1-10-20 Snowpack Update

After the winter solstice, we got a small snowstorm that deposited a few inches of snow and a crust layer on the top of the snowpack. Before the New Year storm cycle, we developed a surface hoar, crust, near-surface combo, which has been the cause of many human and natural triggered avalanches.

The 2019-2020 ski and sled season truly started on January 1st, 2020. Since the start of the New Year, our forecast region has received between 3 to 9.7 inches of Snow Water Equivalent (SWE) at respective Snotel sites. We currently have about 2-7 feet of snow between 5000-6500 feet. 6500 feet to the highest peaks have 3-7 or more feet of snow. It hasn’t stopped snowing since the New Year besides a couple of quick breaks in the action.

Let’s take a look at the numbers. The Bitterroot Range is at 92% of average in regards to SWE. To get a better idea of the distribution of SWE throughout the region, we can examine the Snotel sites to look at SWE since the New Year. Saddle Mountain is at 85% of average and has got 3.0 inches of SWE. Twin Lakes is at 92% of averages and has received 7.6 inches of SWE. Lolo Pass is 82% of average and has got 5.2 inches of SWE. Those Snowtel sites are in the Bitterroot Range running south to north.

The Upper Clark fork is at 102% of average in regards to SWE. The Upper Clark Fork has the Missions, Rattlesnake, and Swan ranges in it. The Swan has the most snow in our forecast area. The North Fork of the Jocko Snotel site, which is in the southern Missions, has got a whopping 9.7 inches of SWE since the New Year. This Snotel site correlates well to how much SWE the Swan and Missions has received. Stuart Peak Snotel site in the Rattlesnake is at 98% of average and has got 5.3 inches of SWE since the New Year.

The start of our ski and sled season was rather grim, especially in the Rattlesnake and Bitterroot Mountains. Before the New Year, we were below average in snowfall, and there was barely enough snow to ski or sled unless you were in the Swan Range.

The new load of snow fell on a weak snowpack with two distinct layers of concern. The first being a depth hoar or facet layer found in the first 40 or so cm of the snowpack. These layers can be found in most areas throughout the region and are still a concern. We have observed natural and human triggered avalanches failing on this layer.

Another concern is a layer of surface hoar, a thin-crust, and near-surface facets buried below 2-5 feet of heavy, dense snow that has fallen since the New Year. This layer has been reactive and has been the culprit of many natural and human triggered avalanches. The catch is that it is not found in all regions of our forecast area and can vary drainage to drainage and slope to slope. Watch out.

Before the New Year, our snowpack was above about 6500 feet and 1.5 to 3.5 feet in depth. Between about 5000 feet and 6500 feet, we had about a foot of faceted snow. The recent snowstorm put a massive load of SWE on our weak faceted snow sitting between 5000 and 6500 feet. The elevations between 5000-6500 may be a concern with the new load of snow we get and potentially later in the winter. Only time will tell.

We are expecting seasonable temperatures and above-average snowfall, which is exciting news. The deep buried weak layers are starting to heal, but we are not there yet, especially with multiple feet of snow forecasted in the new few days. Large loads of snow and wind may make the buried weak layers reactivate. It should start to be a low probability and high consequence season, with the potential of triggering a giant avalanche always lurking. I would expect large naturals during weather events, and less skier triggered slides breaking into the buried weak layers. Skiers and riders may not see as many red flags, but the danger may still be there. We need a break from the snow to let the buried weak layers heal. Stay safe out there!