Avalanche Forecast
Regions: Cascades - South West.
You may still find and trigger a slab avalanche in isolated areas where recent storm snow has consolidated and sits upon recently buried weaker snow. Loose avalanches, particularly loose wet avalanches, may release naturally or be human triggered as the sun comes out Monday. Take care around terrain traps that would amplify the consequences of a small avalanche.
Discussion
Snow and Avalanche Discussion
While loose avalanches were the most common avalanche seen or triggered over the weekend, a few small natural and skier triggered slab avalanches were reported in the Crystal backcountry (Pickhandle Point/Crown Point area) on Friday and Saturday covering a variety of aspects.
The 3/6 weak layer (buried surface hoar) was identified in at least one avalanche crown. This weak layer was identified and reactive in some test pits in the Paradise and Crystal areas 8-14 inches from the surface. A NWAC forecaster traveling above treeline on Mt. St. Helens Saturday reported large whumpfs on a SE aspect at 7600'. Facets over a sun crust showed strong over weak snow 14" from the surface. The lingering storm slab problem is likely confined to isolated areas near and above treeline as the recent storm snow on shaded aspects is still uncohesive (loose) in most areas.
It's been several weeks since avalanches released down to the early February crust. After more observations targeting this interface several feet down on Saturday, we feel we have enough information to drop deep persistent slab from the avalanche problem set. We'll continue to monitor this interface as we get deeper into the spring and weather patterns change.
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.Â
Moving Forward
As we move further into March, there are two points to consider:
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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.
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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.
Avalanche Problems
Storm Slabs
In isolated areas, storm slabs may still be reactive where they sit over older weak snow (likely surface hoar buried 3/6). Use snowpack tests to see if you can identify this interface about a foot down, not to green light any particular slope. If you observe collapses or whumpfs in the snowpack, dial back your terrain use and avoid slopes steep enough to slide. Given the amount of sunshine over the weekend, this problem may linger longest on shaded aspects at mid and upper elevations. We will definitely be watching how this interface evolves as more snow and wind arrives Monday night and Tuesday.
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: Possible
Expected Size: 1 - 1
Loose Wet
With sunny skies and mild temperatures forecast for at least the first half of Monday, loose wet avalanches are most likely to release from steep, rocky, sunny slopes. Monitor changes in the upper snowpack, looking for natural pinwheels or small loose wet avalanches as clear signs of increasing danger. Avoid terrain traps like gullies where even a small avalanche can bury you.
In the same vein, on very steep shaded slopes loose dry avalanches continue to have the potential to run fast and far.
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.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1