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Archived

Avalanche Forecast

Dec 15th, 2018–Dec 16th, 2018
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Snoqualmie Pass.

You could still trigger dangerous avalanches at upper elevations. Avoid steep slopes where you see pillows of drifted snow above treeline. If you experience collapsing, cracking in the snow, or see recent avalanches, avoid all open slopes 35 degrees or steeper.

Snowpack Discussion

Regional Synopsis 20181214

We made it through our first strong winter storm and are headed into a weekend with dangerous and fickle avalanche conditions. Thick slabs of new snow (2-5 feet) are perched above a weak layer of faceted crystals. Avalanches triggered on this layer could be very large and life threatening.

Reports continue to come in of very large natural and explosives triggered avalanches in the northern and eastern zones. For perspective, several of these slides have been classified as D3, or large enough to destroy a house.

Anyone accessing alpine areas should limit their exposure to areas where avalanches start, run and stop. In some places the weak snow will talk to you by whumpfing and cracking. In other places, the heavy blanket of new snow is thick enough that it can give a false sense of stability while it masks the dangerous layering below.

Approximate snow totals from 12/10 - 12/14:  

  • Mt. Baker: 55”

  • Washington Pass: 29”

  • Stevens Pass: 37”

  • Snoqualmie Pass: 28”

  • Paradise: 38”

  • Mt. Hood Meadows: 13”

  • Olympics: mostly rain

The change in the snowpack is pretty dramatic with elevation. Height of snow decreases rapidly below 4500’ at Baker and Washington Pass, 5500’ at Crystal/Rainier. The Passes have better low elevation coverage, but it's still pretty thin below 4000’. With additional warm storms in the forecast, this pattern is expected to continue for awhile.

Be cautious and get home safe.

Avalanche Problems

Persistent Slabs

You could still trigger avalanches big enough to bury or kill you. There's uncertainty about how easily you can trigger these avalanches. Persistent slabs are difficult to predict and often break much larger and wider than you expect. The best way to stay safe is to avoid open slopes 35 degrees and steeper where persistent slabs are suspected. Assume that you could trigger a dangerous persistent slab avalanche on any slope where you see wind-sculpted features. Put an extra buffer of terrain between where you travel and anywhere avalanches could start. Stay well out from under large avalanche paths and do not stop near these features.

You will find a layer of weak, sugar-like, faceted snow down 3-4 feet below the surface of many slopes. Where this snowpack structure exists, you may see signs of instability such as collapsing (a whumpfing sound), recent avalanches, snowpack tests that show propagation, or cracks that travel away from where you impact the snow. These are all indicators that you should avoid avalanche terrain.

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.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Possible

Expected Size: 1 - 1

Loose Wet

New snow and rain will cause wet avalanches at lower elevations starting midday.

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.

Elevations: Below Treeline.

Likelihood: Possible

Expected Size: 1 - 1