Avalanche Forecast

Issued: Dec 13th, 2018 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The snowpack has grown substantially this week around Paradise, Crystal, and White Pass, but with that come a risk for avalanches. Avoid steep open slopes above 6000’ where underlying old weak snow may produce dangerous avalanches.

Summary

Discussion

Avalanche Summary

Avalanche professionals in the Crystal area have reported large avalanches, whumphing, shooting cracks, and collapses this week. All this information points to one thing, unstable snow. Observations from Paradise have been limited due to dangerous weather and poor visibility. We know the snowpack is weak and recent storms have deposited significant new loads in some areas. This is a recipe for avalanches.

Snowpack Discussion

Regional Synopsis 20181213

Winter is here.

A productive winter storm pattern has elevated the avalanche danger and provided an interesting start to the season.  

A quick breakdown:

  • Tuesday and Wednesday saw a significant localized avalanche cycles.

  • We have a lot of new snow...2’ to 5’ above 4500ft.

  • We’ve gotten a lot of wind.

  • We have weak layers near the ground.

Dangerous avalanche conditions will persist over the next couple of days. That said, it is early season and the flavor of avalanche you may run into is likely a function of elevation, timing, and dumb luck.

Here are some basic emerging patterns:

  • Snowfall totals so far this week (Monday morning to Thursday evening) illustrate more recent snowfall in the north than the south:

    • Mt. Baker: 55”

    • Washington Pass: 29”

    • Stevens Pass: 37”

    • Snoqualmie Pass: 28”

    • Paradise: 38”

    • Mt. Hood Meadows: 13”

  • Upper versus Lower Elevations: The change in the snowpack is still 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.

  • East versus West: Loading along the East slope has been more incremental, and a  variety of buried facet and surface hoar layers may be found. This is most pronounced near WA Pass. Although significant snowfalls and precip totals have resulted in thicker, more homogeneous snowpack in the western zones, lingering weak layers near the ground will persist through the week.

The uncertainty of how reactive our buried weak layers are will carry into the weekend. Be cautious and get home safe.

 

Problems

Persistent Slabs

An icon showing Persistent Slabs

Persistent Slabs can be difficult to assess and predict. We have a high degree of uncertainty concerning the exact distribution of buried persistent layers in this region. This is a time to stay out of startzone in areas above 6000 feet where you are more likely to find old weak snow. Around Paradise 3 feet (Crystal 1-2 feet, White Pass 1-2 feet) of settled snow now sits atop a layer of weak sugar facets and/or surface hoar. If you trigger an avalanche on this layer it could be large and deadly.

You may encounter areas where the wind has deposited more snow over this layer into firm slabs. If you see evidence of wind transported snow, such as snow drifts, fresh cornices, or firm snow surfaces, stay off of nearby startzones.

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

Likely

Expected Size

1 - 1

Storm Slabs

An icon showing Storm Slabs

Below 6000 feet you will find storm slabs formed during this Thursday’s storm. These slabs are gaining strength but are not fully healed. You are most likely to trigger lingering storm slabs on convex rollovers, unsupported slopes, or in locations affected by the wind.

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.

Elevations: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Dec 14th, 2018 10:00AM