The Bottom Line: Avalanche danger will rise quickly rise on Friday, as a burst of new snow is expected. The ingredients are ripe for slab formation; New snow, rising temperatures, and a weak layer. Careful snowpack evaluation and navigation will be necessary to stay safe in the mountains on Friday.
Snow and Avalanche Discussion
Just a couple of inches fell on the 17th with little to no wind. This likely preserved the old snow surface that consisted of a mix of facets, crusts, and wind packed snow. Towards the end of the dry weather period, shaded aspects harbored surface hoar and facets between 3,000ft and 6,000ft in the area. As the snow accumulates overnight into Friday, the stage will be set for our next round of avalanche activity. More on the recently buried snow surface in this observation here.
Regional Synopsis
January 16, 2019
Since Friday January 11, we’ve enjoyed a spell of generally nice weather in the mountains. This allowed for some great views, enjoyable outings, and lots of snow observations. A more active weather pattern beginning Thursday January 17th will bring this nice weather to a close.
During this time period, the snow surfaces around the area have changed dramatically. This forms the foundation for a few current trends we are seeing in the mountains.
New Snow Problems
Reports from around the area indicate a wide variety of snow surface conditions prior to new snow on January 17th. We’ve heard about breakable crust, very icy surfaces, sugar facets, surface hoar, and rime. What snow surface you encounter can depend on aspect, elevations, and general location.
As a series of winter storms impact the area, how will the new snow bond to the old snow surface. This can be tricky to predict. As the snow starts to pile up make lots of shallow snow observations. Shovel tilt test, hand shears, and small slope test can all help you track how the new snow is bonding to the old snow at different aspects and elevations. Stop and take a look at where the snow is failing. Do you see large grains of snow? Do they look like feathers? Do the act like sugar? How far below the current snow surface are they?
Old Snow Problems
There are still lingering persistent weak layers in the eastern zones of the cascades. You are most likely to find weak older snow in areas further east from the Cascade crest where snowpacks are shallower, more variable, and generally weaker. In some locations weak snow near the ground can still be found. These basal facets have hung around all season. The only way to gain information about these old persistent weak layers is to get out your shovel and dig. Because of the size of our forecast zones and the variability in the snowpack, it's important to make snow observations as you travel. We’ll keep watching these old layers, but let us know what you see while you are in the mountains.
Avalanche Problems
Storm Slabs
The avalanche danger will increase quickly as new snow will contribute to slab formation. We anticipate that the new snow won’t bond very well with the old surface. Slides may be easily triggered, even in low elevation areas and could be surprisingly wide, encompassing much of the terrain feature. Warming temperatures during the day will help to create the strong over weak recipe that is needed for slab avalanches. Winds may drift the new snow into areas of thicker, cohesive slabs as well. When you find more than 6” of new snow has drifted in or accumulated, it’s time to evaluate for slab characteristics. Look for shooting cracks within the new snow. Check how the new snow is bonding by using small, inconsequential test slopes and quick hand pits. Is there strong over weak? Is the new snow cohesive enough to crack? If noticing these signs of danger, it is time to step back and avoid slopes steep enough to avalanche.
If the temperatures warm and the snow surface becomes wet and heavy, consider loose wet avalanches. Look for rollerballs and small fan shaped avalanches. If these begin, stay off steep slopes and out of gullies and terrain traps.
