Avalanche Forecast
Regions: Cascades - West.
You can trigger a dangerous avalanche in wind-affected snow that has piled up at high rates overnight. Wind-affected areas where you experience firm or punchy slab layers are where you will trigger an avalanche with more widespread triggers as you ascend. Avoid wind-affected snow and unsupported slopes steeper than 30 degrees where you may trigger a large slab avalanche.
Discussion
On Sunday there was a multiple burial incident above Skyline Lake in the neighboring Stevens Pass zone. Skiers triggered the slope from below, reporting a large collapse just before the avalanche. All three party members were caught, 2 were fully buried but able to clear their airways. The third member was buried up to his shoulders with one arm free and was able to self-rescue. Thankfully, no injuries were reported and everyone was ok. The slide occurred on a southeast aspect at appx 5400 ft. and was 2ft to 3ft deep.
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Southern parts of this zone should align well with Stevens Pass, which picked up over 2 ft of snow since a front moved through on Thursday with a steady moist flow allowing showers and heavy convergence bands to keep piling up the snow. The northern part of this zone received less snowfall and should have a slightly lower danger, perhaps more similar to the West North zone.
Snowpack Discussion
February 13, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
Heart of Winter
The action has been non-stop so far in 2020 with several widespread natural avalanche cycles and a few recent close calls. The active weather pattern has kept us all on our toes, especially January’s barrage of storms bringing seemingly endless precipitation and dramatic snowpack growth. Ongoing snow, wind, and rain continued into February, and a not-so-ordinary atmospheric river event recently left its mark on the region. The second week of February brought the first stretch of high pressure in weeks, allowing the snowpack to gain strength and the avalanche danger to ease between storms. Now, in the heart of winter, we have a deep and healthy snowpack with snow depths throughout the Cascades and Olympics near 100% of normal. Looking ahead, each day brings new changes to the upper snowpack, and a dynamic pattern with direct action events (storm-driven avalanche danger) will likely be par for the course.
Atmospheric River AftermathÂ
Model simulation for February 5-6th, 2020 showing an Atmospheric River (AR) with a less than common northwest-southeast orientation as it impacts the region. This orientation allowed for strong westerly winds and more favorable upslope flow than a more typical AR approaching from the southwest. Image courtesy of the Center for Western Weather and Water Extremes, UC San Diego. (Link)
An atmospheric river impacted the region on February 5th-8th, causing a string of notable events. This storm favored the Central Cascades and Stevens Pass in particular, which experienced continuous heavy snow and rain for 86 hours, amounting to almost 70in of snow with about 7.5in of water equivalent. Not surprisingly, atmospheric rivers often go hand in hand with avalanche warnings, which were issued for 3 consecutive days at Stevens Pass from February 5th-7th, along with high danger in all other zones. Heavy rain fell at low elevations and even caused a significant mudslide on SR 410 between Enumclaw and Crystal Mountain, closing the road for 4 days and knocking out communications to 9 mountain weather stations for a week. As the AR exited the Northwest, and natural avalanche activity tapered off, conditions still remained touchy to human traffic on February 8th and 9th. Several triggered avalanches were reported that weekend, most notable of which was a close call near Mt. Baker Ski Area:
On February 8th, a skier was fully buried in an avalanche adjacent to Mt. Baker Ski Area. The avalanche was triggered by a traveler from a different party. Mt. Baker Ski Patrol was on the scene immediately, located the victim quickly, dug them out, and cleared the airway. The individual survived and reported no injuries. The avalanche was about 1ft deep and eventually broke up to 500ft wide. NNW aspect 5500ft. Photo: Mt. Baker Ski Patrol
Clear skies on Sunday, February 9th gave observers a chance to document the widespread avalanche cycle in the Stevens Pass zone that occurred February 5th-8th, including this view of crowns from large natural avalanches in the Berne Camp Chutes with Glacier Peak in the background. Photo: Matt Primomo
High Pressure before President’s Day Weekend
The week of February 10th brought the longest stretch of dry weather so far in 2020. A notable northwest wind event redistributed snow throughout the region and drove an isolated wind slab problem in most zones. Generally, it was the quietest few days avalanche-wise in weeks. However, a significant human-triggered avalanche occurred near White Pass on February 12th. Fortunately, no one was caught or injured. The incident provided a good reminder that even during periods of lower avalanche danger when avalanches are unlikely, outlier events can and do happen. The winter snowpack will always pose some level of uncertainty, and big triggers like cornice fall can produce surprising results.  Â
The crown of a human-triggered avalanche on a northeast aspect at 6700ft in the Hogsback area near White Pass. Two travelers unintentionally triggered a cornice, which dropped onto the slope below and triggered a very large avalanche. 2/12/20 Photo: White Pass Ski Patrol
Avalanche Problems
Storm Slabs
Heavy convergence snowfall should create reactive snow and we think that these instabilities should decrease throughout the day with the snow spigot shutting off. Instabilities may exist within the unconsolidated fresh storm snow, but the wind will be the key ingredient to get cohesive snow and a reactive slab which will be more prevalent above treeline where the moderate wind will also make fresh and reactive slabs larger. Look for signs of wind-affected snow in the terrain around you, including wind-drifts, textured snow surfaces, recently formed cornices or wind-lips, firmer layers beneath the surface, or tree branches that have lost their snow. Avoid all above treeline slopes steeper than 35 degrees and any wind-affected or unsupported slopes steeper than 35 degrees near treeline or below.
While we don’t have direct evidence of a persistent weak layer in the West Central zone, knowledge of persistent grains in the Stevens Pass zone combined with the recent avalanche incident described above have us concerned that the weak layer present there may be in this zone as well. Consider digging down 2-4 ft and looking for persistent grains or a crust that may have persistent grains surrounding it before exposing yourself to large slopes steeper than 30 degrees.
If the sun makes an appearance, expect rollerballs and small loose wet avalanche activity on southerly slopes.
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: Likely
Expected Size: 1 - 1