Avalanche Forecast
Regions: Snoqualmie Pass.
Stormy conditions Sunday night and Monday morning should continue to keep you on your toes as you travel around the Snoqualmie Pass area on Presidents Day. Steer around convex rollovers, steep wind affected slopes, and areas below cornices. Avalanches could be larger and easier to trigger as you ascend in elevation and near the tops of the prominent peaks surrounding the Pass.
Discussion
This storm cycle continues to produce snow for the Snoqualmie Pass area. A forecasted Puget Sound Convergence Zone Sunday night could add an additional 6-12†to the already impressive snow totals since Feb 13th. The tricky part about Convergence zones is snow accumulations can vary widely over a short distance. You’ll need to make observations to see if the avalanche forecast fits with the snow you find in front of you. If you think more snow piled-up than expected, dial back your terrain selections and seek out lower angled slopes.Â
As of Sunday afternoon, 18-24†of generally right-side-up settled snow exists over the prominent 2/13 crust. Observation on Sunday indicated the most reactive snow was in wind-affected areas. Travelers triggered small soft wind slabs on lee slopes near treeline.
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
Look for storm slabs to be the largest and easiest to trigger in areas where the most snow falls Sunday night, in wind drifted locations, and at upper elevations. Take time to evaluate the new snow before jumping into steeper terrain. Look for signs of cracking in the surface snow, small slopes that produce slabs, and measure the overnight snow. Steer around convex rollovers, areas where the wind drifted the snow, and steep terrain at upper elevations.
In nearby Stevens Pass, weak old snow produced several human triggered avalanches including one with three individuals caught and carried over the weekend. So far we have not seen direct evidence of a persistent slab in the Snoqualmie zone, however, a variety of snow surfaces including surface hoar and facets were reported prior to this storm cycle. When in doubt, seek out lower angle terrain and give yourself plenty of space from steep overhead slopes.
If the sun comes out on Monday, anticipate the potential for cornices to fail naturally and small loose avalanches to occur on steep and rocky slopes. Don’t let either of these issues catch you lingering on slopes below.
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