Avalanche Advisory Archive 2016 – 2018

Date Issued:2017-04-06 11:26:12
Primary Trend:2
Primary Probability:6
Primary Likelihood:2
Primary Size:1
Primary Description:

Expect to see wet loose avalanches at lower elevations...

Also expect to see increased activity if we see much sun on south to Southwest facing slopes later in the day.

Most of this activity should be small and yet remember this could still be enough to trigger deeper weak layers where they are present.

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

With decreased precip and decreased temperatures this layer is a little tougher to trigger yet be aware this deep instability remains... its a meter or more deep in most places and could be quite large if triggered.

Continue to use extreme caution in steep open areas and avoid extreme terrain especially with convex breakovers.


The National Weather Service Forecasts-

Today- Numerous rain showers. Snow level 1200 feet. Highs around 47. Southeast wind 10 mph until afternoon becoming light and variable.

Tonight- Mostly cloudy. Colder. Lows 26 to 32. North wind 10 to 15 mph.

Friday- Mostly cloudy in the morning then becoming sunny. Highs around 52. North wind 10 to 15 mph.

Temperatures remain quite warm around the region. This morning Eaglecrest was showing 31f while Mt Roberts was showing 33f. This is right at the freezing point after several days of warm weather.

At uppermost elevations today avalanche danger is more Moderate to Low... yet at mid elevations where the snowpack has not frozen in days avalanche danger remains more Considerable.

Natural avalanches are possible at mid mountain to lower mountain elevations with wet loose avalanches as the layers of the onion continue to peel back. If we see much sunlight today expect danger levels to increase at elevations on South to Southwest facing slopes later in the day.

Human triggered avalanches remain a concern due to the persistent weak layer that remains in place... With not much precip in the last few days and cooler temps today this danger has reduced but the mouse trap is still set with weakness in places... just don't go to that place... ;o)

With not much precip in the forecast and cooler temps danger levels are dropping slightly... yet with spring conditions... we may continue to see small wet slides in places.


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 are

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.


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.


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