Avalanche Forecast
Regions: Cascades - North East.
New snow and wind will create heightened avalanche danger at mid and upper elevations. Watch for fresh drifts, wind stiffened snow, and slick old snow surfaces. Beware of places where persistent weak layers still linger such as high, north facing terrain, and in areas with a shallow snowpack.
Discussion
Snow and Avalanche Discussion
A week-long stretch of dry, spring-like weather is coming to an end. Beginning Thursday night, a storm will impact the region bringing moderate snowfall and strong southwest winds to the Washington Pass area. Avalanche danger will steadily increase over the next 48 hours as snow accumulates. The new snow may not bond well to old snow surfaces. Observers recently found a mix of supportable and breakable sun and wind crusts, crunchy styrofoam, and recycled powder. Monitor changing conditions throughout the day Friday, and assess how the new snow bonds to these variable surfaces.
Snowpack Discussion
January, 31, 2019
As we turn the corner to February we're coming out of a week-long high pressure ridge and into unsettled weather. The snowpack survived extremely warm temperatures and sunny skies over the week with minimal new wet avalanche activity reported. This break in the weather allowed for avalanche danger to steadily decline in all regions. With stormy weather, the danger is once again elevated.
We’ve heard a variety of stories from backcountry travelers over the past week. There have been reports of extremely firm slopes creating slide-for-life conditions. Others reported perfect spring like snow. Some encountered difficult breakable crust. And, for a lucky few, softer, drier, mid-winter snow has been found. A common thread in most zones is where precipitation falls as snow, it likely will be landing on slick surfaces. It's time to pay attention to the new old interface formed by our most recent storm.
North-South:
While a high elevation rain event, around January 23rd, formed surface crust in many regions, it’s the constant melt-freeze cycles from the past week, that caused a divergence in the Northerly and Southerly snowpacks.
North: On shady slopes, things haven’t exactly been soft. The crust formed at the end of the last storm extends to high elevations (Mt. Hood 7000+ft, South Cascades 6500 ft, Passes and Central Cascades 6000 ft. and West-North 5500 ft.). Only areas in the East Cascades seemed to escape the wrath of this breakable crust. Without the help of the sun, shady slopes haven’t been softening even during this period of warm weather. Instead, the surface crust underwent some weakening. Observations found faceting on top of and below this crust. In some locations, this caused the crust to begin to degrade, becoming less supportive. Surface hoar has also been reported from the typical valley bottoms and sheltered terrain near water sources. At low elevations, rain may melt any weak snow on the surface. Slopes receiving significant dry snow should be suspect for a poor bond at eh interface buried on Thursday night.
South: On sunny aspects, the sun drove warming and melting of surface snow. Long, cool, winter nights allowed for the surface to freeze again. This repeating melt-freeze pattern created a thicker, firmer, and more supportable surface. On many days, weak surface snow, such as near surface facets or surface hoar, melted during the day limiting its development. On cooler days, very firm travel conditions were reported. Crusts may provide a poor bond for any snow falling on them.
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Near surface facets developed on shaded snow surfaces in many areas. These facets are from near Mt Baker. Photo: Lee Lazzara
East-West:It’s not uncommon for our east-side forecast zones to experience lingering persistent weak layers (PWL’s). This season, we’ve also seen several different PWL’s in our western zones. This break in the weather gave the snowpack time to gain strength in all zones.
West: While you may find some weak snow in the upper few inches of the snowpack, the mid and lower snowpack has been found to be quite strong. Firm rounded grains, stout  crust, and strong frozen melt-forms make up the majority of the snowpack at this time.
East: The east-side snowpack continues to be highly variable. You may find deep strong snowpacks closer to the crest or you could encounter shallow weak snowpacks areas further east. While there are number of potentially weak interfaces, there are two more common layers we’ve got our eyes on.
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January 22nd surface hoar and small facets. You can find these just under the recent storm snow, about a foot down. .
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December 26th surface hoar. This layer can be found from 16†to 40†down and is still producing clean, planar shears with tests.
You are most likely to find these layers to be preserved on wind sheltered, shady, and open slopes above 5,500ft. You can find more defined weak layers where snowpack is less than 4 feet deep and variable especially east of Highway 97. Persistent weak layers have been “dormant†or unreactive during the week of high pressure. They may or may not become reactive with the next storm. As new snow accumulates, it will begin stressing these weak layers. We’ll keep tracking them to watch their progression..
The lower eastern slopes and the Columbia River. Snow exists at low elevations, but snowpack depths are shallow. Photo: Matt Primomo
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Avalanche Problems
Wind Slabs
New snow and wind will create fresh wind slabs near and above treeline. These slabs will become thicker throughout the day Friday as more snow accumulates. The most likely place to trigger wind slabs will be near and above treeline on lee slopes and terrain features 35 degrees and steeper. Look for sudden changes in snow surface texture and hardness to identify wind-drifted areas of snow. If you trigger a wind slab avalanche, it has the potential to step down on to deeper buried weak layers resulting in a larger avalanche. Stick to scoured or wind-sheltered terrain to avoid the problem.
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
In the Washington Pass area, the snowpack is generally strong and well settled. However, a few persistent weak layers still exist in the top three feet of the snowpack. Keep these layers in mind as we receive more snow in the coming days. You are most likely to find these preserved on shaded and sheltered open slopes. Near and east of Mazama, the snowpack may be shallow. Where you find less than 4 feet of total snow, suspect a weaker snowpack structure and lingering weak layers near the ground. Continue to watch for cracking or collapses within the snowpack from the weight of a person or a sled. Avoid areas with where thick slabs are adjacent to shallow, rocky, steep slopes.
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: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 1