Avalanche Forecast
Regions: Cascades - South West.
UPDATE: Increased below treeline avalanche hazard to reflect storm totals.
The West-South zone picked up a decent bit of snow with this storm and it’s got our attention. Be leery of convex rollovers, unsupported slopes, and places where the wind affected the snow. If you see cracking or feel collapsing, stay off on nearby slopes greater than 35 degrees.
Discussion
Snow and Avalanche Discussion
Storm totals as of Thursday afternoon are generally in the 8-10â range around the West-South. A little more snow is expected Thursday night/Friday. The showery nature of the remaining precipitation means some areas may pick-up a bit more snow than others. This leaves a bit of uncertainty in the avalanche forecast.
In some areas, the new snow buried facets and surface hoar. A public observation from the Crystal backcountry on Wednesday reports a few small avalanches, that may have failed on persistent grains. If you find areas where the new snow sits over these buried weak layers, you may find that avalanches fail in unusual ways. You could see wide propagation, remote triggers, or thin crowns. This snowpack structure may not exists everywhere, but keep it on your mind as you travel. If you see surprising observations, dial back your terrain use and avoid anywhere an avalanche could start.
We donât expect much sunshine in the West-South Friday, but if the sun comes out expect the snow to change quickly. You will see loose avalanche occurring naturally on steep sunny slopes. Donât linger below these areas during periods of prolonged sunshine.
Snowpack Discussion
March 4, 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 Februaryâs 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, weâve experienced three weeks of very cold and snowy conditions. Backcountry travelers frequently reported, âbest 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 recording near or just 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 many areas, there has been a lack of avalanche activity and snowpack tests have been less reactive. These two pieces combined have allowed us to drop persistent slabs from some forecasts and shift others to âunlikely.â The exception is the East Cascades where a shallow snowpack has preserved buried facets, and persistent slabs will remain possible until the sensitivity drops further. So, weâre 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 havenât confirmed any avalanches on these shallow weak layers, but they have our attention and weâre tracking them.
Recently our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow. Several wind events, predominantly from the east, 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, itâs anyoneâs 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 avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.Â
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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 donât 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 donât line-up with the avalanche advisory, you may be experiencing a different snowpack. Itâs 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⦠thereâs still plenty of winter left. Stay safe out there.
Avalanche Problems
Storm Slabs
You are most likely to find storm slabs in places that received more than 10 inches of snow, the wind drifted the snow, or the snow overlays surface hoar/facets. You can use simple tests as you travel to identify the storm slab. Do you see cracking while breaking trail? Do small slope tests produce small avalanches? Can you feel strong snow over weak snow when you dig with your hand? Has the wind drifted the snow in the area? If you answer yes, be leery nearby slopes greater than 35 degrees.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Deep Persistent Slabs
There are still lingering questions about the deep persistent weak layer in the West South. While we’re beginning to find observations to indicate this layer is gaining strength, we also have data from this zone this week where snowpack tests still indicate this layer is an issue. What can we make out of all this? Don’t let this layer drop from your mind. Be suspicious of complex avalanche terrain. Stop, discuss, and consider this low likelihood but high consequence event with your team before you commit to traveling in large complicated avalanches paths.
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: 1 - 2