Avalanche forecast

West Slopes South

Northwest Avalanche Center
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Avalanche danger

Tue
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

On Monday, areas of wind-drifted snow will continue to be the most likely location to trigger an avalanche at upper elevations. Lower elevations may get damp from rising freezing levels and some rain, and could still produce surprising avalanches. Consider dialing back your terrain choices from what you may normally pursue at Considerable hazard.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Strong winds on Monday should create new Wind Slabs or thicken existing slabs throughout the zone. Due to rising freezing levels, you may have to travel to higher elevations to start to find cold dry snow for the wind to transport into new slabs. At lower elevations, warm temperatures or possible rain could help to destabilize recently created slabs, possibly triggering wet avalanches. Look for Wind Slabs near rocky features such as gullies and cliffs, on convex rollovers, or on leeward slopes near ridgelines. Some of these slabs may be sitting on weak snow grains, causing them to break in surprising ways. Other slabs may have multiple density changes from the past few days to fail on. You should use test slopes to check the reactivity of all of the snow sitting atop the 1/13 crust to see how it reacts to your weight. As you travel, look for textured surfaces, smooth pillow-like piles of snow, or fresh cornices to key you into wind loaded slopes you should avoid.

    (1/28/2021) This avalanche was remotely triggered by a skier in the Crystal backcountry on Thursday in wind loaded terrain. You can see the shooting cracks initiating from where the skier is standing.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Persistent Slab avalanches are very dangerous and difficult to predict. We have had good confirmation of their existence in the Crystal area. Observations from Paradise on Sunday revealed that weak snow grains are buried in the snowpack, but at this time we have not heard of any avalanches on them. On Friday, Mt. Saint Helens was very reactive, with remotely triggered avalanches up to size D2.5 which we believe to have failed as Persistent Slabs. These slabs can be triggered from long distances, break in surprising ways, and propagate widely across terrain features. You should look for red-flag signs of instability such as shooting cracks and whumpfing collapses as you travel. If you notice these signs of instability or find weak snow grains in your pits sitting above the 1/13 crust, put plenty of space between yourself and slopes 35 degrees and steeper.

    Just like we mentioned with the Wind Slabs, if we do get a good shot of rain, water could percolate down and destabilize the bonds between these weak persistent grains, causing slab avalanches.

    (1/29/2021) This slab was remotely triggered by a snowmobiler on Mt. Saint Helens on Friday (1/29). [Photo: Kurt Heine]

Avalanche Discussion

Sunday brought some rising temperatures and a bit of rain to the lower elevations, with wind and snow showers at upper elevations in the West South. Monday should bring a similar pattern, so don’t be surprised to find some rollerballing or a few loose wet avalanches if light rain falls on dry snow at low and mid-elevations. If we end up with a good shot of rain, wet avalanches could be more pronounced. 

On Sunday, snow was observed to be thick and wet up to around 6000 feet by a NWAC forecaster at Paradise, with drier snow found at higher elevations. This has aided in slab development above the 1/13 AR crust and should be prominent around the zone. Another NWAC forecaster was at Crystal on Sunday and experienced whumpfs and cracking in wind-loaded terrain during his travels, while also confirming slab development due to warming. In areas that contain weak persistent grains above the crust, we now have the 3 elements needed for creating slab avalanches: a slab, weak layer, and bedsurface. We know that near-surface facets are sitting atop the 1/13 crust in the Crystal backcountry, while at Paradise buried surface hoar and near-surface facets are sitting atop a few different crusts in the upper snowpack. Mt. Saint Helens has recently produced avalanches which appear to have been persistent in nature.  We received a good public observation that small facets may exist at White Pass atop the 1/13 crust and are reactive in snowpit tests. This problem may not exist everywhere, but if you happen to be in a location where a persistent slab avalanche is triggered, it could be large and costly. Digging down to the 1/13 crust and positively identifying these weak snow grains is the only way to tell if the problem exists in your area. If you do not find them, you should not immediately rule persistent slabs out, continue to collect more information and ease into terrain. Thanks to everyone who has submitted public observations this weekend as we navigate a bit of uncertainty around the snowpack. We’d love it if you kept them coming! Submit a public observation here. 

During the ongoing Covid-19 pandemic, continue to follow the latest guidance from local, state, and federal health officials. Avoid unnecessary risks that could impact first responders and stress medical resources.  Wear a mask when appropriate, practice social distancing, and consider recreating closer to home.

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