Turnagain Pass

There were a handful of slab avalanches that appeared to be triggered by the sun near Girdwood yesterday afternoon. These were on southerly facing slopes. The slabs were likely barley hanging on when the sun warmed them enough to push them over the balance. Photo below of one taken from the Forest Service office.

There was also a skier triggered slab in the upper Girdwood Valley, no one caught. This was reported to be up to 2 feet deep. The Girdwood area had less snow from the storm, around 2 feet vs the 3 to 5 feet seen in Turnagain and Placer Valley.

One of several storm slabs that released yesterday. This one seen from the Forest Service office in Girdwood at 5pm. 3.13.24.

Skier triggered storm slab in the upper Girdwood Valley. No one caught. Thanks to Matt Yoder for the photo. 3.13.24. 

What a storm that just wrapped up. Over the past three days, from Monday morning through last night several feet of snow has fallen. Estimated storm totals listed below. Early this morning skies began to clear and winds turned northwesterly. Although the wind is bringing in cooler air, daytime warming today should be a factor. With the time change last Sunday, solar noon is now just after 2pm. This means the hottest part of the day for the rest of spring will be 2 to 5pm. More on the sun effect in Problem 2.

For today, the message is simple: A lot of new snow in a short time falling on questionable weak snow surfaces equals the potential for very large and dangerous human-triggered avalanches. These avalanches could be triggered remotely, meaning from the side, top, or bottom of a slope.

Storm Slabs:  There is just too much uncertainty as to how well the new snow will bond to the underlying surfaces. Below the storm snow the lower elevations have a mix of facets over slick crusts, the southerly slopes have possibly buried surface hoar on top of a sun crust, shaded slopes have a mix of facets over wind-hardened snow. It’s a mixed bag of weak snow that sits under all this storm snow and some slopes may heal and bond quickly while others do not. Regardless, snow science tells us that the first day after the storm is inherently dangerous, even if there is no weak snow underneath to deal with.

Wind slabs:  The winds were strong enough to load slopes in the higher elevations and will only make these slab avalanches larger. It will be great to see how the winds affected the snow as today will be our first look.

Cornices:  We should be extra careful along ridgelines. Cornices have likely grown and changed shape. These could be teetering on the brink of failure and if one falls off, which is dangerous in itself, could also trigger a large slab below.

Estimated storm totals from Monday morning through last night:

Turnagain Pass:  3-4 feet of snow
Girdwood Valley:  2 feet of snow
Placer Valley:  4-5 feet of snow
Summit Lake:  6-10 inches of snow

Over 4 feet of snow fell in this location that has settled to just over 3 feet. This is around 1,200′ in the trees just to the north of the Turnagain motorized parking lot. 3.13.24.

Sunshine today is likely to warm the snow surface enough to create wet loose avalanches on solar aspects. A small wet loose avalanche isn’t very exciting, but with so much new snow, it could be either quite large in bigger terrain and/or also trigger a slab below. Additionally, daytime warming can penetrate the snowpack and make the new snow that’s trying to bond to the old snow more unstable. Just another reason to play it safe today and let the steeper slopes adjust on their own.

Several inches to a foot below the new storm snow are various layers of even older weak snow and crust mixes. Essentially, all this new snow is overloading multiple questionable layers and if one of these fails it could propagate widely and produce a large avalanche. This is something we’ll be sussing out in the days to come.