The bottom line: A significant winter storm sill create dangerous avalanche conditions Friday at all elevations. No matter if it’s raining or snowing, it would be prudent to avoid open slopes greater than 30 degrees where will be able to trigger avalanches.
Snow and Avalanche Discussion
The timing of the highest avalanche danger will be tricky Friday. We expect avalanche danger to peak during the overnight and early morning hours and slowly decrease through the day coinciding with a decrease in precipitation. That’s all dependent on how the weather pattern evolves.
On Thursday, in the Snoqualmie Pass backcountry, we found weak snow buried by this recent storm. Surface hoar, facets, rime, and firm crust were all reported in this area over the past week. We don’t know yet if these layers will become active and produce avalanches, but they should be on your mind if you enter the mountains.
Buried surface hoar near Cottonwood Lake at Snoqualmie Pass. Photo: Dallas Glass.
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
Warming temperatures during Thursday night and Friday will create up-side-down storm snow. As a result, you will be able to trigger slab avalanches on slopes greater than 30 degrees. Avalanches may be larger and more destructive at higher elevations and in places where the wind drifts the snow into deeper slabs. If the new snow sits on top older weak snow, you may find that avalanches propagate wider than you’d expect, fail on slopes near 30 degrees, and trigger remotely from non-avalanche terrain. If you see shooting cracks, experience collapses, or see natural avalanche, dial it back, and put large buffers between you and anywhere an avalanche can start, run, or stop.
Aspects:
All aspects.
Elevations:
All elevations.
Likelihood: Likely
Expected Size: 1 - 2
Loose Wet
A shift to rain is likely Thursday night and Friday near Pass level as the warmer west wind replaces the colder easterly flow. Where it rains, the surface snow will become wet and weak. You will be able to trigger loose wet avalanches on slopes greater than 35 degrees. Even small loose wet avalanches can hurt you, if then carry you into a terrain trap, such as a creek, a gullie, or trees.
