Northern New Mexico

Northwest through East facing terrain above treeline continues to harbor a persistent slab avalanche problem. These avalanches are difficult to trigger but buried hard slabs resting over very weak faceted snow present the concern for producing large dangerous avalanches. Mitigate the uncertainty by choosing less committing terrain that provides options to avoid the avalanche problem or allows you to ski lower-angle terrain. Near and below treeline the majority of our snowpack is fully faceted cohesion-less snow. The greatest concern here remains ground obstacles. 

Okay, it's happening! We're getting a storm. Our last chance of decent snowfall fizzled faster than the models could even keep up, but that's not the case with this one. We should see the snow start flying around 6 pm this evening and this will persist through most of the day Tuesday. By the end of this event, we're looking at receiving  7 to 11 inches of new snow from 0.7 to 0.9 inches of snow water equivalent. We can expect to see moderate to strong southwest winds transporting snow onto the leeward sides of ridges and terrain features as well, adding to the overall stress. We've got a pretty perfect recipe for increasing avalanche danger. Let's talk about it a little more. 

So by the time the snow starts falling this evening, it will have been 20 days since our last significant snowfall. Yea, you read that right. The last time we saw snowfall of more than an inch was back in December! That's nearly three weeks of high pressure. During that time there is metamorphism of our snow grains constantly occurring within our snowpack. A shallow snowpack harboring a steep temperature gradient is constantly trying to equalize. The area's in our snowpack with more moisture are trying to move water vapor to areas with less, as this water vapor travels it forms sharp, angular snow grains called facets. Our snowpack is full of them because this process has been allowed to continue for nearly three weeks uninterrupted. 

Let's narrow down where we feel our greatest concerns are. Above treeline we have buried hard slabs that have been presenting a persistent slab avalanche problem. The facets resting just underneath this slab are extremely fragile and the addition of a new load will undoubtedly stress these to failure. This layer sits 40 -60cms from the surface. Facets over time have the ability to chain and bond vertically within the snowpack, this can result in increasing compressive strength which may provide some insight as to why it's been difficult to trigger these avalanches. At the same time, however, their shear strength is and always will be total garbage. Take the game Jenga for example, a tower of stacked blocks, push directly down on the tower and it's not likely to topple. Give it a moderate tap from the side and the tower's ability to withstand that force is far far less. The addition of a new load will stress not only the compressive strength but the shear strength as well. An avalanche release on this layer will produce large avalanches that will be capable of entraining the entire snowpack to the ground. 

Our second major layer of concern is near-surface facets that exist just below the surface. In many places, these likely rest just underneath a thin and fragile layer of wind crust formed from recent strong winds. Without diving too deep into the science of it all, these form from a consistent and rapid diurnal change within the upper snowpack. Warming during the day followed by cold clear nights that advocate rapid cooling and longwave radiation suck the moisture right out of the top 20cms. This process has also been allowed to persist for nearly three weeks. This layer is also incredibly weak and likely won't be able to handle the addition of a load and cohesive slab. Avalanches that fail on this layer can step down into those deeper facets under the hard slab producing a large destructive avalanche. 

Near and below treeline terrain is entirely faceted. The missing component here for producing an avalanche is the presence of a cohesive slab which we'll be adding with this incoming storm. We'll likely see a concern for storm slab avalanches initially. Wind will likely aid in developing a persistent slab avalanche problem again for most near treeline terrain but we'll see if we are able to maintain a slab below treeline. All season long, we haven't been able to keep a slab at lower elevations. With a more active period of weather for the week and some cloudy nights that stunt the faceting process, we could see things get interesting below treeline, but we'll wait and see. 

So to wrap all of this up, we can expect our avalanche danger to increase rapidly overnight into tomorrow morning. We'll need to break some of the terrain selection habits we may have formed over the previous few weeks and get back to looking at our terrain a little differently. Cautious route-finding and careful snowpack evaluation will become essential moving forward. I hope everyone is excited about the new snow, I know we sure are here at TAC! Let's give our snowpack some respect and time before we ask for too much too soon. Stay safe everybody!