Avalanche Advisory Archive 2016 – 2018

Date Issued:2017-01-03 06:54:31
Danger:1
Primary Trend:2
Primary Probability:4
Primary Likelihood:1
Primary Size:2
Primary Description:

These deeper weak layers are hard to trigger... but the layer that is only about 45cm off the ground remains weak in pit tests. Be aware that a slide at this depth would be a hard slab and not easy to manage. It would also be quite large. Limit your exposure to super exposed steep convex rolls. Limit your stress on the snowpack by riding one at a time.

The odds of this layer sliding are not good and yet the consequences would be quite high... take this into your hazard stability analysis.

Secondary Trend:2
Secondary Probability:5
Secondary Likelihood:1
Secondary Size:2
Secondary Description:

These deeper weak layers are hard to trigger... but the layer that is only about 45cm off the ground remains weak in pit tests. Be aware that a slide at this depth would be a hard slab and not easy to manage. It would also be quite large. Limit your exposure to super exposed steep convex rolls. Limit your stress on the snowpack by riding one at a time.

The odds of this layer sliding are not good and yet the consequences would be quite high... take this into your hazard stability analysis.

Discussion:

The National Weather Service Forecasts-

Today- Partly cloudy in the morning then becoming sunny. Patchy freezing fog in the morning. Highs 22 to 32. Light winds.

Tonight- Mostly clear becoming partly cloudy. Patchy freezing fog. Lows 10 to 20. Light winds.

Wednesday- Sunny. Patchy freezing fog in the morning. Highs around 30. Light winds.

Temperatures remain quite warm around the region. The Mt Roberts Tram peaked at 43f yesterday and came down to 36f overnight where it is holding now. This inversion is creating much warmer temps at elevation in the region.

Eaglecrest has been cooler showing 30f on summit. Which is also right about their daily high.

These near freezing temperatures allow the snowpack to settle and bond more rapidly. This is a healing process for the snowpack which often removes some of those weak layers over time.

Winds have been light around the region for several days and remain light today. The forecast is calling for 5-15mph winds which shouldn't change conditions too much in the short run.

We have not seen any precipitation over the last several days also leading to a more stable snowpack.

Danger is LOW in the region right now. Natural avalanches are Unlikely and human triggered avalanches are also unlikely...

We saw some weakness throughout the region late last week with several natural avalanches spotted over several days. My fieldwork yesterday showed this layer has stabilized for the most part where I looked... but be aware that due to spatial variability this weakness may still be present in places.

This weak layer may allow for large triggers to start slides. Remember LOW danger doesn't mean NO Danger... Be safe out there and have a great day.

Tip:

Deep Persistent Slab

Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.

Deep Persistent Slabs are destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.

How they form

Deep Persistent Slabs form when a persistent weak layer is deeply buried under a thick hard slab of snow created by numerous storm events. This problem commonly develops when an early season persistent weak layer like depth hoar or crust-facet combinations become more deeply buried over time.

Where they form

The spatial distribution of Deep Persistent Slabs (like Persistent Slabs) is dictated by the distribution of the thick slabs and the culprit weak layer. The slab and weak layer distribution are dictated by the weather patterns responsible for their creation. Thus, Deep Persistent Slab distribution can range from widespread across terrain to specific terrain features, but are confined to areas with a deep snowpack. They can occur at all elevations and on all aspects, and can be triggered on low-angle to steep slopes. Consult the backcountry avalanche forecast to determine where in the terrain this problem exists.

Timing and Triggering

Deep Persistent Slabs can be a problem during any time of the snowy season but the most common period is mid-winter through spring since it takes some time to develop a thick slab via multiple snow events. The problem takes weeks to months to develop, and can then persist for weeks or months.

Recognition

Unfortunately, there is often little direct evidence of a Deep Persistent Slab problem until a large destructive avalanche releases. The lack of avalanche activity even with dozens of tracks on a slope does not indicate the absence of a Deep Persistent Slab problem. Observing deeply buried weak layers in snow profiles, and gauging sensitivity with appropriate deep layer snowpack tests like Deep Tap Tests and Propagation Saw Tests can be helpful in assessing the presence of this problem.

Treatment and Avoidance

Deep Persistent Slabs are very difficult to predict and manage. They are low-probability high-consequence events. If you are caught in one, you are unlikely to survive. Often the only evidence of the problem arrives too late as a large, deadly, and unexpected avalanche. The only real effective risk management strategy is to avoid areas where you suspect a Deep Persistent Slab. They are most commonly triggered from shallow spots in the snowpack. Avoiding these areas is one way to reduce risk. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.

Forecaster:Tom Mattice