Avalanche Forecast
Regions: Stevens Pass.
Unfortunately, it is not quite open season yet. Mid and upper elevation exposed slopes are the most suspect to produce wind slabs. Very large, deep slabs have become unlikely, but not impossible. Seek out sheltered, well-supported terrain and minimize your exposure to steep slopes that appear variable with wind slab.
Discussion
Snow and Avalanche Discussion:
An observer on March 1 got test results that indicated propagation was possible on the early February layer on a northwest aspect at 5,200ft near Lanham Lakes. This early February facet/crust is kind of like an outbreak of the measles. It can't be ruled out for some time after the last known case (avalanche) because evidence still points to the fact that the virus is out there, and someone could still turn up with it....I still find myself choosing my terrain very cautiously. A couple of easterly wind events with the most recent one tapering off on the 27th have left thick drifts and slabs in many places. Recent storms buried a mix of surface hoar and near surface facets on shaded aspects, and melt-freeze crust on southerly aspects. The most recent report of a slab was on the 27th when a skier triggered a large hard slab off the northwest aspect of Mt McCausland and went for a 600ft ride. He was the second skier on the slope. The skier pulled his airbag and ended up on top of the debris. Luckily the skier was ok and got away with just losing a pair of poles. This was in a couloir at 5,600ft, and the crown was 18-24" deep. More information here. Another slide was reported in the Stevens Pass backcountry from the 26th on the obs page here. We are uncertain what the weak layers were for both of these slides, perhaps the most recently buried interface, or perhaps the early February facet/crust layer.
Natural slab in Christmas Tree Bowl, spotted on 26th. Photo: Kyle Horner.
Snowpack Discussion
March 1, 2019
Here we are at the beginning of March. The days are noticeably longer, the sun feels warmer, but the winter is far from over. This may be a good time to take stock of February’s events, where we are now, and what we might expect in the future.
A February to Remember
Many longtime Cascade travelers continue to remark about the long span of high-quality travel conditions in February. Indeed, we’ve experienced three weeks of very cold and snowy conditions. Backcountry travelers frequently reported, “best conditions in yearsâ€. February also saw a few rounds of natural and human triggered avalanches. These avalanches were different from our more frequent Cascade storm slabs; here today and gone tomorrow. Most avalanches failed on buried weak persistent layers. As such, most zones spent much of February with Persistent or Deep Persistent Slabs in their forecasts.
Natural avalanche in Icicle Creek Feb 13, 2019. Photo: Matt Primomo
With the notable absence of high elevation rain events, snowpacks around the northwest grew remarkably. On the west slope, most weather stations added 2-4 ft of snow depth. East-side locations added 1-2 ft. Mt Hood locations added 4-5 ft. Even with these impressive snowpack growths, many Cascade Snotel sites are still recording below average snow depths for the winter. Â
During most of February, snow conditions were soft and powdery making for fun recreating in many areas. Photo Matt Primomo.
Where We Are Now
Lack of avalanche activity on the layer of buried facets from Feb 8 allows us to gain confidence in many areas. In some areas, snowpack tests are showing less than sudden results. These two pieces combined have allowed us to drop persistent slabs from some forecasts and shift others to “unlikely.†The exception is the East Cascades where a shallow snowpack has preserved buried facets, and persistent slabs will remain possible through the weekend. So, we’re out to the woods, right? Well, maybe. The same cold and stormy weather that brought us excellent snow quality, also allowed us to bury new weak layers. So far we haven’t confirmed any avalanches on these shallow weak layers, but they have our attention and we’re tracking them.
Recently our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow. Several wind events from various directions placed wind slabs on a variety of aspect. With the colder than normal temperatures, wind slabs may not heal as quickly as normal.
A small natural wind slab avalanche in the Crystal Mt backcountry. Photo; Dallas Glass
What Might We Expect
As we move into March, it’s anyone’s guess what specific weather patterns lay in store of us. However, there are two items that stick out in our minds.
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The strength of the March sun: You can already feel it just walking around town. As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see slab avalanches become easier to trigger, natural loose avalanches originating from sunny slopes, or surface snow become thick and heavy. Remember, the sun frequently brings change.
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High elevation travel: Frequently March begins to usher in a period where we push higher and deeper into the mountains. Remember, we don’t have as much information about these far-flung locations. If you use the longer days of March to travel to bigger objectives, keep your eyes open. When observations don’t line-up with the avalanche advisory, you may be experiencing a different snowpack. It’s times like these we need to stop and reevaluate.
Mt Baker on a nice day in early February. Photo: Lee Lazzara
February was amazing! but March is here… there’s still plenty of winter left. Stay safe out there.
Avalanche Problems
Wind Slabs
A couple of recent wind slabs have been reported. Observers on Mt. Mastiff found cracking underfoot with recent wind slabs on Friday. Firm, sometimes hollow wind slabs have been formed from low density cold snow and sustained winds earlier in the week. These slabs may be resting on a layer of near surface facets, surface hoar, or facets over a melt freeze crust. It may be easier to find wind slabs than to avoid them at mid and upper elevations. Alpine terrain and slopes exposed to the highway corridor where winds funnel through the pass are likely to hold the most substantial slabs. Look for uneven snow surfaces, hard drum-like sounding snow, and lens shaped drifts. Don't get lured out onto a wind slab on a slope steeper than 35 degrees, as they make break above you. Avalanches within upper snowpack layers have the potential to step down and create much deeper persistent slab avalanches. Seek out sheltered terrain, avoiding places where you may be taking your chances on a slab.
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
Deep Persistent Slabs
You are unlikely to get any collapses, or whumphs anymore that alert you to this instability on this layer. The only way to find it is to dig, or, if you're in the wrong place, you may find one of these pockets. The most likely place to get one of these to release would be on a place where the slab is relatively thin. I would seek to avoid places that appear to have been frequently wind stripped and are now loaded, such as exposed northwest to northeast facing terrain. Choose conservative uphill routes and descent options to minimize your risk. Travel one at a time from well defined safe zones if you do choose to enter consequential terrain.
This facet/crust layer that was initially buried on February 8th can be found about 2-4 feet beneath the surface on most aspects and elevations. This layer is still quite reactive in certain places in the East Central Zone, where it was nearer to the surface most of the month and now recently buried by wind slab. It is unknown if the skier triggered slide on the northwest aspect of Mt. McCausland and the natural in Christmas Tree Bowl were on these layers.
Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 2 - 2