Avalanche Advisory Archive 2016 – 2018

Date Issued:2017-01-28 07:16:58
Primary Trend:1
Primary Probability:3
Primary Likelihood:2
Primary Size:2
Primary Description:

With a moisture saturated snowpack down low and new snow at upper elevations that will warm a bit this morning wet loose, wet slab, and glide avalanches remain a concern.

Most of the snowpack is quite strong but in areas with fewer anchors and a history of glide look for those cracks opening up as a tell tail sign of increasing danger.

Secondary Trend:2
Secondary Probability:9
Secondary Likelihood:2
Secondary Size:2
Secondary Description:

With a moisture saturated snowpack down low and new snow at upper elevations that will warm a bit this morning wet loose, wet slab, and glide avalanches remain a concern.

Most of the snowpack is quite strong but in areas with fewer anchors and a history of glide look for those cracks opening up as a tell tail sign of increasing danger.


The National Weather Service Forecasts-

Today- Windy, rain. Highs around 44. South wind 10 to 20 mph increasing to 20 to 30 mph with gusts to around 40 mph in the morning, then becoming northeast 10 to 15 mph in the afternoon.

Tonight- Widespread rain and snow showers. Little or no snow accumulation. Snow level 1000 feet in the evening. Lows around 33. Southeast wind 10 to 15 mph.

Sunday- Cloudy. Scattered snow showers. Highs around 37. East wind 10 mph

Temperatures remain quite warm around the region. The Eaglecrest Mid Mountain gauge at Powderpatch has been at or above freezing for 3.5 days now. This morning the Mt Roberts Tram is showing over 35f. Eaglecrest is showing 39f at the bottom. 34 at the weather station and 32 on top.

Precip volumes were moderate with the tram picking up 9.6mm and Eaglecrest closer to 12mm (almost .5\"). This translated to an overall loss of snow at the tram of 3cm. Eaglecrest picked up a little at mid mountain and above showing +3cm of snow. With 12mm of precip you can assume the new snow volumes are greater at upper elevations near summit.

Wind remains a concern around the region. Windloading occurs very rapidly even with low snow volumes when moved by the winds. This morning Eaglecrest is blowing 20 gusting into the 30's and the tram is closer to 40 gusting 50. These are high windloading winds regardless of how much new snow we received. If we picked up any snow at all there will be large buildups in windloaded areas.

Recognizing that we have been building continued windslabs at uppermost elevations over the last several days, and recognizing that wet loose and wet slab avalanches remain possible at lower elevations where the snowpack hasn't frozen in days and is very moisture saturated, Avalanche Danger remains CONSIDERABLE today.

Natural avalanches are possible and human triggered avalanches are possible as well.

Precipitation rates are forecast to fall, as are temperatures (by as much as 7 degrees in the next 24 hours). This should start to lower the danger over the next 24 hours.


Avalanche safety ? The Slab Avalanche!

There are two common types of avalanche: the ?loose snow? avalanche and the ?slab? avalanche.

Loose snow avalanches start at a single point and grow as they flow down the mountain. They are easily recognised by their triangular shape, and most commonly occur immediately after fresh snowfall (look for that tell-tale sluff) or in the spring, when warmer temperatures release heavy ?wet slides? on all gradients.

Slab avalanches occur when a large, well-bonded slab of snow releases en-mass from a weaker layer buried in the snow pack. They are distinguished by the wide horizontal crown wall across the top, and the smooth sliding surface left underneath

Whilst both types of avalanche can have devastating consequences, the slab is the one most commonly ?triggered? by skiers and snowboarders, and the one that poses the greatest danger.

Slab Avalanches

How slabs are formed?

1.Like the rings of a tree, the snow beneath your feet is made up of many layers that tell the history of the changing winter conditions, snowfall, wind and temperatures. These different layers don?t always bond well together, creating weaknesses that affect the stability of the snowpack‑

2.Wind slab is a dense, strongly bonded layer that forms quickly as tiny grains of windblown snow come to rest on a sheltered slope. In itself, a slab can be very strong, but if it doesn?t bond to the layer underneath ? or there are weaknesses buried further down in the snowpack ? it can create highly unstable conditions.

3.Wind accelerates over mountain ridges, carrying snow with it, and so slabs o‑ en form on the sheltered ?lea side? of a ridge. On sheer drop-off s above concave slopes, the snow will build into overhanging cornices and load the slope below. Over rounded ridges, it will create dangerous, avalanche-prone convex slabs that extend into the steeper slope below.

How do I avoid a slab avalanche?

1. Timing? The highest risk of avalanche is in the 24 hours following fresh snowfall. If the fresh snow was influenced by wind, then the risk of a slab avalanche is extremely high!

2. Wind? Even when it hasn?t snowed for many days, if it is windy then older snow can be blown around and form dangerous slabs. Look for signs of current and previous wind activity, and think about where and how the snow will have been deposited.

3. Terrain ? Terrain shape is extremely important when it comes to risk assessment. Imagine that gravity is trying to pull the snowpack down the hill, and look for the areas of most support, least support and greatest tension. Use this information to plan your line and make your decisions. Convex rollovers are especially dodgy!

4. Recent Slabs? Note previous signs of slab activity and look out for new ones. Think why they occurred. Slopes of similar aspects and altitude might also be at risk

5. Appearance? Wind slab is smooth but dull in appearance and has the feel and consistency of polystyrene. Learn to recognize it and keep checking the consistency of the snow as you go to help you map out a mental picture of what is going on and where.

6. Temperature? Temperature greatly affects the stability of the snowpack. Early season and overnight cold temperatures can freeze the snow crystals, causing unstable layers that can then be buried. Sudden temperature rises will also make the snowpack unstable and more prone to slides, so be wary of slopes getting strong sunlight and noticeable air temperature increases. This is why north-facing slopes tend to be more stable.

7. Noise? ?Whumping? or fracture lines signal both snow slab and weak bonds. The scary ?whumping? sound of the snowpack settling is a sure sign of danger hidden beneath your feet, and if you hear it you?re either fortunately on terrain too fl at to slide or you?re getting avalanched! Cracking snow layers tell of dangers ahead.

8. Pitch? The optimum angle for avalanches is between 35 and 45 degrees ? as sod?s law would have it, the perfect riding angle. If it looks good to ride, you can bet the risk is there!
Most riders get avalanched on moderate risk days when they are caught off guard. Remember, safe passage off -piste is all about good decision-making. Imagine the risk is there, increase your awareness and question everything!

Forecaster:Tom Mattice