Avalanche Forecast

Issued: Dec 17th, 2018 12:16PM

The alpine rating is high, the treeline rating is high, and the below treeline rating is considerable. Known problems include Storm Slabs, Persistent Slabs and Loose Wet.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Heavy precipitation, warming temperatures, and strong winds are the perfect set-up for avalanches Tuesday. When conditions are like this, it is time to stay away from areas where avalanches can start, run, and stop.

Summary

Discussion

Avalanche Summary

We received reports late Monday of a backcountry traveller caught and carried in the Crystal Mountain backcountry. The involvement occurred on a NW aspect near 6400'. Avalanche danger is expected to increase on Tuesday.

This area has not seen the same natural avalanche cycles as regions farther north. However, we received observations of a similar weak snowpack structure. While storms in this area have deposited good amounts of snow, the loading of the snowpack has been more incremental than in the Baker area. Tuesday’s storm will again significantly stress the snowpack. Will it be enough to wake up this buried persistent layer? It’s hard to say. We are expecting significant new precipitation, warming temperatures, and strong winds. This is a perfect set-up for avalanches.

Snowpack Discussion

Updated Regional Synopsis 20181216

Dangerous and fickle avalanche conditions remain as active weather continues into the week. 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 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. You can find similar snowpack structure responsible for these avalanches in many other locations throughout the Cascades, including Stevens and Snoqualmie Passes.

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 - 12/16:  

  • Mt. Baker: 75”

  • Washington Pass: 35”

  • Stevens Pass: 42”

  • Snoqualmie Pass: 36”

  • Paradise: 51”

  • Crystal Mt Base: 29”

  • Mt. Hood Meadows: 21”

  • Olympics: mix of rain and snow, no net gain

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. Mt. Hood and Hurricane Ridge have low snow coverage below 5000’.

If you're out in the mountains, please let us know what you see.

Be cautious and get home safe.

Problems

Storm Slabs

An icon showing Storm Slabs

You will be able to trigger new storm slabs Tuesday as warming temperatures and strong winds create an up-side-down snowpack. These avalanches can occur on any open slope greater than 35 degrees. In some locations winds will transport snow creating deeper and firmer slabs. Travel in any avalanche terrain is not recommended. Be aware that these relatively smaller avalanches could also act as the trigger for the much larger and deeper persistent slabs. If an avalanche steps down into older snow layers it could be unsurvivable.

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1 - 1

Persistent Slabs

An icon showing Persistent Slabs

A high degree of uncertainty surrounds our persistent slabs in the West South zone. Put a large buffer between you and any terrain where this avalanche problem could occur. We have observations of buried surface hoar and/or facets 2 to 4 feet below the snow surface in areas above 6000 feet. Persistent slabs can act in surprising ways with remote triggers and wide propagation. Avoid traveling in locations where avalanches can start, run, and stop during this large loading event.

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: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 2

Loose Wet

An icon showing Loose Wet

Small loose snow avalanches will occur at lower elevations receiving rain. Don’t let these small avalanches fool you. They can be dangerous if they carry you over cliffs, or into trees and rocks. Avoid steep terrain with wet surface snow. 

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

Likely

Expected Size

1 - 1

Valid until: Dec 18th, 2018 12:16PM