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Avalanche Forecast

Archived

Mar 17th, 2018–Mar 18th, 2018

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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You can trigger avalanches on wind loaded slopes at higher elevations, in areas of weak older snow deeper in the snowpack, or on steep sunny slopes slopes. Even though Persistent Slabs are becoming less likely, watch out for shallower areas of snow where you can still trigger these avalanches. Weak layers buried deep in old snow have been responsible for fatalities and serious injuries each of the past four weekends.

Detailed Forecast

We have been talking about Persistent Slabs in the Cascades for a month now. These difficult to assess avalanche problems have been the cause avalanche fatalities or serious injuries every weekend for the past 4 weeks. Persistent Slab avalanches are now difficult to trigger. If you manage to trigger one, the avalanche may be large enough that you will have little chance of surviving. Currently, you will see significant backcountry traffic in avalanche terrain, but little signs of avalanches. Test results and recent avalanches show that these layers can still be triggered. The best way to stay safe from the potential consequences is to limit your exposure to large avalanche paths. Stay out of start zones and complex terrain in large avalanche paths.

The threat of Loose Wet avalanches lingers today. If the skies clear and the sun makes an appearance, watch for uncohesive, wet snow on steep slopes. While Loose Wet avalanches are easier to predict and avoid, they can still be powerful enough to push you into trees, over cliffs, and through rocks. Use caution near these terrain traps on steep sun-exposed slopes. Until southerly slopes get softened by the sun they will remain hard and slick. If you travel on these slopes while they are still firm it may be difficult to stop a fall.

Small winds slabs have been reported at higher elevations. You are most likely to trigger a wind slab on convex rollovers or steep unsupported slopes where wind loading has occurred.

Snowpack Discussion

Light snow accumulations this weekend are falling on recent crusts and wind affected surfaces. In general 2-6 inches of new snow fell Wednesday along the East Slopes of the Cascades. On sunny, lower elevation slopes this snow melted and formed a crust. Winds during and immediately following the storm built shallow wind slabs on lee slopes at higher elevations. 

Below the most recent snow and crust, several older persistent weak layers continue to be reported. Observations from the Eastern areas of the Cascades have found these layers reactive and 3 avalanche fatalities occurred on persistent weak layers in early March. The exact persistent weak layer depends on your location.

Commonly seen persistent weak layers are:

  • Surface hoar and facets buried on 3/8. This layer is typically found 8-14" below the snow surface near and below treeline. The distribution of this layer is not widespread. 
  • A more widespread persistent weak layer is found just above a firm crust layer buried on 2/5. This layer of weak sugary facets (2/13 or 16) can be found 2-4 feet below the snow surface.
  • Other shallow and very weak snowpacks may be found further east of the crest or in areas with less snow.

Observations

North

On Thursday, North Cascades Heli reported 6 inches of new snow over a variety of old snow surfaces. Where found, the 3/8 buried surface hoar layer continues to be reactive in snowpack tests. This layer is 10-15 inches below the snow surface in this location. The 2/5 firm crust was observed down about 2.5-3 feet below.

Central

NWAC professional observer Matt Primomo traveled in the North Fork of the Teanaway River Thursday (just north of Cle Elum). Matt found 2-4 inches of new snow over a generally supportable crust. The 3/8 buried surface hoard was found about 12 inches below the snow surface. It was reactive in some snowpack tests. The 2/5 layer was also found in this area about 3 feet below the snow surface.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Loose Wet

Loose Wet avalanches are the 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. 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.

Wind Slabs

Wind Slab avalanches are the 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.