Avalanche Forecast
Issued: Jan 3rd, 2019 10:09AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Heavy rain, snow, and wind will maintain very dangerous avalanche conditions. Avoid all avalanche terrain.
Summary
Discussion
Several natural and triggered avalanches were reported Thursday in the Mt. Baker area. These avalanches failed within new snow breaking 10-12” deep with wide propagation. Avalanches were especially easy to trigger near and above 5000ft where the snow stayed dry. Recent avalanches ran far distances and created large wet debris piles.
Although temperatures are forecast to cool over the next 24 hours, wet avalanches remain a concern especially below 4500ft. Heavy rain like we are experiencing can create glide avalanches - the release of the entire snowpack as a result of gliding over the ground. These hazardous conditions are very difficult to forecast. Avoid steep, lower elevation terrain where smooth ground cover exists.
The Mt. Baker area received over 3” of water since Wednesday night. This fell as rain up to 4000ft. A mix of rain and 14” of snow fell up to 5000ft. Above, snow totals are significantly greater, but unverified. Up to 3” of additional water is forecast over the next 24 hours. Expect avalanches at all elevations.
Snowpack Discussion
Happy New Year!
Thanks to all of you who volunteer, send observations, and support NWAC in various ways - we appreciate it.
December of 2018 was fun (from a forecasting perspective) with three pronounced avalanche cycles, a couple different persistent weak layers, some rain events, and a flurry of human-triggered avalanches to ring in the New Year. Most importantly, it seems that we made it through the last days of 2018 without anyone getting seriously hurt by an avalanche.
The deep (Dec 9) layer responsible for many of the avalanches early in the month no longer seems to be a problem in the western zones. That said, it is still possible to trigger an avalanche on its counterpart (or basal facets) in the eastern areas.
A widespread layer of surface hoar formed around Christmas. Late December storms preserved this layer in areas above the rain line and we have numerous (more than a dozen) reports of people triggering avalanches on it in the last three days. At least 4 people were caught and carried during this period, but so far we have no reports of serious injury. Most of these avalanches were soft slabs, D1-D2+, but there were several harder wind slabs in the mix.
It appears that the layer is most reactive and/or prevalent in the Crystal Mountain backcountry and in the mountains around Leavenworth and west of Mazama.
Surface Hoar can be an especially tricky and persistent weak layer. Read more about it here.
Problems
Storm Slabs
Heavy snowfall will continue to build fresh storm slabs over the next 24 hours. Avalanches remain very likely within new snow. Wind-loaded terrain at mid and upper elevations will develop thicker and more reactive slabs. Avalanches involving new snow have the potential to step down and trigger deeper weak layers in the snowpack. Storm slabs will become more problematic as temperatures cool and more snow accumulates on Friday. These avalanches may break widely across terrain features and run far distances downslope. Watch for shooting cracks, collapses, and recent avalanches. Give the storm snow time to stabilize. Avoid travel on or below slopes 30 degrees and steeper.
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
Expected Size
Persistent Slabs
New snow and wind will thicken existing slabs resting on a layer of feathery surface hoar that formed around Christmas. Recent observations from this area indicate the surface hoar has a spotty distribution and can be hard to find. Regardless, impressive and unusual recent avalanches still have us concerned. This layer can be found in the upper snowpack between 4500-5500ft primarily on north and east aspects. Be especially mindful of steep slopes near treeline that are sheltered from the wind.
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
Expected Size
Valid until: Jan 4th, 2019 10:09AM
