Turnagain Pass

There was one confirmed human triggered avalanche yesterday on the buried surface hoar that is the main concern this weekend. A snowmachiner on Seattle Ridge triggered a 1.5-4′ deep avalanche on a small WNW slope at 3000‘.  The snowmachiner was on the slope and it broke above him but he was able to ride off to the side. This incident really illustrates the potential of this layer and the reason to avoid steep terrain and large or high consequence slopes (ones that slide into terrain traps). The layer of surface hoar and/or near surface facets buried under the Valentine’s Day Storm has been found in almost every snow pit that has been dug throughout the advisory area since the storm. Observations from Friday continue to confirm that the layer is reactive and triggering a slab avalanche on the steeper slopes is likely. On Wednesday, there were several remotely triggered slabs on this layer in the Girdwood Valley.  This could be a tricky situation since the pack may ‘seem like it feels fine’ and people could get away with riding/skiing steeper terrain before a slope breaks. Slopes that saw higher traffic last weekend could remain intact while an adjacent slope slides. Triggering this set-up in spots where the slab is thinner will be more likely and adds to the overall spookiness. In one spot the layer could be a foot below the surface and in another 4′ deep, the slab being connected across the slope and the weak-layer being fairly uniform is why the avalanche danger remains elevated. 

What to keep in mind if headed to the mountains:

1. These slabs can be triggered remotely, from the ridge, the side or below
2. In terrain that saw high traffic last weekend, this layer will likely be more stubborn to trigger, but don’t let that lure you into thinking the pack has stabilized
3. No obvious signs on instability, or red flags, may be present before a slab is triggered
4. This is the kind of avalanche that could have several tracks on the slope before someone finds the right trigger point (thin areas of the slab). 

What can we do? Terrain management is the key for hedging our bets. This is simply sticking to lower angle slopes, under 35 degrees with nothing steeper above you.

Snowmachine triggered avalanche on Seattle Ridge that occured yesterday. Photo: Bryan Pfaender

The February 9th buried surface hoar layer (with some preserved stellars) was the culprit in this avalanche. Photo: Bryan Pfaender

Solar effects from SUNSHINE??

If the sun shines and winds are calm, solar radiation can be significant this time of year. Wet/damp loose sluffs on steep South aspects are likely when we do get this first shot of warmth. Additionally, the warming surface snow can enhance the likelihood of triggering a slab. Avoiding sun baked slopes this weekend will be recommended. These Southerly aspects likely have a crust under the new storm snow with either buried surface hoar or near surface facets on top – creating a perfect slab/weak layer/bed surface set up.

Surface warming on Seattle Ridge yesterday. 

Cornices have grown and changed shape following the Valentine’s Storm. These could be teetering on the balance and could break further back than expected. If one does fall it may trigger a slab avalanche below, potentially creating a very dangerous situation if a person is involved. Yesterday there were a few places where chunks of cornice had released. If the sun comes out again today, warming can loosen these beasts and increase the potential for them to break. Remeber these often break much further back than expected. 

Cornice hanging over steep terrain on Wolverine ridge. Note the slab that was triggered looker’s right. 

Another concern that needs to be avoided is travel under glide cracks. A new one was observed yesterday on Seattle Ridge.  Remember these release without warning. 

Lurking at the bottom of the snowpack are various layers of facets with varying degrees of strength. In the Summit Lake zone and some areas in Girdwood Valley and Johnson Pass depth hoar has been found. Last week’s storm cycle tested these layers and only a few avalanches that we know of broke in the deeper layers (Girdwood Valley, Portage Valley and Summit Lake). These layers will be tough for people to trigger, but possible in shallow snowpack zones. More likely is the case where an avalanche occurring in the upper layers of the pack will have the potential to step down and release the entire snowpack. If this does happen the volume will be large and could run long distances. The possibility of these large avalanches is another reason for conservative terrain choices.