Avalanche Forecast

Archived

Mar 14th, 2018–Mar 15th, 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

Stevens Pass.

You may be able to trigger avalanches in specific locations Thursday around Stevens Pass. You are most likely to trigger an avalanche on steep sunny slopes with wet surface snow, on wind loaded features above treeline, and on slopes where older weak persistent layers still exist.

Detailed Forecast

A weak layer of surface hoar and/or facets was buried on March 8th in the Stevens Pass. This resulted in several natural avalanches on the 9th. Observations continue to find this layer 12-18 inches below the snow surface in this region. Snow profiles and snowpack tests can be used to identify this layer but cannot prove its absences. Avoid triggering a persistent slab by staying off of large open slopes greater than 35 degrees.

Expect new roller balls, pinwheels, and loose wet avalanches Thursday. The sun will create unstable wet surface snow conditions. Expect these conditions to develop first on steep sunny slopes and move from eastern to southern to western aspects. Stay away from steep slopes where you see signs of wet surface snow. Loose wet avalanches can seem common and are often small, however they can have large consequences if they carry you into gullies, over cliffs, or through trees.

If you travel above treeline, use visual clues to identify and avoid areas where recent winds have deposited snow. Snow drifts, wind textured snow, and uneven snow surfaces with cracking all indicate that wind slabs may exist nearby. You are most likely to trigger a wind slab on steep convex rollovers, unsupported slopes, or cross-loaded features.

Even though it is not listed as a problem, Deep Persistent Slabs may still occur in the Stevens area. This is a classic low likelihood-high consequence scenario. Remember tracks on a slope do not mean it is safe. You can avoid this difficult to predict weak layer by simply staying out of large avalanche startzones.

Cornices may grow weak and fail during day time warming and periods of direct sunshine. Select travel routes and techniques that limit your exposure to cornices above you.

Snowpack Discussion

Six inches of snow fell in the Stevens area Tuesday night and Wednesday. At higher elevations new shallow wind slabs formed on lee slopes. The new snow bonded well to the old snow surface. There are no reports of newly buried weak layers at this time.

Avalanche activity Wednsday was limited to very small loose wet avalanches and rollerballs that occurred with day time heating. One skier triggered loose wet avalanche was observed on an east facing test slope at 5200’.

Below the recent snow a new crust is strengthening as it freezes and grows. On Wednesday this new crust was generally not supportable. It ranged in thickness from 1-4” depending on location.

A layer of weak surface hoar and facets buried on March 8th can be found 12-18 inches below the snow surface in some locations. While this persistent weak layer has been observed on most aspects near and below treeline.

An older deeper persistent weak layer can be found 3 to 5 feet below the snow surface. Weak sugary facets are located just above a very firm and widespread crust (2/8).

There are no significant layers of concern below the 2/8 crust.

Observations

NWAC forecaster Dallas Glass was in the Stevens backcountry Wednesday. Dallas found 6 inches of new snow well bonded to the old snow surface. Snowpack tests in several locations continued to identify the 3/8 persistent weak layer.

Several professional avalanche observations from all around the Stevens Pass area Friday through Sunday reported avalanches and snowpack tests confirming the presence of a buried persistent weak layer (3/8) on most aspects near and below treeline.

Numerous snowpack tests from the Stevens Pass area gathered over the last several weeks by multiple avalanche professionals continue to suggest that the 2/13 facet layer can produce avalanches. The most consistent test has been the Propagation Saw Test.

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.