Avalanche Forecast
Regions: Cascades - West.
Use visual clues to identify and avoid steep wind-loaded terrain. Watch for snow surfaces to become wet and weak on sun-exposed slopes, especially during the first half of the day Sunday. The new snow still needs time to settle and stabilize. Don’t let the powder snow and sunshine lure you into big terrain without first gathering plenty of information and carefully evaluating the snowpack.
Discussion
Friday night’s storm delivered 8-14in of new snow to the adjacent Stevens Pass and West North zones. The storm started out warm and windy and finished cold. Storm snow is right-side-up, but reactive density breaks within new and wind-loaded snow still need time to heal. Several natural and triggered avalanches were reported Saturday in wind-loaded, upper elevation terrain. Most were shallow (4-6in) and small (D1), although a couple of very large natural avalanches were spotted high in the alpine that broke quite wide and several feet deep in recent wind-loaded snow.
In addition to lingering storm instabilities, strong March sun will also drive the avalanche danger Sunday. Expect clear and sunny skies in the morning and increasing clouds in the afternoon. A round of wet loose activity should begin mid-morning on steep, rocky, sun-exposed slopes. The strong solar radiation could also cause cornices to fail. Steer clear of these heavy and precarious features. A cornice fall could act as a huge trigger, resulting in a large and surprising avalanche. Â
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
You can trigger avalanches in wind-loaded terrain 35 degrees and steeper. Wind slabs will become thicker and more reactive as you climb up in elevation. Most recent wind slab avalanches have been shallow, but if you venture into aggressive high alpine terrain, you could trigger fresh slabs several thick that could produce large avalanches. Pay attention as you transition into wind affected terrain looking for cornices, textured snow surfaces, and obvious signs of unstable snow like shooting cracks, whumphing collapses, and recent avalanches. Ease into terrain slowly and use small slopes to test the recent storm snow. Also keep in mind that in non-wind loaded areas, storm layers are still settling and stabilizing. Even if you don’t see evidence of wind-drifted snow, test the upper snowpack and gather information before committing to steeper terrain.
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: Possible
Expected Size: 1 - 1
Loose Wet
Clear skies in the morning will allow strong solar radiation to rapidly weaken snow surfaces. It may not take much time before the March sunshine causes wet loose activity on solar aspects. Be especially careful of east and south aspects that are steep and rocky. Loose wet avalanches could even trigger slab avalanches as they run downslope, or entrain significant snow and pack quite a punch on their own. Even a small loose wet avalanche could have serious consequences if it were to knock you off your feet and carry you over rocks or into confined terrain. If you witness pinwheels or rollerballs, know that wet avalanches could follow soon after. Time your travel carefully, and stick to colder, shady slopes to avoid the problem.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1