Avalanche Forecast
Regions: Cascades - North East.
Look to avoid that combination of slab over facets, especially on steep, shaded slopes. You may still trigger a slab in unsupported terrain and in areas where the snowpack is shallow. Seek sheltered, well-supported terrain and minimize your exposure to large, convex open slopes where avalanches may initiate. Loose dry avalanches may initiate easily on steep, shaded slopes.
Discussion
Snow and Avalanche Discussion
South facing slopes have become crusted, and shaded aspects hold cold, weakening powder. The area has been largely cold and dry, but a few inches of fresh snow on Wednesday has helped keep the surfaces fresh and soft. No new avalanches have been reported except for a few small loose avalanches in steep terrain. The most recent slab avalanche occurred over the weekend on a northeast aspect at 6500ft. This skier triggered slide was 50ft wide, ran 100ft downslope, and failed on a layer of facets below a stiff, 1ft thick wind slab. Older weak layers from early February have produced inconsistent results in snowpack tests recently. Continue to dig down 1-2ft and assess these layers of facets above and below crusts. In some areas, they may still be problematic. Significant snowpack variability exists from west to east within the zone. Avalanche Danger in the Washington Pass vicinity may be closer to low near and below treeline. You may find a weaker structure in areas further east and south of Mazama. On Thursday I found an area near Blewett Pass which had that combination of recently drifted snow over large facets, and was getting blatent signs of instability. Similar conditions may be found in outlying areas near and south of Mazama.Â
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
Persistent Slabs
Continually check for strong over weak structure with the handle of a ski pole or quick hand pits. Try to get a sense of where cohesive slabs begin. If you are traveling and all of a sudden only sinking in an inch or two, you may be on a slab that is sitting on faceted snow. Look for uneven snow surfaces, hard drum-like sounding snow, and lens shaped drifts. Firm wind board that rests on weak faceted snow have been known to break above people. Terrain management is key. Choose well supported slopes, avoiding convex rolls and crossloaded gullies. Travel one at a time from well defined safe zones if you do choose to enter consequential terrain.
Plenty of weak faceted grains can be found at, or near the surface. In some cases they sit above and below crusts buried in early February, found about 1-2ft below the snow surface. The recent slide on a northeast aspect at 6500ft suggests a patchy layer of persistent grains in upper snowpack layers near the pass as well. Shallower snowpack zones further east, or closer to the Columbia River, have a more defined weak snowpack structure.
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: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1