Avalanche Forecast
Regions: Cascades - West.
Variable storm snow amounts will create storm slab hazard in some areas and not in others, but notably in areas that experience a convergence band if it forms Thursday evening. If you find more than 6” of snow sitting on wind-affected snow, crusts, or weak sugary facets, choose slopes less than 35 to mitigate your hazard. Also, steer around exposed terrain features at higher elevations where you could still trigger a wind slab.
Discussion
Snow and Avalanche Discussion
The adjacent West-North zone rung-out almost a foot of new snow with this weather system. Observations from Thursday found the new snow loose and unconsolidated until late afternoon when increasing winds and snowfall rates created subtle slabs in some locations. Avalanches were limited to very steep convex slopes where the wind drifted the snow into slightly firmer slabs.
A shovel tilt-test highlights the interface between the old and new snow. You may find old weak facets at this interface in some locations. Photo: Andrew Kiefer.
When the sun comes out expect the snow to change quickly. Loose wet avalanches may release on steep sunny slopes. Donât get surprised by this easy to predict and avoid avalanche problem. Stay out from under steep sunny slopes during periods of prolonged sunshine. Â
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
This storm system largely wraps up for the West Central area Thursday evening, but the last legs of the storm could bring 6” of fresh snow to some locations. This leads to some uncertainty with the avalanche forecast. If you experience more snow than the weather forecast suggests, you may be dealing with locally hazardous avalanche conditions and a storm slab problem which will be reactive and small. The new snow buried a variety of old snow surfaces including weak facets, settled powder, and firm crust. If you find a location where the new snow rest over facets and a crust, an avalanche may act in unusual ways. You could see wide propagation and/or thin crowns. If you see more than 6” of storm snow or signs of such unusual avalanche activity, avoid all slopes steeper than 35 degrees.
In areas with less than 6" of storm snow, watch steeper lee terrain features near ridgelines for signs of wind transport, where a wind slab could catch you.
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