Avalanche Forecast
Regions: Stevens Pass.
Dangerous conditions may be found due to a layer of weak snow near a crust that is getting overloaded by new snow. Numerous human triggered slab avalanches have occurred in the zone this weekend, and more are likely on Monday, even though the storm is winding down. Exercise conservative travel techniques to stay off of, and well out from underneath steep open slopes.
Discussion
In the past couple of days there have been numerous skier triggered avalanches in the Stevens Pass zone. On Sunday there was a multiple burial incident above Skyline Lake. 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 5,400ft, and was 2ft to 3ft deep.
On Saturday, observers reported triggering slabs from 1 to 2ft deep near Lichtenberg and Arrowhead. At the same time on both Saturday and Sunday, a number of other folks report few signs of instability. With the snowfall and winds tapering, and a fair amount of uncertainty with the reactivity and distribution of persistent grains, Monday will warrant cautious backcountry travel to stay safe.Â
A 2ft to 3ft deep slab that buried three people, as described above. Southeast aspect at ~5,300ft. above Skyline Lake. Dustin Riggs photo, February 16, 2020.
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
Persistent Slabs
Look, listen, and feel for signs of instability. Shooting cracks and whumphing collapses are signs that you should stay off, and out from underneath avalanche terrain. This persistent slab situation is sneaky, because you may not get direct observations of instability before triggering an avalanche. Keep it simple, and play it safe by avoiding steep, open slopes.
Recent avalanches have been triggered from below, with audible whumphing collapses, and with impressive propagation. There is more than one potential failure interface; A layer of buried surface hoar from February 13th that may exist on northwest to southeast aspects in open sheltered locations. This layer may be found down about 2ft from the surface. Another potential weak layer is a layer of facets above or beneath a thin crust from February 7th. This layer may be found down 2 to 3ft deep. Look for these layers at and above 4,500ft or so. It appears that southeast aspects have been the bullseye for recent activity.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
Wind Slabs
New snow and strong westerly winds will continue to form slabs through Sunday night. Use caution on wind-exposed upper elevations slopes. Steer around drifted pillows of snow. Defer to lower angled slopes if you find instabilities like shooting cracks or recent, shallow avalanches. Any shallow wind slab in the surface has the potential to step down to a deeper, weak layer and become a larger avalanche. Use visual clues like drifts and wind-sculpted features to identify areas of wind deposition. Use your pole or probe to feel for stiff, drifted snow under softer surfaces.
Cornices remain large and prominent on high alpine ridges. Use caution near or under these features and avoid going near cornices where they are overhung.
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