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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, December 30th, 2012 - 7:00AM
Expires
Mon, December 31st, 2012 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

Dangerous avalanche conditions remain in the backcounty. A strong pacific storm bringing rain, snow and strong wind will keep the avalanche danger HIGH. Human triggered avalanches are very likely both above and below treeline and natural avalanches are likely. Anything, or person, in an avalanche path, including the runout zone, is at risk. Travel in avalanche terrain is not recommended.

Sun, December 30th, 2012
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Dangerous avalanche conditions. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Wet Loose
    Wet Loose
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
More info at Avalanche.org

Rain on snow will impact the below treeline areas today as this storm is bringing in warmer temperatures with rain up to 1500ft and heavy, wet snow below 3000ft. It is downright soggy on Turnagain Pass. This will be a good shock to the snowpack as it is the first significant load with warm temperatures at the mid-elevations. Natural avalanches are likely to release and send debris down gullies and into runout zones. There were 3 of these natural slides 24 hours ago on the front (east) side of Seattle Ridge – two of them covered highmarks across from the Turnagain Pass motorized lot. Today is not the day to play anywhere near or under slopes. In fact, it is a good day to stoke the fire and hunker down.

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the 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. 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.
More info at Avalanche.org

For the past week now we have seen several “Alaska style” storms that are putting down dense snow that is quickly bonding to itself. This normally would be a good thing, but this year the tabels are turned. The problem is we have a “Colorado style” base of weak faceted snow sitting underneath all this sticky Alaska snow. This has set the stage for the deep slab problem that will haunt us for a while. As of yesterday, we had a strong slab 3-6’ thick overlying 1-2′ of weak snow from October and November. Today, we will add another couple feet to that – making it a 4-7’ slab. Any avalanche triggered with a slab this thick is unmanageable and likely unsurvivable.

As far as storm snow concerns go, such as fresh wind slabs and instabilities within the new snow, they are out there at the upper elevations. However, the wet avalanche and deep slab problems are the most concerning today.

 

Weather
Sun, December 30th, 2012

It has been six full days now with accumulated precipitation and varying degrees of strong east winds. Today the onslaught intensifies with a powerful storm churning in the Gulf. This is ushering in warmer temperatures with rain falling below 1500′. We can expect around 2 €“ 2.5 € of rain (20-30 € of dense snow above treeline) from overnight into this evening. So far this morning we have around 0.5″ of rain on Turnagain Pass with 1″ at mid elevations in the Girdwood Valley. Wind has been relentless from the east and is averaging 60mph with gusts over 100mph €“ which should remain though the day. Ridgetop temperatures are, and will continue to be, in the mid to upper 20’s F and mid to upper 30’s below treeline.

We are in a nice progressive pattern and should see a break Monday with another low moving in for Tuesday. This system looks to be a bit cooler but fairly potent.


Fitz will issue the next advisory on Near Year’s Eve – Monday morning, December 31st.

Observations
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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.