Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - North West Avalanche Forecast

Jan 4th, 2019 10:00AM

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

There’s a lot of new snow on the ground from the last two days. This recent storm produced several large and destructive avalanches. Give the snowpack time to adjust and strengthen by staying off of open slopes greater 30 degrees.

Summary

Discussion

Avalanche and Snowpack Discussion

We are just coming out from under an avalanche warning in the West-North zone. The Mt Baker area has received 6.52 inches of water since Wednesday afternoon. That is a lot of water in 48hrs! At higher elevations most of this fell as snow. Several large, natural, and triggered avalanches were reported Thursday.  A break in the weather Saturday should allow the avalanche danger to slowly decrease.

At lower elevations a wet snowpack exists. In some locations glide cracks have been reported. Even though the temperatures have cooled, it will take several days for the snowpack to freeze. Stay out from under any slope where you see large crevasse-like cracks in the snow.

Snowpack Discussion

January 4, 2019

The first few days of 2019 were active here in the Northwest. A strong weather system impacted the region bringing warm temperatures, heavy precipitation, and strong winds. This weather system did not impact the forecast areas equally. Even within the same forecast zone we can see wide discrepancies in precipitation numbers. The snowpack you encounter this weekend will be largely dependent on where you go and the elevation at which you travel

Storm Precipitation Totals as of Friday Afternoon

Hurricane Ridge: 2.41”

Mt Baker: 6.52”

Stevens Pass: 2.58”

Snoqualmie Pass: 2.27”

Crystal Mountain: 0.52”

Paradise: 2.23”

White Pass: 0.55”

Washington Pass: 1.05”

Mission Ridge: 0.31”

Mt Hood Meadows: 0.51”

A few big stories stand out in the current snowpack: recent avalanche warnings in the northern zones, persistent slabs in the western areas, and a complex and weak snowpack in the eastern zones.

The northern zone experienced the brunt of this latest weather system. This led to two days of avalanche warnings and at least one large natural avalanche cycle. It's tough to say what the snowpack looks like in areas near and above treeline, but we know those areas received substantial new snow.

Photo: Large natural avalanche at Mt Baker Ski Area during the recent storm. -Mt Baker Ski Patrol

Earlier in the week we began forecasting a new persistent slab in our west-slope zones. A layer of buried surface hoar produced avalanches last Saturday, Sunday, and Monday. How did that layer fair after this recent round of weather? In locations like Mt Baker and Paradise, it was well tested with heavy precipitation. In other locations, less water may not have adequately stressed the weak layer. As visibility improves and more observation come-in the picture may become more clear.

Photo: Large remotely triggered persistent slab avalanche in the Crystal backcountry: Jeremy Allyn

In the eastern zones a complicated and weak snowpack exists. Several persistent weaklayers have plagued these regions most of the winter. Don’t expect this to change anytime soon. Snow profiles and snowpack test can give you a glimpse into the persistent layer. Remember, snow profiles cannot prove the absence of a weak layer or that a layer has “healed.”

Photo: Large remotely triggered slide on buried surface hoar from Christmas above Leavenworth on 12/31: Matt Primomo

Problems

Storm Slabs

An icon showing Storm Slabs

Likelihood

likely

Expected Size

1 - 1

You will still be able to trigger avalanches within the new snow Saturday. Give these storm slabs time to heal by avoiding open slopes greater than 30 degrees. Storm slabs will be larger, more destructive, and harder to assess at higher elevations and in locations where the wind drifted snow. You can use simple observations such as measuring the depth of the new snow, small slope test, and simple hand pits to find strong snow (slab) over weaker snow (weaklayer). These observations can confirm storm slabs are present in nearby terrain.

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.

Persistent Slabs

An icon showing Persistent Slabs

Likelihood

possible

Expected Size

2 - 2

A layer of buried surface hoar was observed in several avalanches last week. This layer was present in the Mt Baker backcountry. How did this layer fair in the most recent storm? It's hard to say. Without a doubt, the significant precipitation stressed this layer.  At this point, the surface hoar is deeply buried. This will make it more difficult to trigger, but if you do, the avalanche would be very large and destructive. You are most likely to find this layer on N-E aspects above 4500 ft.

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.

Valid until: Jan 5th, 2019 10:00AM