Avalanche Forecast

Issued: Feb 7th, 2019 10:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Loose Dry and Persistent Slabs.

NWAC, Northwest Avalanche Center

Cascades - East.

Watch for triggering loose dry avalanches on slopes steeper than 35 degrees where you find soft snow. You could trigger avalanches on old, weak layers in areas with a shallow snowpack, such as the Wenatchee Mountains and Blewett Pass. Expect dangerous avalanche conditions to develop Friday night and continue into the weekend.

Summary

Discussion

Snow and Avalanche Discussion

2-3 inches of light, dry snow could add up throughout the day. Expect snow to ramp up overnight with possibly 1-2 feet of total accumulation with stronger winds on Saturday. If 1 foot of snow accumulates in the valleys, avalanches may be possible in unusual places like on roofs and steep lowland slopes.

Friday's snow will likely be light and uncohesive. Expect a poor bond where the new snow falls on weak old layers and slick crusts. Watch for a dramatic rise in avalanche danger for Saturday. We'll keep an eye on how the storm snow and underlying layers bond. We may be tracking this interface for some time into the future.

Surface hoar and weak facets blanketed many slopes as of Thursday. This could become a weak layer buried by Friday's storm. Photo Josh Hirshberg

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

Problems

Loose Dry

The new snow and underlying surfaces are light and uncohesive. Watch for triggering loose dry avalanches on slopes 35 degrees and steeper. These avalanches are easy to predict as they break at your feet. Use caution around these terrain traps like cliffs, gullies, or rocky slopes. Use small test slopes to check if the surface snow slides easily. If you find more than 6 inches of cohesive snow on the surface, expect that you may be able trigger a small slab avalanche.

Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.

Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.

Loose Dry avalanche with the characteristic point initiation and fan shape.

Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Persistent Slabs

The new snow is beginning to stress weak layers at the old snow surface and deeper into the snowpack. 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.

In the Wenatchee Mountains and east of Highway 97, the snowpack is often less than 4 feet deep and highly variable. The most recent layer of buried surface hoar (January 22) may be found about a foot beneath the surface. A pair of thin freezing rain crusts associated with the early January and late December weak layers are still a concern in this area as well. You may find these to be best preserved on shaded, and open slopes above 5,500ft. Further west, 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.