Avalanche Forecast
Regions: Cascades - North East.
Winds will continue to redistribute the recent storm snow and form fresh wind slabs. Use caution as you transition into wind affected terrain, and take time to identify slope scale features you want to avoid. Old weak layers may still be reactive in areas with a shallow snowpack.
Snowpack Discussion
February 5, 2019
The active weather pattern this past weekend brought most areas a shot of new snow. Sadly, it seems like areas close to Seattle may have gotten more snow than the mountains. As a result, we are generally finding lower avalanche danger in most of our forecast zones. The general exception to this rule is the mountains closest to the Canadian border. The Baker area received 4†of water since January 31st. Here are a few general trends we see at this point.
The surface is where it’s at:
A combination of high elevation rains in January, a week of warm high pressure, and minimal new snow loads allowed most of our snowpacks to gain strength. In many locations, we are finding that proverbial Cascade “brick.†It’s not that you can’t find any weak snow in the mid- and lower snowpacks, but as a general rule, the snowpacks in our forecast regions are strong. Consequently, most unstable snow will be limited to the snow surface and the last interface buried in the most recent storm. For example, this week observers are primarily tracking the snow surface and the interface buried on February 1st or 3rd, depending on your location.
Very strong surface snow. This crust was buried in most locations Feb 1 or 3. Photo: Chip Daly
How can you use this? Keep your eyes open as you travel and look for areas that appear different. Why are they different? Could this be a rogue wind slab? Is the sun affecting the snow in this location more than other areas? Is the snowpack thinner and weaker in this area? When you find unique areas, it may be worth giving them a wide berth.
Still watching the far-east:
There is a lingering exception… areas east of highway 97 and east-slopes nearer to the Columbia River. As you travel further east in our forecast area, the snowpack gets thinner, weaker, and more variable. So, why are we seeing the avalanche danger go down in these locations? In general, there just isn’t a substantial slab over the buried weak layers. Two things could change that. 1. A wind event. One quick way to get a slab is to build one with some dry snow and moderate winds. That’s why we have wind affected areas on our mind when we travel to areas with thinner snowpacks. 2. A big storm. While areas like the Wenatchee mountains have seen their fair share of snow this season, these areas haven’t seen a big loading event. How would this weak snowpack handle a big storm? It’s hard to say at the moment. Just remember, if you go to mountainous areas around Twisp, the Wenatchee Mountains, or the East-South zone, you may encounter snowpacks with lingering persistent weak layers.
A shovel tilt test highlights weak snow interfaces in a very shallow east-side snowpack. Photo: Josh Hirshberg
The danger is real:
Even though the snowpack is strong and the avalanche hazard is generally low, there are other risks in the mountains that may pose a very real danger to backcountry travelers. The one we keep hearing the most about is very firm and slick surfaces. You are most likely to find icy surfaces in locations that did not receive much snow during this recent storm. When conditions are like this, it’s tough to gain traction, and any slip may result in a long uncontrolled fall.
Icy snow surfaces have been making travel conditions difficult, and sometimes dangerous. This firm, slick layer is buried below the most recent snow in many locations. Photo: Dallas Glass
What’s next?:
Given the strength of the snowpack, it will likely take a larger storm to ramp the avalanche danger back up. When will that storm arrive? We’ll keep watching the weather patterns and let you know. Make sure to check the latest weather and avalanche forecast before you head into the mountains.
-NWAC team
Avalanche Problems
Wind Slabs
Plenty of low-density snow is available for transport. Fresh wind slabs have formed in leeward areas near and above treeline. Wind slabs that formed over the weekend still exist at upper elevations. These thicker slabs may be more difficult to trigger, but a resulting avalanche could be large. Snowfall from Monday night masked evidence of previous wind loading in some areas making it difficult to identify older wind slabs. You are most likely to find these drifts on leeward sides of ridges and in wind exposed upper elevation terrain. Watch for blowing snow, textured snow surfaces, and lens-shaped pillows to help identify wind slabs. Avoid freshly wind loaded slopes and terrain features steeper than 35 degrees.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
Persistent Slabs
Multiple persistent weak layers can be found in the top three feet of the snowpack. Although these layers have not produced an avalanche in several days, they still linger and need to be evaluated. Watch for cracking, listen for collapses, and dig down to identify these layers of weak, sugary snow with snowpack tests. Use the terrain to your advantage, avoiding areas with thick slabs adjacent to shallow, rocky slopes. Use extra caution around large terrain features, and unsupported convex rolls.
Near and east of Mazama, the snowpack is often less than 4 feet deep and highly variable. Here, a layer of buried surface hoar and facets may be found a few feet beneath the surface. You may find these to be best preserved on shaded, and open slopes above 5,500ft. Further west near Washington Pass, the snowpack is generally strong and well settled.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, South East, South West, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Unlikely
Expected Size: 1 - 1