The snowpack continues to evolve in West Central Montana. In late January, our primary avalanche concern was persistent slab avalanches failing on facets at the bottom of the snowpack. During this time period, several natural persistent slab avalanches failed near the ground. Although the exact timing remains uncertain, most of the avalanches failing on this layer occurred sometime before January 24th. During this time period, natural avalanches were reported in Bass Creek and above Moose Lake in the north-central Bitterroot. The most recent natural persistent slab avalanche failing on this layer occurred on February 8th in the side-country adjacent to Lost Trail Powder Mountain. During this time frame, there were no human-triggered persistent slab avalanches reported.
In mid- to late January, the persistent weak layers slowly began gaining strength in areas with a deeper snowpack (4-5+ feet deep). The combination of deep snowpack and warm temperatures resulted in a decreased temperature gradient within the snowpack. This promoted sintering, a process by which bonds between snow crystals strengthen, and weak layers begin to heal. Areas with a shallow (less than 3-4 feet) snowpack maintained a large temperature gradient, which is not conducive to sintering. This led to a difficult situation in late January to early February, where the likelihood of triggering a persistent slab avalanche was decreasing, but the consequences were not. It was still possible to trigger an avalanche on the weak snow at the ground from late January to early February, and the avalanche danger remained moderate.
At that time, you could walk around with a probe checking for depths to see if the terrain you were in was problematic for persistent slab avalanches. Slopes with a deeper snowpack were safer, and shallow areas were not. Even in deeper areas, we were concerned with likely trigger points such as thin areas around cliff bands or convexities.
On February 1st, we had winds gusting to 100mph on the highest peaks and record temperatures approaching 60 F in Missoula and the 40’s at upper elevations. The unseasonably warm temperatures made it possible for deep persistent weak layers to begin to heal throughout most of our advisory area. The snowpack gained strength remarkably quickly, and by February 6th we removed the persistent slab avalanche problem from the forecast.
We were thrown a curveball on February 8th, when a natural avalanche failed on depth hoar near Lost Trail Powder Mountain in the southern Bitterroot. Nobody was caught in the slide, but this was a clear sign that the depth hoar layer had not healed at all locations in our advisory area. On our next avalanche advisory posted February 11th, we once again added persistent slab avalanches as a concern in the Southern Bitterroot. Only time will tell if and when this concern will go away.
Between February 4th and February 12th, we saw weak layers develop during breaks in the storm at isolated locations within our advisory area. One such layer is a layer of facets located on top of a crust that is currently buried about 2′ deep at Wisherd Ridge in the Rattlesnake. We have also seen a layer of small surface hoar buried 2-3′ deep in Gash Creek, Downing Mountain, and outside of Lost Trail. While we have been able to get these layers to propagate in stability tests, we have not seen any avalanches on these layers as of February 20, 2020.
At this point, we are primarily concerned with new snow instabilities including wind drifted snow, dry loose and wet loose avalanches. We are also finding isolated persistent weak layers in the upper 2-3′ of the snowpack throughout the advisory area. These layers are able to propagate in some stability tests and they remain a concern, although the likelihood of triggering an avalanche on one of these layers is decreasing. The southern Bitterroot is the only part of our advisory area that harbors weak snow at the ground.
Weather Discussion from January 24th until February 21st
From January 24th until January 30th we received a few inches of snow here and there, adding up to about a foot of new snow. Then on February 1st, we had historic warm temperatures and high winds gusting up to 100mph in the mountains. The following day we received a few inches of high-density snow, which helped to cover the wind- and sun-effected surface.
Between February 4th and February 18th, we were in a steady NW flow. During those two weeks, new snow accumulation totaled 2-3 feet, equal to 4-8 inches of water. Storm totals were the highest at the Twin Lakes Snotel site in the central Bitterroot, which received 36″ of snow equalling just over 8″ of SWE (Snow Water Equivalence). Jocko Creek came in at a close second with 27″ snow equal to just under 8″ water. Most of this snow has come in at a steady rate of 4-6″ of snow per day, making for great riding conditions without loading the snowpack past its breaking point. There were a few brief dry spells within that period that promoted weak layer development at isolated locations within our advisory area, but we have yet to see any avalanches failing on those layers. Recently, powder enthusiasts have been thoroughly enjoying steep terrain in most locations without minimal concern of triggering a persistent slab avalanche.
We are currently situated under a high-pressure system which has brought with it clear skies and warm temperatures since February 19. Mountain temperatures have been reaching the mid-30’s during the day and dipping into the single digits at night, and winds have been calm to light. This daily temperature fluctuation will develop stout crusts on solar aspects, and the cool, clear, calm nights will build surface hoar throughout our advisory area. While the riding conditions may be fantastic during this period of fair weather, we are most likely building the weak layers that will be our primary concern in the weeks to come.
Snow Water Equivalent (SWE) and Weather and Avalanche Outlook
SWE totals are currently above average throughout our forecast area. We are at 117% of average SWE in the Bitterroot Mountains and 118% of normal in the Upper Clark Fork, which includes the Rattlesnake Mountains, Southern Missions, and Southern Swan Range.
It is noteworthy how cumulative SWE correlates to the deep persistent slab avalanche problems in our advisory area. We are still concerned about certain areas with a shallow snowpack, and now the only place with a deep persistent weak layer problem is in the Southern Bitterroot. Saddle Mountain in the Southern Bitterroots has reported the least amount of SWE in the past few weeks as well as the entire season. Low SWE totals, accompanied by strong winds scouring the snow, have resulted in a thin snowpack that is still conducive to deep persistent weak layers in the Southern Bitterroot.
The temperature outlook from mid-January to late March shows equal chances of seasonable temperatures, and the precipitation models predict above-average chances for precipitation. Let us hope that proves true for skiing and riding conditions.
It is tough to predict what the rest of the season will hold in regards to avalanche problems. If we continue to get snow, we should only be worried about new snow avalanches such as wind drifted snow, loose dry, loose wet, storm slabs, and wet slabs. However, there is a chance we could have another persistent weak layer problem if we develop surface hoar or near-surface facets during the current dry spell and they get buried by new snow. It will also be interesting to see how the depth hoar behaves in the Southern Bitterroot throughout the season. The best scenario for that area would be a few inches of snow each day without wind, which will build a deeper snowpack and help promote healing.