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Avalanche Forecast

Archived

Mar 16th, 2018–Mar 17th, 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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It may look and feel like spring, but you can still trigger avalanches on specific slopes Saturday. You are most likely to trigger an avalanche on steep slopes receiving direct sunshine, on wind loaded slopes at higher elevations, or in areas where weak older snow still exists within the snowpack. Even though it is becoming more difficult to trigger a persistent slab avalanche, they may still initiate from large triggers or shallower locations in the snowpack.

Detailed Forecast

We have been talking about persistent and deep persistent slabs in the Cascades for a month now. These difficult to assess avalanche problems have resulted in fatalities each of the last three weekends. This is a classic low likelihood-high consequence scenario. Tracks on a slope do not indicate that it is safe. It may take a very large trigger or finding a shallower spot in the snowpack to cause a deep persistent slab avalanche. We continue to receive reports from professionals around the region demonstrating that the snowpack structure for these weak layers is still present. These same professionals are conveying that they are still avoiding large avalanche start zones as they travel. If you want to avoid triggering a persistent slab, stay out of large steep open areas.

Expect spring like conditions on steep sunny slopes. You may encounter firm surface conditions in the morning. If you travel on these slopes while they are still firm it may be difficult to stop a fall.

The sun will soften surface snow until clouds increase in the afternoon. Expect this to occur first on steep sunny slopes moving from eastern to southern to western aspects as the day progresses. If you see new loose wet avalanches, notice new roller balls, or experience soft surface snow deeper than your ankle, it’s time to change slopes. Use slope aspect to stay ahead of any loose wet avalanche problems. Be aware of high consequence slopes such as above cliffs or creeks where even a small loose wet avalanche may have higher consequences.

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

In general 2-6 inches of new snow fell Wednesday along the East Slopes of the Cascades. The highest totals have been reported closest to the Cascade Crest. Winds during and immediately following the storm built shallow wind slabs on lee slopes at higher elevations. This new snow fell on a new melt-freeze crust in some locations. Observations show the new snow is generally well bonded to this most recent crust.

On sunny aspects, the sun created wet surface snow conditions Thursday and Friday. On shaded slopes cold dry snow still exists.

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) 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.