Turnagain Pass Avalanche Forecast

Another large slab avalanche was triggered yesterday in our forecast zone; three others were triggered in Seattle Ck Drainage Friday and one on Fresno in the Summit Lake area Saturday. Yesterday’s slide was on the West face of Magnum (2,900′) and triggered by a skier at the top of the slope. The avalanche took out previous tracks and is believed to have released in a weak layer buried during Solstice. It was triggered at a thin spot in the slab that can be seen in the middle of the crown in the photo below. No one was caught and the skier was able to get off the slab at the top. A big thank you to the group for sharing their details and photos.

Looking up at the Magnum slide from yesterday, 1/5/19, West aspect, 2,900′. Note how the crown is thin in the middle, to the right of the skier in the photo. This thin area is where the skier reported to have triggered the slide. Photo from group involved.

Looking down from the crown at the debris, which covered previous tracks. Photo from group who triggered the slide.

Although the weather continues to be cold, calm and clear, triggering a large slab, similar to the one above, that breaks in weak snow 1-3′ below the surface remains our primary concern. Buried just over two weeks ago, during Solstice, was a layer of surface hoar and near surface facets. This layer now lurks between 1 and 3 feet below the soft snow surface and is still causing us grief as seen yesterday.

As long as this weak layer remains a concern, things to keep in mind:

• Signs of instability are not likely to be present before a slope releases.
• Thin areas are likely to be the trigger spots. For instance, on top of rollovers, near rocks protruding and areas that have seen past scouring by the wind.
• It may be the 3rd or 10th person on the slope before someone hits the thin spot and triggers a slide.
• Remote triggering a slab is possible, meaning from the top, sides or below.

Using good travel habits is key with this problem as it can be quite tricky. Exposing one person at a time, watching our partners, having escape routes planned and considering the consequences if the slope does slide are good ways to help stack the odds in our favor. For instance, avoiding slopes that end in terrain traps, where debris can pile up.

Snowpack at 3,000′ in Seattle Ck Drainage. Snow pit tests showed this weak layer took a lot of force to fail, but once it failed, slid easily. Less force is needed in thinner sections of the slab however.  See VIDEO. 

The cold temperatures are faceting and loosing the surface snow more and more each day. Sluffs have been reported to be getting larger on steep sustained slopes. Watch your sluff.

Glide cracks continue to open with the cold temperatures which is another interesting factor with this unpredictable hazard. Limit time spent underneath glide cracks as they could release into a dangerous glide avalanche at any time.

Cornices:  Avoid traveling on cornices and limit exposure when passing beneath. Even with the cold benign weather, they can still be teetering on the brink of failure and break farther back than expected.