Avalanche Advisory Archive 2016 – 2018

Date Issued:2017-04-01 08:05:22
Danger:4
Primary Trend:1
Primary Probability:3
Primary Likelihood:3
Primary Size:2
Primary Description:

We continue to see avalanches on this deeper weakness. Behrands Avenue Avalanche path slid yesterday...

Of interesting note is it also slid 3 weeks ago... so what was a large slide on Behrands yesterday would be even larger in areas on Douglas that have not slid yet... as the the Berhands slide was only the surface of our two deep layers.

These slides are becoming VERY LARGE in nature in places.

Use extreme caution out there until temps cool off and precipitation rates decline.

Secondary Trend:2
Secondary Probability:4
Secondary Likelihood:3
Secondary Size:2
Secondary Description:

We continue to see avalanches on this deeper weakness. Behrands Avenue Avalanche path slid yesterday...

Of interesting note is it also slid 3 weeks ago... so what was a large slide on Behrands yesterday would be even larger in areas on Douglas that have not slid yet... as the the Berhands slide was only the surface of our two deep layers.

These slides are becoming VERY LARGE in nature in places.

Use extreme caution out there until temps cool off and precipitation rates decline.

Discussion:

The National Weather Service Forecasts-

Today- Widespread rain showers. Highs around 44. South wind 10 mph.

Tonight- Widespread rain showers mixing with snow late. Little or no snow accumulation. Snow level 1400 feet decreasing to 800 feet late. Lows around 35. Southeast wind 10 mph.

Sunday- Numerous rain showers mixing with snow early. Little or no snow accumulation. Snow level 300 feet. Highs around 42. southeast wind 10 mph.

Temperatures remain quite warm around the region. Mt Roberts Tram remains above freezing again. This makes all but about 6 hours of above freezing temps since last Sunday morning at tram elevations. You can assume snowpack lower than 1800' is quite moisture saturated and we could see wet loose avalanches in that zone.

We have received 76mm of precipitation at the tram in the last 6 days. This left only 5cm of new snow. Eaglecrest is showing 92mm of precip in the last 6 days with about 25cm of snow.

For a 6 day loading period these are not high volumes... yet at near freezing temperatures and with the previous persistent weak layer in place this was just enough to continue our natural avalanche cycle yesterday with a fair amount of sizeable slides around the region.

Currently avalanches remain easily triggered both naturally and by man on this deep persistent weak layer.

This morning Eaglecrst is 33f at the Powerpatch Wx Station... and Mt Roberts is closer to 35f. Temperatures are forecast to rise a little through the middle of the day then cool overnight and be cooler tomorrow back to below freezing... So temperature wise we still have a concern... with above freezing temps at mid mountain that will surely rise to above summits through the day this will add increased stress to the snowpack until the cooling takes place late tonight into tomorrow.

We received another shot of moisture in the last 24 hours. Not quite the projected amount or we may have seen additional natural avalanches. Eaglecrest received about 14mm of precip and nearly 10cm(4\")of new snow. The Tram picked up 16mm precip for essentially no new snow.

This new snow at higher elevations may cause wet loose avalanches... as snow cleans off of trees, rocks and steep faces it wont take much to get these wet loose slides going. Be aware that this can be the trigger for much bigger avalanches as well.

Winds remained high around the region again yesterday with winds from 20-45mph out of our traditional SSE. These winds have created large windslabs on the lee slopes. They will be easily human triggered on steep convex slopes and may also be enough to trigger the deeper persistent weak layers.

It is important to recognize that danger today may be a little lower than yesterday due to the fact that precip volumes are light... with around .6\" in the forecast for the next 24.

Avalanche danger is Considerable to HIGH today. Both Natural and human triggered avalanches remain likely.

Be aware that we have wet loose potential. Wind Slab Potential... and Deep Persistent Slab Potential. Play it safe out there today folks and remember good habits.

Please continue to avoid the Flume and Perseverance Trails.

Tip:

Managing Valley Bottom Runout Risk

Full path avalanche potential even with reduced danger ratings below treeline

In the last few days, we're seeing the start of the next phase of the low probability/high consequence scenario. That is , the peristent weak layer(s) that produced a pattern of deep persistent slab avalanches in BC and responsible for the recent major cycle in Alberta are slowly adjusting to the load of the overlying snowpack. Cornices and the upper layers of the snowpack are stabilizing as well and less likely to be triggers for the deeper weak layers. All this results in what's referred to as a \"dormant\" period.

Dormancy does not mean that no large avalanches will occur. But the probability decreases and it takes some kind of change to \"wake up\" the dormant layers which starts a new cycle. My previous blog discusses what to watch for in terms of factors that increase the sensitivity of PWLs and potential triggers for deep persistent slab avalanches.
in this blog, I want to talk about this:

As the likelihood of avalanches decreases, danger ratings come down. In particular, Below Treeline ratings often drop to Low in the dormant phase of an LP/HC pattern. However, as illustrated in the photo above, large \"full path\" avalanches can still occur in these conditions.

To manage risk at times like this, we need to adjust our approach to travelling through and recreating under overhead avalanche terrain when we are below treeline.

Even with reduced danger ratings, as long as deep persistent slabs are listed as a problem in the forecasts or if there's ongoing talk of low-probability/high-consequence events in other venues, it's advisable to eliminate or at least significantly limit exposure to valley-bottom runout zones. Especially when the weather is changing or has recently changed or when there's a potential triggering mechanism, go around runout zones if you can--well around--like in mature timber a couple hundred meters away from the trimline (sides and bottom) of the avalanche path. If you can't do that and are willing to accept the risk then expose only one person at a time, do not stop in the runout zone at all, never group up there, and make sure everyone's across safely before continuing.

It takes training to recognize valley-bottom runouts. Basically, they are open swaths that run through treed areas with large alpine slopes above. If this description isn't clear to you or the terminology I'm using here is a mystery, consider taking a course, travelling only with someone who has the training and experience to know what a runout zone looks like, or hiring a professional to help you manage the potential risk of full-path avalanches.

Karl Klassen
Avalanche Canada Warning Service Manager
kklassen@avalanche.ca

Forecaster:Tom Mattice