Avalanche Forecast
Regions: Olympics.
Shallow wind slabs should continue to build near and above treeline and with light additional snowfall and continued wind transport onto N and E aspects, the danger will be slightly higher than on Friday. You are most likely to trigger in steep, rocky start zones or on slopes steeper than 35 degrees that roll away from under you. On southerly slopes in particular, prepare for firm crusts, and keep a keen eye out for open moat-like holes in the terrain.
Discussion
The Olympics picked up 3" of fresh snow Thursday night that generally was distributed 2-4" deep, except on N-E aspects where wind-deposits were up to 8-10" deep and created isolated wind slabs (5200-5600 ft) that were stubborn to trigger (you could trigger on a steep slope, but with minimal propagation).
Test slope fractured 8" down near Maggies, NE aspect, 5200 ft. Wind deposits were deepest just below ridgelines on NE aspects. Photo: Sam Luthy
Firm crusts particularly on S aspects above 4800 ft and breakable crusts below that, make travel difficult.Â
Keep a keen eye out for moat-like features that are opening up on these steep south-facing slopes as you could fall into one.
Forecast Schedule
For the 2019-20 winter season, avalanche danger ratings will be issued for the Olympics every Friday through Sunday and during mid-week holidays. During the week, No Rating will be issued but forecasts will include expected conditions and relevant travel advice. If you are out in the Olympics, share your backcountry observations.
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
Wind Slabs
Wind slabs should grow slightly as more light snow along with light to moderate winds continues to transport snow onto generally N and E aspects, building upon wind deposits formed Thursday night and Friday so we are confining the problem to generally NW-N-NE-E-SE aspects. You can trigger these slabs on specific terrain features, such as steep, convex slopes in the lee of ridges or potentially well below the lee of ridges if you head into more exposed alpine terrain. Use test slopes such as the one pictured above to assess the wind slab problem in your area. Look for active snow transport, 6" or more of snow on a crust, cracking or firm snow surfaces and textured snow surfaces as a sign that wind deposits and potentially slabs lurk in the terrain. If you identify any of these warning signs, avoid the steep and unsupported terrain features where you might trigger a slide.
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: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1