Avalanche Forecast
Regions: Snoqualmie Pass.
East wind has formed wind slabs over the past couple days. You may be able to trigger avalanches in stiff wind-drifted snow near and above treeline. Avalanches in older weak layers are unlikely. Use caution if you're venturing into upper elevations or very steep terrain, as there's more uncertainty about conditions there.
Discussion
Snow and Avalanche Discussion:
The deep persistent slab avalanche problem was recently removed from the Snoqualmie Pass forecast zone. A weak layer of facets is buried 3-4 feet deep under stout slabs and sits over a crust. It's been over two weeks since observers reported avalanches on this layer. The facets have undergone significant rounding (strengthening) and most snowpack tests indicate that the layer is difficult to trigger. In isolated areas, some snowpack tests are more reactive.
Keep in mind that at Low danger avalanches are unlikely, but not impossible. When avalanches are "unlikely" it means that there's somewhere in the realm of a 10% chance that you could trigger them. You may find unstable snow in isolated and extreme terrain. If you're traveling in steep or remote terrain, consider the consequences of an avalanche, a fall, or a gear failure.
Wind-textured snow at 5,000ft on a north aspect of Chair Mountain. Snoqualmie Pass. Photo: Dallas Glass
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
Wind Slabs
East wind built shallow slabs on exposed slopes on Sunday and Monday. Look for wind-affected surfaces and drifts as an indicator of wind slabs. Use inconsequential test slopes to check if you can trigger avalanches in the drifted snow. Cracks shooting through the snow are an obvious sign of instability showing that you can trigger a wind slab. The easiest way to avoid these avalanches is to stay off of leeward slopes near and above treeline. Steer around areas of stiff, drifted snow.
There are a couple of interfaces in the upper snowpack (about a foot below the surface) that could make it easier to trigger wind slabs. Use extra caution if you find buried surface hoar or facets, under the drifted slabs. Observers reported wind slab avalanches at Snoqualmie Pass on March 3rd and 4th. On February 27th, a skier triggered and got caught in wind slab avalanche on Stevens Pass.
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