Avalanche Forecast
Regions: Cascades - South West.
Give the snowpack time to gain strength and to adjust to the impressive recent storm totals. Avoid traveling on open slopes greater than 35 degrees where you could start an avalanche. If you are traveling in the Crystal area, be aware very large and destructive avalanches have been observed over the past two days.
Discussion
The West South zone is demonstrating its diversity as several very different snowpacks and avalanche hazards emerge from this storm cycle. You may experience locally lower avalanche danger in locations that did not receive the impressive water amounts of Paradise and Crystal. In all locations, this was a potent winter storm and current conditions deserve respect. The raging storm and limited visibility hampered observations resulting in a high degree of uncertainty.
Very large avalanches were reported again Saturday from the Crystal Mountain area. These avalanches illustrate the potential for very destructive events failing on layers deep in the snowpack.Â
A very large (D3) avalanche triggered with explosives on Saturday by Crystal Mountain Ski Patrol. E aspect 6700ft. Photo: Crystal Mountain Ski Patrol
An NWAC Professional Observer traveling in the Paradise area did not observe any recent avalanches.Â
Snowpack Discussion
December 19th, 2019Â (The regional synopsis is updated every Thursday @ 6 pm)
Let’s take a moment to recap what happened over the past week:
A potent winter storm arrived on the 12th, adding to the very shallow snowpack throughout the region. Anywhere from 6-36†of snow fell between the 11th and 15th. Places like Mt Baker, Paradise, Mt Hood, and White Pass were the winners regarding snowfall, with quite a bit less for areas further east of the crest like Mission Ridge, Blewett Pass, and Washington Pass.Â
The most notable avalanche activity occurred in the Mt. Baker backcountry where numerous human triggered storm slab avalanches occurred (with several big enough to injure, bury, or kill a person). The majority of these occurred on upper elevation, north and east facing terrain. Besides the danger posed by the size of the avalanche, many slides quickly revealed rocks and other obstacles barely hidden below the snow surface.Â
The storm layer took time bond with the underlying snow surface, and we saw the likelihood of triggering slab avalanches slowly decrease over a handful of days. The avalanche danger was at Considerable for many places on Saturday the 14th, then tapered to Moderate on Sunday, and eventually reached Low in many places by Tuesday and Wednesday.Â
A large, human triggered avalanche on the old snow interface. East aspect of Table Mountain (West-North Zone) at 5,500ft. December 15,2019. Photo by Brooks Broom.Â
Backcountry travelers have noted generally thin and shallow snowpacks, with a range of structures across the region. Many folks were skeptical of the layering they found. People were able to dig down and observe weak layers in many areas. These were buried in late November and early December. In some places they presented as weak snow over a crust, in others, a layer of feather-like surface hoar. Cold temperatures have likely preserved many of these layers, and will be worth considering as the snow piles up.
Below is a list of commonly visited locations and their snowpack depths in inches (as of December 19th at 4am). Check the weather station hourly data feed and watch as the subtropical javelin of moisture (also known as a strong atmospheric river) changes conditions dramatically over the next few days.Â
Total Snow Depth in Inches as of December 19th at 04:00 AM.
-Matt Primomo
Avalanche Problems
Storm Slabs
You can still trigger avalanches in the recent storm snow on slopes greater than 35 degrees. Several weak layers formed during the storm as a result of changes in precipitation rate, wind speed, and temperature. The storm just ended and these layers need time to heal. Use small low consequence slopes to investigate the storm snow. When you see cracking or find stronger snow over softer snow, storm slabs may be present. In areas affected by the wind, steer around any deeper drifts or slopes below fresh cornices
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: Likely
Expected Size: 1 - 1
Persistent Slabs
As we transition out of the active storm cycle period, a buried layer of old weak snow continues to rear its head and cause very large avalanches. You are most likely to find this layer in the Crystal area on W-N-E facing rocky slopes above 5800’. Stay out of large open terrain in these locations. Snowpack tests on Thursday, natural and triggered avalanches Friday, and triggered avalanches Saturday all reveal a layer of weak sugary facets and/or feathery buried surface hoar about a foot above the ground. Avalanches originating in the new snow may step down into these deeper layers creating very large and destructive avalanches 3-6 ft deep. So far, we have only confirmed this layer near Crystal, however, that does not mean it is absent from other locations.
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: Alpine, Treeline.
Likelihood: Possible
Expected Size: 2 - 2