Turnagain Pass

Yesterday was the first day since the beginning of the Valentine’s Storm Cycle (beginning Monday, Feb 13th) that no natural avalanches were seen. The storm cycle is fading, but we did get another shot of snow last night, adding another 2-6″ to mountains. This last storm cycle has brought a total of 2-3′ of dense new snow to the area and simply put, the new snow is not bonding well with the old snow (seen in video). This is due to a weak layer of buried surface hoar along with near surface facets that is sandwiched between the new snow and old harder snow//crusts. Until proven otherwise, this weak interface is suspect to be on all aspects and elevations. 

Things to keep in mind with this ‘persistent slab’ avalanche problem:

1. Human triggered slab avalanches (2-3′ deep) breaking in this layer are likely in terrain over 35 degrees
2. Slabs can be triggered remotely, from the ridge, the side or below
3. In terrain that saw high traffic last weekend, this layer will be more stubborn to trigger, but don’t let that lure you into thinking the pack has stabilized
4. No obvious signs on instability, or red flags, may be present before a slab is triggered
5. This is the kind of avalanche that could have several tracks on the slope before someone finds the right trigger point (thin ares of the slab). 

What can we do? Terrain management! Sticking to lower angle slopes (under 35 degress with nothing steeper above you) is recommended.

 
Photo below: A look at the bottom of the slab and the weak layer (buried surface hoar) shining in the sun.

WIND SLABS:
Strong winds have been impacting the mountains for 6 days now. Although we should see a decrease in ridgeop wind today, the Easterly flow should still be strong enough to load leeward slopes. Wind slabs from the past few days have been around 6″ to a foot thick and showing signs of bonding fairly well. However, the problem is this has made for a more solid slab out of the Valentine’s Storm snow that sits over the aforementioned buried surface hoar and facets. Nonetheless, fresh winds slabs should still be something to look for and if triggered, could step-down to the weak layer below. Watch for current wind loading and stiffer, hollow feeling snow over softer snow. 

CORNICES:
Cornices have grown substantially during the past week and could release naturally with the ongoing wind/snow. If you find yourself along a ridgeline, give them a wide berth as these could be quite touchy and have a good chance of triggering a slab below. Also, keep in mind if there are other groups underneath you in valley bottoms.

 Wind plume on the NW ridge of Magnum on Turnagain Pass. This was the theme of yesterday’s weather.

SUN?
If the sun comes out today, watch for quick changes in the snow surface and an increased change for triggering a slab. Moist to wet loose snow avalanches on Southerly aspects should be expected with the first shot of sunshine.

Weak snow (facets and depth hoar) in the lower layers of the snowpack continues to be a concern in our advisory area. Avalanches occurring in the upper layers of the pack have the potential to step down and release the entire snowpack in some places. If this does happen the volume will be large and could run long distances.  A few of the naturals throughout this last storm looked to have run into older faceted snow. The possibility of these large avalanches is another reason for conservative terrain choices. 

Several natural slab avalanches on a SE aspect of Fishes Breath in Girdwood Valley, including one that stepped down into an older layer of the snowpack. Photo courtesy of Mike Welch