Turnagain Pass

Widespread natural wet loose and wet slab activity was seen yesterday along the mountains around Turnagain Arm, Girdwood, and the Kenai Mountains around Moose Pass and Seward. These avalanches occurred on easterly, southerly and westerly aspects during the heat of the afternoon and evening hours. A large avalanche off south facing Goat Mountain was caught on camera in Girdwood (video below). Take a look at the recent observations as well for images from the natural cycle last Wednesday into Thursday.

There were also three known human triggered avalanches on west and southwest slopes on the backside of Seattle Ridge. These were snowmachine triggered storm slabs (16-24″ deep) that were becoming moist and more reactive during the heat of the day. No one was caught.

Goat Mountain natural avalanche caught on camera, thanks to Tarryn Megan for the video! 4.16.21.

Snowmachine triggered slab avalanche in -3 Bowl on the backside of Seattle Ridge. SW aspect at 3,000′. Rider was not caught. Slab was 16-24″ deep and composed of last week’s new snow. Failed around 2-3pm on an old sun crust. 4.16.21

Snowmachine triggered slab avalanches on the backside of Seattle Ridge (-3 Bowl) yesterday. Failed between 4-5pm. They were on a westerly aspect around 3,300′ and composed of last week’s storm snow. 4.16.21. Photo: Chris Yelverton.

The springtime transition is starting. The snowpack saw a shock to the system yesterday with rapid warming and that will only intensify today. As of 6am this morning ridgetop temperatures are already in the mid 30’sF. The high pressure in place is creating a subsidence inversion, which is warming the ambient air at the high elevations. Couple this with 15 hours of solar heating and light winds …. and we have avalanche problems.

Regardless of elevation, any slope in direct sunshine that has not avalanched, will be likely to do so today; especially steep rocky terrain. These sunlit slopes are expected to experience significant surface warming and melting of any crusts that have formed overnight. Once the melting takes place and the surface turns to wet snow, all bets are off and natural avalanches become likely and human triggered avalanches very likely. This will begin first with east aspects, then south, then west as the sun makes its way across the sky. Today is a day to avoid being on, or under, any slope or steep terrain baking in the sun. Be aware of your exit routes. If surface crusts are present in the morning keeping the snowpack in check, those crusts may not last long before turning into a mushy mess, signaling the potential for natural wet snow avalanches.

Wet snow avalanches today may be either wet loose or wet slab, or a combination. They may begin to gouge down into lower layers as water makes its way deeper in the pack today. They may start small and if they are in steep sustained terrain, they could entrain enough volume and momentum to run to valley bottoms. A reminder, that even a small wet sluff can be dangerous as wet or moist snow is much more difficult to escape from than dry snow. There could also be some natural slab avalanches induced by the warming, or triggered by a wet loose avalanche that still harbor a fair amount of dry snow. That all said, the shock to the snowpack will be ongoing today.

Human triggered slab avalanches: Triggering a slab that is still mostly dry, or moist, is likely again today on east, south and west aspects that have not seen a lot of melting below the surface yet. Similar to yesterday, avoid slopes over 30 degrees that are seeing direct sunshine and warming.

Glide avalanches: We have seen a few of these release over the past few days (Skookum and Summit Lake) and are expecting more to start releasing with the warm conditions. Watch for and limit any exposure under glide cracks.

On shaded aspects that are not seeing direct sunshine and surface melting, there are still avalanche concerns. The new snow from last week is essentially a dense slab anywhere from 3-4′ thick in the mountains around Girdwood, and ~2′ thick at Turnagain Pass. Some wind effect and stubborn wind slabs were seen yesterday on the top of this slab. How well the storm slab has bonded to the old hard wind affected snow underneath is a bit of a question. With such warm ambient temperatures, even out of the sun, these slabs may still be reactive and keeping our guard up, looking for signs of cracking or whumpfing, will be key for assessing north aspects.

There are still older weak layers in the pack under the new snow. This is most predominant in areas such as Summit Lake with a shallower snowpack. As we move forward and watch what this spring transition does, we should keep in mind these layers exist and avalanches could step down on northerly slopes as well as southerly.

Cornices:  These grew during last week’s storm and are becoming more tender with the springtime warming. Be sure and steer well away from them along ridgelines and limit exposure under them.

Cornice break triggered by a snowmachiner. Smaller cornice over a moat, but you get the idea. These have not only grown last week, but are becoming less stable with the springtime warming. 4.16.21.