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Forecaster Mindset on the Persistent Weak Layer - 27 Dec 2022

Dave Kelly
We're in for a run of large storms that began on Tuesday December 27, 2022 set to impact the state throughout the next week.  See Trent and Mike's weather discussion HERE
The buried persistent weak layer (PWL) has plagued us since it formed during high pressure in November. It was subsequently buried at the end of November and we saw low elevation avalanches on this persistent weak layer at elevations below 8000' including a few close calls with people getting run through forested avalanche terrain. Since the first avalanche cycle ran it's course in mid December this PWL has been less of a factor. Avalanche results on this PWL layer have been less the norm. At some aspects and elevations observers have been noting reluctant to no results with extended column tests while at other aspects and elevations there have been extended column tests with propagation. This is what we would call Spatial Variability.  
The question this morning was not if the PWL would avalanche today, but how much weight (water weight of snow/rain) would it take to overload (cause avalanches) on this weak layer.  The factors that come into play are precipitation intensity (how quickly weight is added to the snowpack from atmospheric conditions), wind (how quickly wind transports snow from one slope to another), and how weak the weak layer is. The mountain locations with weaker snowpacks like mid elevation Provo and Ogden areas, Upper Mill Creek and the Park City Ridgeline still harbor weak facets that pour out of the snowpack. These are typically thinner snowpack areas where snow metamorphism (temperature gradient) is occurring and these are places of greater concern for overloading the PWL and seeing avalanches (Snake Creek video below). Areas with a stronger snowpack like the upper Cottonwood Canyons (Red Pine Gulch video below) will probably be able take more water weight before they avalanche and the subsequent avalanches will be much bigger if they occur at all. The only way to distinguish between areas with shallow and deep snowpacks is to take a minute and dig to the ground, which may save your life in the long run. 
What I think is that this upper elevation weak layer will support more weight before it breaks and causes avalanches. The tipping point depends on speed of loading.The uncertainty lies with how quickly new weight is added. The snowpack is similar to my 14-year-old dog who dislikes like rapid change.  Anything that happens too quickly leads her to be unhappy and either eat plants or vomit (sometimes both). So, like a 14-year-old husky let's hope that we have nice steady loading.  The last thing we want is the mountains to puke avalanches and eat people. 
All the factors mentioned above; wind, new snow/rain, and the current weakness of the PWL all play into the uncertainty. In an ideal world we'd have 6" of snow or .5" of water a day for the next 2 weeks and the weak layer would adjust and we'd be playing in the Greatest Snow on Earth with less uncertainty over the reactiveness of the PWL. Looking at the Plumes for Alta Collins from the University of Utah's Atmospheric Sciences Department, they don't line up for nice gradual loading and thus the uncertainty. Most likely we won't see avalanches on the PWL today, maybe after this storm-or the next-or the snowpack adjusts and we don't see avalanches. For now I am wary of this buried PWL and will avoid traveling on or below slopes over 30 degrees. 
For now only time will tell.  Assess, reassess and let the mountains lead. 
Snake Creek December 9, 2022
Greg's snowpit from Red Pine on December 24, 2022 shows a strengthening snowpack
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