Avalanche Forecast
Regions: Cascades - South West.
Calm, cold weather is allowing the snowpack to slowly gain strength. That doesn’t mean you can’t find a lingering wind slab. You are most likely to trigger an avalanche near convex rollovers, on unsupported slopes, or in complex terrain at higher elevations.
Discussion
Snow and Avalanche Discussion
Cold temperatures and light winds are keeping the recent snow soft and dry. This means it’s generally loose and unconsolidated on sheltered, shaded slopes. We received reports of a few loose dry avalanches on Monday. You may continue to trigger small sluffs on steeper terrain. Don’t let these small avalanches catch you off guard if you are traveling in high consequence terrain.
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
The wind slabs are now a few days old. This should mean they are more difficult to trigger. When we get into situations like this, we start thinking about slope features where it is easier to trigger an avalanche. You are most likely to trigger a lingering wind slab near convex rollovers, on unsupported slopes, or in more complex terrain. You’re more likely to find old wind slabs at higher elevations, and in areas that are more prone to wind, such as near ridges and passes. Keep your eyes open as you travel, and take a moment to identify slope scale features you want to avoid.
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
Loose Wet
Low-level clouds kept the sun at bay in many locations Tuesday. That means that the loose wet problem didn’t materialize. The first time the sun influences the recent snow is when we expect it to be the most reactive. In some locations, the first time the new snow sees the sun could be Wednesday. If the sun comes out, you may see new rollerballs or loose wet avalanche on steep sunny slopes. Expect these conditions to develop first on steep, rocky slopes receiving direct sunshine. When you see signs of wet surface snow, you may trigger a loose wet avalanche on similar slopes.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1