01-24-20 Snowpack Update Image

Avalanche conditions from January 11th until January 24th have been at considerable danger, trending towards moderate danger. We are currently most concerned with persistent weak layers, failing near the ground on depth hoar and facets. The second problematic persistent weak layer is a combination buried surface hoar, near-surface facets, and crusts. This weak layer is found at different depths, depending on location. It is hard to pinpoint exact areas where you will find this layer, but the best way to find out is to dig a snowpit. In locations with a shallower snowpack, this layer is more pronounced. Throughout our forecast area, we have significant spatial variability in regards to depth and persistent weak layers.

Areas with the deepest snowpack are found in the Southern Missions and the Swan Range. These areas have received the brunt of the storms throughout the winter. The Central Bitterroot, from Twin Lakes to Gash Point, has been favored for snow and currently has a deep snowpack. Lolo pass has a reasonably deep snowpack, but at lower elevations, it is still thin. The Rattlesnake and Southern Bitterroot hold the least amount of snow in our forecast area. Locations that are most susceptible to persistent weak layer avalanches are areas with a shallower snowpack or thinner areas near rocks, cliffs, or convexities.

A few days ago, northern locations in our forecast area got a crust that is capping near-surface facets and surface hoar. This layer is not yet buried deep and does not pose much of a problem. Areas, where it might currently be problematic, are places where new snow has drifted, burying this layer deeper. We do not yet have a good grasp on how widespread this weak layer is and how it will react in the future, but it is something to keep an eye on.

Since the New Year, we have received a tremendous amount of SWE. Over the past week, precipitation has been tapering off, giving our snowpack a chance to adjust. The problem is that we have now entered a low probability but high consequence situation. We currently have moderate avalanche danger, which means that human triggered avalanches are possible. You have the possibility of triggering a large avalanche, encompassing the entire seasons’ snowpack.

We have had numerous human and natural triggered avalanches failing on persistent weak layers. Places with the greatest concern right now are in areas with a thin snowpack in terrain above about 6000 feet in elevation, thin rocky areas around cliff bands, or on steep rollovers. We are less concerned about persistent weak layers in areas with a deeper snowpack, meaning about 4-5 feet or deeper. The reason we are less concerned about deeper areas is that the large amount of snow overlying the persistent weak layers has allowed them to heal.

Let us take a look at Snowtel sites, beginning the morning of January 11th until the morning of January 23rd. In the Rattlesnake Mountains, Stuart has accumulated 2.6 inches of SWE and is 97% of average. The North Fork of the Jocko, which correlates to the Seeley Lake Region, has received 4.2 inches of SWE and is 112% of average. Twin Lakes in the Central Bitterroot has got 4.0 inches of SWE and is 95% of average. Saddle Mountain in the Southern Bitterroot has received 2.2 inches of SWE and is 88% of average.

Today we are at 103% of average in regards to SWE in the Upper Clark Fork, which is where the Rattlesnake Mountains, the Swan Range, and the Southern Missions are. In the Bitterroot, we are at 96% of average in regards to SWE. The outlook for the next three months predicts that we will have above-average precipitation and equal chances for seasonable temperatures.

Hopefully, we get more SWE to help burry our persistent weak layers. We need a deep snowpack in all areas to help our persistent weak layer problem heal. I wish it were that simple of a scenario. I would expect that many slopes that have already avalanched this season may do so again. To hammer it in, we have a low probability but high consequence situation right now with our avalanche problems. Choose terrain wisely as the consequence of getting caught in a persistent weak layer avalanche is high.