The avalanche danger may rise to HIGH tonight, possibly earlier, IF we see more rain than expected. Rain on snow will create very dangerous avalanche conditions. Avoid being on or beneath avalanche terrain today. Any triggered avalanche will run long distances and be especially destructive as it entrains wet snow at lower elevations.
Rain on snow and warm temperatures will cause a round of destructive avalanches. These avalanches will run long distances, well into the lower elevation band. Plan your day to avoid being in the runout of avalanche terrain. This terrain may not be obvious since debris may flow into mature forests.
Warm temperatures and rain may increase the likelihood of slab avalanches failing on weak layers buried near the bottom of the snowpack. We have a poor snowpack structure on all aspects above 5000 feet. I am more concerned today about southerly aspects where the snowpack is thinner and at elevations below 6500 feet, where we will likely see rain today and tonight. During the last loading event, natural avalanches were observed at the middle and upper elevation bands. A triggered avalanche failing 1 to 4 feet thick that entrains wet snow below will be unsurvivable.
(12/22/2022)
Crowns from one very large or multiple large to very large natural avalanches at the head of Sheep Creek, south of Mt. Cameahwait. These likely involved wind drifted snow.
Photo: Adam Clark/ BNSF
Southwest winds are rising this morning and will increase through the day as a potent storm approaches our area. Aeneas Ridge reports sustained winds of 36 mph with gusts up to 56 as of 0600. Sunday, the BNSF avalanche team reported drifting snow above 6000 feet. Expect fresh snow drifts on the downwind side of ridgelines and saddles. Wind features such as dunes, pillows, and scalloped surfaces indicate that you are in wind-affected snow. Cracking beneath your feet or machine is a sign of instability. Sheltered terrain above the rain line will still hold soft snow and is a safer, higher-quality experience.
(12/25/2022)
Triggered a small slab of drifted snow a test slope.
There is a high level of uncertainty regarding today's avalanche danger. We've rated it as HIGH, though we expect those conditions to develop late in the day and possibly overnight.
An atmospheric river is set to impact our forecast region today, with rain levels reaching 6000 feet. This elevation will vary throughout the day and the forecast region. The main uncertainties of the day are: What elevation will we see rain? How much rain will fall before it turns to snow? What kind of surface will rain fall on?
Typically, the most direct-action avalanche cycle we can experience is the initial onset of rain on a dry snowpack. This creates nearly instant and widespread avalanching. The primary ingredients to this are obviously rain, but also the liquid water content at the surface and surface texture. A dry, unconsolidated snow surface would lead to a more unstable snowpack. On the flip side, avalanches may not be as predictable when surfaces have already become moist or if there is a crust. In order for it to avalanche in this scenario, it typically requires more rain to wet the snow surface or break down the crust. This is where we have a lot of variability and uncertainty throughout the forecast area.
We have confirmed reports of a thick freezing rain crust in the valley floor of the Middle Fork. This crust formed on Christmas Eve. Since then, we’ve received nearly consistent rain at lower elevations. This crust may have already broken down due to rainfall. We received reports of blowing snow at 6000 feet on Christmas Day in John F. Stevens Canyon, leading us to believe the rain crust does not extend to that elevation. I observed a dry snowpack above roughly 5200 feet in the southern Whitefish Range. A number of reports in the middle elevation band described the snow surface as “upside down,” meaning harder over softer snow. Blase observed moist snow between 5000-6000 feet in the Middle Fork on Christmas Eve. Areas that have a crust or are moist may not avalanche as quickly as areas that still hold a dry snowpack.
Given a high degree of uncertainty regarding liquid water content and the precipitation forecast, we rated the avalanche danger HIGH at middle elevations. Whether we see a widespread natural avalanche cycle today or not feels like a crap shoot. The consequences are not worth wagering. We strongly recommend avoiding all avalanche terrain today.
