Avalanche Forecast
Regions: Stevens Pass.
You may still be able to trigger avalanches in recently drifted snow near and above treeline. Steer around pillowed wind drifts on slopes steeper than 35 degrees and defer to lower angled slopes if you see signs of danger like cracks shooting through the snow. For Sunday, expect continued light snow and cooling temperatures.
Discussion
Another 5 inches of snow accumulated by the end of Saturday on top of Friday's 17-inch blustery storm. Filtered sun and warm temperatures over the past 2 days have settled the recent snow at low elevations and on sunny slopes. On Friday, observers reported easily triggering long-running, avalanches in the new snow and drifted wind slabs. While most avalanches were small, Stevens Pass Patrol reported a couple large (D2) avalanches resulting from control work.  On Friday, a party triggered a slab avalanche with a cornice drop on a wind-loaded southeast aspect at 5,600ft on Arrowhead Mtn. Also on Friday, a party on Lichtenberg Mtn reported triggering avalanches at the new/old snow interface on east aspects at 5400ft.Â
Clear skies and colder overnight temperatures last week formed weak snow surfaces, especially on northerly, shaded, and wind-protected aspects. A layer of small facets and surface hoar may be at the interface below the recent storm snow.Â
Large (4mm) surface hoar grains found near Arrowhead Mtn on a NNW aspect around 4500ft on Thursday.Â
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 Januaryâs 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 Presidentâs 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
Use caution on wind-exposed upper elevations slopes 35 degrees and steeper. Steer around drifted pillows of snow. defer to lower angled slopes if you see cracks shooting through the snow. 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.
Strong westerly winds formed wind slabs as snow fell Thursday night into Friday. More recent snow has covered these slabs. While warm temperatures over the past couple of days are helping them heal, you may still be able to trigger these avalanches. Cornices remain large and prominent on high alpine ridges. Use caution near or under these features and avoid stepping onto 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: All elevations.
Likelihood: Likely
Expected Size: 1 - 1