Avalanche Forecast
Regions: Snoqualmie Pass.
This weather system isn’t impressive, but it will be enough to create unstable snow and increase the avalanche danger. Be on the lookout for places where the wind drifts the snow into slightly firmer and thicker slabs. If you see blowing snow or textured snow surfaces, steer away from any nearby slope greater than 35 degrees.
Discussion
Snow and Avalanche Discussion
The snowfall totals expected in this storm arenât likely to draw too much attention, but they will be enough to create some new avalanche problems around Snoqualmie Pass. Most of the new snow should be soft and unconsolidated. This may lead to a few loose dry avalanches on very steep slopes. On Wednesday, a few loose dry avalanches were reported from a nearby zone. These avalanches ran further and faster on southerly aspects where they traveled on a newly buried melt-freeze crust, formed during the recent sunny weather.
Prior to this storm observations reported a variety of snow surfaces including icy slopes, old firm wind slabs, facets, and surface hoar. Itâs these last two, facets and surface hoar, that have our attention. We suspect these layers were buried on Wednesday, but there just isnât enough new snow for them to be a problem. If you find a combination of wind slab over buried facets or surface hoar it could fail in more unusual ways including thin avalanches or wide crowns.
We recently removed the Persistent Deep Slab from the Snoqualmie Pass avalanche forecast. A lack of avalanche activity combined with recent snowpack data gave us confidence that this layer gained strength and is not a current problem. However, donât let this layer slip from your mind. Be suspicious of complex terrain and more remote locations where we have a higher degree of uncertainty.
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
Even though significant snowfall is not expected with this storm, the combination of wind and new snow will be sufficient to create new wind slabs. When you see blowing snow, textured snow surfaces, or drifts behind trees, expect wind slabs to be nearby. As more snow falls during the day, some wind slabs may grow large and more hazardous. Be leery of slopes greater than 35 degrees where you see signs of wind transported snow. Due to the changing winds during the storm, wind loading may occur on multiple aspects.
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