Avalanche Forecast
Regions: Cascades - North West.
Heavy snowfall and strong winds Friday night will create increasingly large and dangerous slabs at higher elevations where winds drift the new snow more deeply onto lee slopes. Use small test slopes to look for reactive layers in the upper snowpack and frequent hand shears to check the new/old interface; if you see signs of instability such as recent avalanches, shooting cracks, or whumping, dial back your terrain selection. Avoid slopes steeper than 35 degrees in wind-loaded areas and steep convex rollovers at all elevations.
Discussion
On Thursday, we received a report of a very large and relatively fresh natural cornice-triggered avalanche released off the E face of Bearpaw Mountain, triggering a slab 3-8 ft deep and 300 ft across. The reporting party has not determined what layer the slab ran on, but it could have been deep wind-loading on top of the 2/22 interface, or perhaps it ran on an older interface.Â
Although we’re primarily concerned about the new snow arriving Friday night and early Saturday, this incident reminds us that with a very large load it may still be possible to trigger these isolated lingering weaknesses at depth and that load may come in the form of a slab avalanche as we transition into a colder and more active weather pattern.
On Friday, Mt. Baker Pro patrol reports small surface hoar surviving increasingly strong winds in wind-sheltered bowls on Shuksan arm (5000-6000 ft, due north aspect). Mild weather and sunshine have contributed to crust formation on most aspects (with a thin crust likely on due north).
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Snowpack Discussion
February 27, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
In the lead up to last weekend, dry, clear and cold conditions produced surface hoar in many areas as well as faceted snow (check out the National Avalanche Center’s encyclopedia for more details). These weak grain types were not difficult to find and recreationists and professionals across the region began to wonder what role these weak snow surfaces would play when the next storm arrived.
Surface Hoar was not difficult to observe during this period of high pressure. This picture is from 2/21 on an SSE aspect, 5,000’ at Stevens Pass just a day before it may have been buried. Photo: Josh Hirshberg
We didn’t have to wait too long, as a major storm arrived on Sunday 2/23, creating dangerous avalanche conditions for most zones. This system dropped close to 2’ of snow at Mt. Baker while areas to the south along the west slopes of the Cascades saw closer to 1 foot. Areas along the east slopes of the Cascades also saw significant new snow amounts with Blewett Pass and Mission Ridge receiving around 6-8†of fresh snow.Â
The wind was also a major factor with this storm from about Stevens Pass southward. Multiple stations recorded winds over 80mph and some went over 100mph. This wind was enough to strip surfaces down to old crusts on windward aspects, leaving leeward aspects with wind slabs and touchy cornices. In the case of Mt. Hood, relentless winds this season have stripped the snow down to blue ice from one of our previous atmospheric river events, creating slide for life conditions on certain slopes.Â
The old snow interface, termed 2/22 for the day it was buried, has us thinking hard about the snowpack moving forward. In the northern part of the region, less wind allowed for more storm than wind slabs, with avalanches that released on both interstorm layers and on the buried 2/22 interface for a day or two following the storm. As you moved further southward or eastward, reactivity on the 2/22 interface was trending downward or non-existent after a natural avalanche cycle that immediately followed the storm. Was the storm potent enough to wipe out the surface hoar and facets? There was a surprising natural avalanche on Stevens Pass during the evening of Monday 2/24, which is suspected to have run on small facets. This avalanche and continued snowpack observations around the region proves that in specific areas, reactive weak snow still existed.Â
This natural avalanche failed 2ft deep on the 2/22 interface on a SE aspect near 5700ft on Cowboy Mountain. 02/25/20. Photo: Stevens Pass Ski Patrol
On Tuesday, the sun started to shine in some locations, bringing back the threat of wet avalanches and again making us wonder about how warming would affect the 2/22 interface. We never reached the tipping point for widespread wet avalanche activity, but rollerballs and snow shedding off trees and rocks were certainly present in many zones. Drizzle or light snow made an appearance on Wednesday along many west-side zones. Thursday saw a return to warm temperatures and sunny skies all the way from Mt. Hood to Stevens Pass with cooler and cloudier conditions for the North Cascades. All in all, it was a quiet week avalanche-wise and the 2/22 wasn't activated.Â
We have one more dry mild day before we move into a stormy Saturday. During this stretch, we’ll continue to monitor buried weak snow grains for distribution and reactivity. Check your local forecast zone for the latest.Â
Avalanche Problems
Wind Slabs
Winds from as recently as Tuesday night and Wednesday created slabs near and above treeline that will be more reactive and larger as you ascend. As these slabs age, they maintain their size but become less likely to trigger. Pay particular attention to specific terrain features where you might find wind-loading on poorly supported slopes steeper than 35 degrees. Look for textured snow or lens-shaped pillows to avoid this problem as you ascend the slope. Recently formed cornices provide clues to wind slab formation on the slopes below.
Prior to the storm, weak snow grains such as facets and surface hoar existed on the surface in many locations. They may be found in isolated areas and should be increasingly stubborn to trigger. Slabs that sit atop these weak grains could break wider or on lower-angled slopes than you may be used to. Look for and perform snowpack tests on the weak grains about 2 ft down and the overlying slab interfaces before committing to consequential slopes.
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: Likely
Expected Size: 1 - 1
Storm Slabs
We’re expecting 8-14” inches of snow by mid-morning on Saturday. The storm slab remains a threat at lower elevations where layers within the storm slab are the primary concern as we expect the precipitation at these elevations to bond well to the old snow crust. You’re most likely to trigger these avalanches on steep, convex rollovers and atop unsupported slopes such as cliff bands. Take advantage of small test slopes to remove support for this layer and if you find reactive snow, choose safer lower-angle slopes.
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.
Elevations: Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1