cold with very active avalanche conditions on persistent weak layers. The month finished with continued cold temperatures but drier weather allowed avalanche activity to taper off. Persistent weak layers that were buried early in the month (Feb 8th) are now unreactive, though you can still find the grains.
Aside from the series of storms in early to mid-February, most regions have measured relatively light snow accumulation in the past 3 weeks. Snow has stayed soft especially on shaded slopes and faceting and surface hoar have been plentiful. While there has been plenty of sunshine since mid-February, very cold temperatures have kept melt-freeze crust to a minimum on sunny slopes.
A natural loose wet avalanche (D1.5) on a southeast aspect of Lichtenberg Mtn at 5,400ft. 3/10/2019. Photo: Josh Hirshberg
Recent Avalanches
Our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow and weak snow surfaces on both dry, shaded slopes and sun-exposed aspects. In some locations, a weak layer of surface hoar and facets was buried on March 6th. Weâll keep an eye on this interface as we move into the future.
A skier-triggered storm slab avalanche (D1.5) on Shuksan Arm, north aspect, 5200ft. 03/09/19 Adam U Photo.Â
Moving Forward
As we move further into March, there are two points to consider:
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The strength of the March sun: As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.
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Weak surfaces: facets and surface hoar have blanketed snow surfaces in many zones. Any significant snowfall will bury well-developed and widespread persistent weak layers. Recently, most storms have not delivered enough precipitation to cause a problem.
Avalanche Problems
Loose Wet
The clouds may or may not be thick enough to keep the snow surface cool and dry. If the sun comes through, a "greenhouse effect" may occur, and loose wet avalanches may begin to run by late morning. These will begin first on east and southeasterly aspects, and continue onto south and southwest aspects as the afternoon rolls on. Look for signs of loose wet instability such as wet, heavy surface snow, rollerballs, and fan shaped avalanches. The larger the slope, the large these slides may become. If you notice these clues, heed the warning signs and avoid going underneath steep slopes where loose wet slides may run naturally.
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: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Wind Slabs
The recent storm snow landed on a variety of surfaces from surface facets and surface hoar on shaded aspects to melt freeze crusts on southerly aspects. The moderate westerly winds built slabs on exposed open slopes. The snow may fracture quite wide on shaded aspects where the recently buried layer is weak and faceted. Any avalanches on the old surface may step down a foot or so to layers from early March. Check for a strong, cohesive layer of snow sitting over weaker snow. If you see obvious clues to instability such as shooting cracks and wind loaded texture, it's a good bet to stay off of steep wind loaded 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: Possible
Expected Size: 1 - 1