Avalanche Forecast
Regions: Snoqualmie Pass.
You’ll need to balance two very different avalanche problems Thursday around Snoqualmie Pass. Lingering wind slabs are becoming harder to trigger, but be leery of convex rollovers, and very steep slopes where you see signs of wind affected snow. If you see new rollerballs or fan-shaped avalanches, steer away from similar nearby avalanche terrain.
Discussion
Snow and Avalanche Discussion
Clouds hung around Snoqualmie Pass for much of the day Wednesday limiting the loose wet avalanche activity. As the skies broke in the afternoon, rollerballs and loose wet avalanches were found on some slopes. Alpental patrol reported a few natural slab avalanches from Tuesday that appeared to fail on a layer of facets buried at the interface of the recent storm snow. Snowpack tests performed by NWAC staff also found this weak layer 10-12 inches below the snow surface. If you dig down you may find small needle-like facets at the base of the new snow. In some locations, these facets sit on top of a thin melt-freeze crust.
Picture: A tilt tests highlight the weak old interface at the base of the storm snow.
Thursday’s avalanche forecast is a bit tricky. The weather pattern at play could bring cloudy skies, which would limit the avalanche danger, or we could see patches of sunshine, where avalanche hazard could rise. Freezing levels are rising to elevations that we haven’t seen since January. While they aren’t exceptionally hot, they are quite warm and should catch your attention.  Make frequent weather observations, and be ready to adapt to changing conditions.
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. We’ll 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 loose wet avalanche cycle only played out in some locations Wednesday. There’s still plenty of cold snow to be affected by warming temperatures and sunshine. Keep your eyes open for new roller balls, fan-shaped avalanche debris, and moist thick surface snow. If you see any of these observations, loose wet avalanches could occur. Steer away from traveling on or under slopes where you see these observations. With a foot of recent snow, any loose wet avalanche could entrain significant amounts of new snow and become large.
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
Even though these wind slabs are a few days old, be on the lookout for slopes where the wind affected the snow. Be aware of features in the terrain where you are more likely to trigger a lingering wind slab such as near convex rollovers, under cornices, and on unsupported slopes. If you find firm, hollow snow under you, see sharp cornices, or experience uneven snow surfaces wind slabs may be nearby. Some of these clues may be hiding under a few inches of soft unconsolidated snow. In some locations, these wind slabs may sit on top of small facets. In these locations, the wind slab may not heal as fast as normal.
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