Avalanche Forecast
Issued: Mar 11th, 2019 11:00AM
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
Expect dangerous conditions to develop at upper elevations, as a strong storm will increase the avalanche danger on Tuesday. The wind will transport a significant amount of snow-both new and old, and it may not bond well to the old surface. Choose a simple route plan for the day, one that allows for avoiding avalanche terrain at upper elevations.
Summary
Discussion
A major change is on the way. This storm headed our way will bring along sustained winds and a decent amount of new snow. More snow will fall in areas closer to the crest like the Salmon la Sac, Teanaway, and the Chiwaukum Range. East of Highway 97, much less new snow will fall. As such the danger may not reach Considerable there. The temperatures have been quite cold recently, and current snow surfaces are a mix of sun crusts on southerly aspects to weak, faceted snow on northerlies. Recently buried layers were also weak and faceted. The slightly warmer temperatures, and winds with this storm are ideal for slab formation. Now is as good a time as any to remember that most avalanches occur during, or directly after a snowfall event.
On Sunday, observers triggered small lingering wind slabs on a north aspect of Wedge Mtn at 4,000ft. We haven't had any reports of collapses since last weekend, when observers reported signs of instability like collapses on Blewett Pass, Nahahum Canyon, and the northeast side of Wedge Mountain. Over this past weekend, the avalanche danger eased, and folks were able to get out and enjoy a couple of very nice days with good stability and great snow in the mountains. This storm may change that for a little while.
Small wind slab triggered by a skier on a Northeast aspect of Wedge Mountain on 3/10. Notice the sparkly (faceted) snow surfaces. Photo: Matt Holland.
Snowpack Discussion
March 10, 2019
February started stormy and cold with very active avalanche conditions on persistent weak layers. The month finished with continued cold temperatures but drier weather allowed avalanche activity to taper off. Persistent weak layers that were buried early in the month (Feb 8th) are now unreactive, though you can still find the grains.
Aside from the series of storms in early to mid-February, most regions have measured relatively light snow accumulation in the past 3 weeks. Snow has stayed soft especially on shaded slopes and faceting and surface hoar have been plentiful. While there has been plenty of sunshine since mid-February, very cold temperatures have kept melt-freeze crust to a minimum on sunny slopes.
A natural loose wet avalanche (D1.5) on a southeast aspect of Lichtenberg Mtn at 5,400ft. 3/10/2019. Photo: Josh Hirshberg
Recent Avalanches
Our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow and weak snow surfaces on both dry, shaded slopes and sun-exposed aspects. In some locations, a weak layer of surface hoar and facets was buried on March 6th. We’ll keep an eye on this interface as we move into the future.
A skier-triggered storm slab avalanche (D1.5) on Shuksan Arm, north aspect, 5200ft. 03/09/19 Adam U Photo.
Moving Forward
As we move further into March, there are two points to consider:
-
The strength of the March sun: As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.
-
Weak surfaces: facets and surface hoar have blanketed snow surfaces in many zones. Any significant snowfall will bury well-developed and widespread persistent weak layers. Recently, most storms have not delivered enough precipitation to cause a problem.
Problems
Storm Slabs
Moderate snow and stronger winds will quickly create storm slab instabilities by early morning on Tuesday. The new snow will be landing on a variety of surfaces from surface facets and surface hoar on shaded aspects to melt freeze crusts on southerly aspects. The slightly warmer temperatures associated with the storm, along with wind should be ideal for slab formation. Slab avalanches may run naturally during the day at upper elevations, where heavy wind loading will transport snow- both new and old, onto leeward slopes. The snow may fracture quite wide on shaded aspects where the recently buried layer is weak and faceted. If you see obvious signs of instability such as shooting cracks and recent slab avalanches on small steep slopes, it is time to stay out of avalanche terrain. Any avalanches on the old surface may step down a foot or so to layers from early March.
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
It has become less and less likely to trigger a deeper slide, however, the poor structure with facets underneath a crust from early February remain concerning in some areas. Will this storm transport enough snow on slopes with this weak structure to become active again? That is a good question. At lower elevations, the snowpack is shallow and weak on shaded aspects. In the foothills, facets can be found near the ground beneath February's storm snow. These deeper slabs are most concerning on isolated, shaded slopes where stronger snow may rest over facets. This structure is more concerning than at upper elevations, where aside from the recent storm snow, the snowpack is generally deep and strong. Persistent slabs are unlikely at upper elevations and in the western portion of the zone.
If you find signs of instability such as cracking and collapsing, avoid steep slopes nearby. Stick to well-supported slopes and avoid places where firm slabs look to exist near shallow rocky zones. Travel one at a time from well defined safe zones when moving through 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: North, North East, East, West, North West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 12th, 2019 11:00AM
