Avalanche Forecast

Archived

Mar 11th, 2018–Mar 12th, 2018

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

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.

Dangerous and complex avalanche conditions still exist in the Stevens Pass area. Continue to stay off of all open slopes greater than 30 degrees. Realize that a Cornice failure or wet snow avalanche could trigger a large and destructive older Wind Slab or a Persistent Slab avalanche. Limit your exposure to overhead hazard Monday, staying away from large avalanche paths.

Detailed Forecast

Several overlapping and dangerous avalanche problems exist in the Stevens Pass area. This is a time to rein it in. Avoid all avalanche start zones as continued warm temperatures and sunny weather maintain dangerous avalanche conditions Monday.

New persistent slabs formed Friday (3/8) and have been observed in several recent avalanches around the Stevens Pass area. Continue to stay off of all open slopes greater than 30 degrees; this includes areas low on the slope and well away from higher start zones. This weak and potentially dangerous layer has been observed on most aspects near and below treeline. Persistent slabs are difficult to manage and have proven deadly in the Cascades over the last several weeks.

You will be able to trigger wet snow avalanches Monday as warm temperatures and sunny skies create wet snow conditions. However, E-SE winds will locally cool the snow surface in the central and south Cascades, creating a more variable loose wet problem than Sunday. Expect new rollerballs, pinwheels, and loose wet avalanches to occur. These conditions will develop first on steep rocky slopes receiving direct sunshine and especially on wind sheltered slopes. Unstable wet snow will spread to other aspects as the day warms. As water reaches deeper weak layers in the snowpack, wet slab avalanches may occur. If you see any evidence of new wet slab avalanches dial back your terrain use and avoid all avalanche terrain.

You are most likely to find and trigger lingering wind slab avalanches on shaded slopes where winds recently deposited snow. Use visual clues to identify and avoid slopes where wind loading has occurred.

Several older deeper persistent weak layers exist in the snowpack. This is a classic low likelihood-high consequence scenario. Just because you see tracks on a slope does not mean it is safe. While it may be very difficult to trigger these deeper layers, any avalanche failing on them will be large and destructive. The avalanche that resulted in a fatality Saturday on Park Butte near Mt. Baker is believed to have released down to the 2/8 crust - 2/13 facet interface. This is a recent example of the low likelihood-high consequence structure that remains in our snowpack.

Limit your exposure to overhead cornices as you travel. They may fail without warning due to daytime heating and the sun.

Snowpack Discussion

Mild temperatures and sunny skies effected snow surfaces Saturday and Sunday. Widespread wet loose activity was reported Sunday across the west slopes of the Cascades on steep sunny slopes. In the Stevens Pass area, several slab avalanches were reported Saturday. Most of them are thought to have occurred on the new persistent weak layer (3/8). Avalanches have occurred on most aspects near and below treeline. Many of them occurred noticeably low on the slope. Observations have found buried surface hoar (4-6mm) and small (0.5mm) facets. These weak persistent layers were found below the 12-15 inches of recent storm snow.

Sunny skies and mild temperatures effected surface snow on most slopes Saturday. On Thursday night and Friday 12-15 inches of new snow fell in the Stevens area. Moderate to strong SW winds transported the new snow including on more exposed open slopes below treeline slopes. This new snow fell on a variety of snow surfaces including surface hoar, near surface facets, and melt-freeze crusts.

Several older persistent weak layers can still be found in the snowpack including a widespread layer of weak sugary facets (2/13) that can be found just above a very firm and thick crust (2/8). Snowpack tests continue to show that if a failure is initiated on this layer it can propagate and cause an avalanche. This layer is generally found 3-4 feet below the snow surface.

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

Observation

Stevens Pass Ski Patrol found buried surface hoar (3/8) below the recent storm snow on both NE and SW aspects at 5500’ Saturday.

An avalanche professional in the Stevens Pass area on Saturday reported new observed slab avalanches on both SE and N aspects. He also experienced one whumph on the 3/8 persistent layer while traveling through lower angled terrain.

NWAC professional observer Matt Primomo traveled near Tye Peak Friday. Matt reported 12-15 inch slab avalanches on Moonlight Bowl (E, 4500), Skyline Ridge (NW, 5200’) and Tye Peak (SE, 4800’). He also experienced several large whumphs while traveling a ridgeline above Tye Lake. Observations in the area found surface hoar and facets (3/8) buried below the recent storm snow.

An avalanche professional near Vahalla Lake Friday also found buried surface hoar 15 inches below the snow surface. He observed a rain crust up to 4300 feet below the most recent snow.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.