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Archived

Avalanche Forecast

Mar 10th, 2018–Mar 11th, 2018
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Stevens Pass.

Avalanche fatalities continue to occur in the Cascades. Do not drop your guard Sunday. Dangerous and complex avalanche conditions still exist in the mountains. Avoid avalanches by staying out of start zones. You will be more likely to trigger avalanches with daytime warming and settling of the snowpack.

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 changing weather creates changes in the avalanche hazard.

New persistent slab avalanches formed Friday and have been observed in several avalanches around the Stevens Pass area. 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. This is time to dial it back.

You will be able to trigger wet avalanches Sunday as warm temperatures and sunny skies create wet snow conditions. Expect new roller balls, pinwheels, and loose wet avalanches. These conditions will develop first on steep rock slopes receiving direct sun and will quickly spread to all aspects as temperatures rise. As water reaches deeper weak layers in the snowpack wet slab avalanches may occur. If you see any evidence of new wet slab avalanches be ready to significantly 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 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.

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

Snowpack Discussion

Several new 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 occurring 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 3-5 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. Snowpack tests on the 2/13 facet layer continue to demonstrate propagation of a potential avalanche.

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. 

Avalanche Problems

Persistent Slabs

Release of a cohesive layer of soft to hard 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 Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Elevations: Treeline, Below Treeline.

Likelihood: Likely

Expected Size: 1 - 1

Loose Wet

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

Wind Slabs

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: Possible

Expected Size: 1 - 1

Deep Persistent Slabs

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 or near the ground. 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. They commonly develop when Persistent Slabs become more deeply buried over time.

 

Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.

 

A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.

Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Unlikely

Expected Size: 1 - 2