The bottom line: Dangerous avalanche conditions are developing. The new snow may be poorly bonded to weak, old snow surfaces. Warming temperatures and winds will contribute to slab avalanches that could be very wide and surprising. Treat all steep open slopes with the utmost caution.
Avalanche Discussion
About 8 inches of snow has fallen in the last couple of days. Avalanche control teams were able to trigger sensitive storm slab avalanches during the morning of the 18th. The old snow surface consisted of a mix of surface hoar, 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, and have been found to be more well preserved on the eastern side of Stevens Pass. An observer reported sudden failures on this layer during the day on the 18th on north and southeasterly aspects near ridgelines. As a hefty new load of snow accumulates into Saturday, a large amount of uncertainty remains about how reactive and how widespread the activity will be on this layer. 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
New snow that falls with rising temperatures is a red flag. Winds will drift the new snow into areas of thicker slabs. Danger will be higher than it was on the 18th as more snow has been added to the slab. Anticipate that whatever new snow does accumulate won’t bond well with the recently buried surface. Slides may be easily triggered, even in low elevation areas and could be surprisingly wide, encompassing much of the terrain feature. It’s time to evaluate for slab characteristics, and how well it bonds. Look for shooting cracks within the new snow. Check how the new snow is bonding by using snowpack tests and quick hand pits. Is there strong over weak? Is the new snow beginning to crack? If so, it is time to avoid slopes steep enough to avalanche. Even small open slopes in the trees could become dangerous if this layer of weak, old snow decides to act up. Be careful to not get too close to the bottom of steep slopes as well.
Aspects:
All aspects.
Elevations:
All elevations.
Likelihood: Likely
Expected Size: 2 - 2
Loose Wet
Temperatures will be quite warm first thing in the morning. Rain on snow often creates avalanches. Loose wet avalanches occur when the snow surface loses cohesion and snow becomes entrained as it continues down steep slopes. Looks for roller balls, fan shaped avalanches, and wet, heavy debris piles to clue you in that these may be happening. If so, avoid getting close to steep slopes, gullies, and terrain traps. Loose wet danger should decrease as the day progresses with cooling temperatures.
