Avalanche Forecast
Regions: Stevens Pass.
Fresh slabs will be primed for human-triggers in leeward alpine terrain on Friday. Avalanche conditions will become increasingly reactive as you climb up in elevation and into wind-exposed terrain. Use visual clues to identify and avoid wind-loaded slopes greater than 35 degrees.
Discussion
A Puget Sound Convergence Zone formed over the Central Cascades Thursday afternoon, and precipitation over the next 24 hours looks to favor Stevens Pass. Expect strong and sustained westerly winds to accompany the cold, low-density snow (2500ft snow levels). If the precipitation forecast verifies, fresh slabs will likely form in all elevation bands, with dangerous avalanche conditions developing above treeline. Carefully evaluate how new snow bonds to old snow surfaces. Observers on Stevens Pass Thursday found variable surface conditions including surface hoar on shaded aspects protected from the wind.
4-5mm surface hoar found on an open, sparsely treed slope near 4500ft on the NNW aspect of Arrowhead Mountain. 02/13/20. Photo: Josh Hirshberg
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
The new snow will easily be drifted into fresh slabs on leeward slopes and terrain features. Fresh slabs will likely become reactive to human-triggers on slopes steeper than 35 degrees by the time 6in or more of new snow accumulates. Be especially careful of convex rollovers and areas below ridgelines. Cornices are also large and looming in the alpine. Steer clear of these heavy and precarious features. Pay attention as you transition into wind affected terrain, and use visual clues to identify and avoid steep wind loaded slopes. Watch for blowing snow, textured snow surfaces, and lens-shaped pillows. Ease into terrain slowly, and use small inconsequential slopes to test the snow. Be suspicious of the variable old snow surfaces getting buried - new snow may not bond well.
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: All elevations.
Likelihood: Likely
Expected Size: 1 - 1