Avalanche Forecast
Regions: Snoqualmie Pass.
Expect conditions to change during the day Wednesday as the snowpack transitions from a cold storm to warmer temperatures and sunshine. Keep your eyes open for places where the wind drifted the snow into deeper slabs, and avoid wind loaded slopes greater than 35 degrees. As the sun comes out, expect natural loose avalanches from steep sun-exposed slopes. Don’t get surprised by these easy to predict and avoid avalanches.
Discussion
Snow and Avalanche Discussion
Wednesday will be a bit of a transition day for the snowpack around Snoqualmie Pass. The snowpack will be changing from a cold winter storm to slightly warmer temperatures, decreasing snow showers, and partly sunny skies. Expect the conditions to change quickly. Slopes you travel in the morning may be very different by mid-day.
This storm didn’t produce quite as much snow as expected around Snoqualmie Pass, but combined with significant winds we did notice a marked increase in the avalanche danger. Alpental Ski Patrol noted a few small natural storm slabs on steep slopes, mainly at the bases of cliffs.
The new snow buried a variety of snow surfaces including firm crusts, wind packed snow, facets, and surface hoar. So far we don’t have any confirmed avalanches on these layers, but they are on our mind. Take the time to look at the base of the new snow. Do you see a thin grey line? Do small slope tests fail and avalanche? If so, you may be dealing with a new buried weak layer. We’ll keep our eye on this layer over the next few days.
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:
-
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.
-
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
Wind Slabs
Observations from Snoqualmie Pass found that the winds efficiently transported the new snow during the day Tuesday. These new wind slabs should still be reactive Wednesday. Be on the lookout for places the wind drifted the snow into firmer and thicker slabs. If the snow feels firm under you, you see fresh cornices, or you find large snow drifts, steer away from any nearby slopes greater than 35 degrees. Some of these wind slabs may be very firm and lure you out onto them before they fail. Give these wind slabs a wide berth.
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: Likely
Expected Size: 1 - 1
Loose Wet
Expect the March sun to quickly change the surface snow. As the sun comes out, you may see natural loose avalanches originating from steep sunny slopes. These avalanches may entrain significant amounts of snow as they travel downhill. Don’t get surprised by one of these avalanches. Stay away from steep sun-exposed slopes and be very cautious around gullies and cliffs where even small avalanches may 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.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1