Avalanche Forecast
Regions: Cascades - North West.
Another wet and windy storm will create dangerous avalanche conditions at upper elevations on Tuesday. You can trigger large avalanches in wind drifted snow on slopes 35 degrees and steeper. Keep wet loose avalanches in mind as rain will continue to weaken snow surfaces below treeline.
Discussion
A few inches of new snow and southerly winds were enough to create fresh slabs up to 6in deep that were reactive to ski cuts in steep leeward terrain on Monday. The next system will bring more snow, rain, and wind over the next 24 hours with the bulk of precipitation expected overnight Monday. Incremental loading and strong SW winds will thicken and densify existing wind slabs, creating dangerous avalanche conditions above treeline. Rain overnight may drive small wet loose avalanche activity at lower elevations.
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
More snow and wind will build fresh slabs and continue to thicken existing slabs in leeward terrain. Avalanches will become larger and easier to trigger as you climb up elevation. 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. Use small slopes to test new snow. Avoid wind loaded slopes greater than 35 degrees at upper elevations, especially convex rollovers and slopes below ridgelines. Reduce your risk by choosing lower angle terrain or slopes sheltered from the wind.
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
Loose Wet
Snow levels should peak overnight at 5000ft, and gradually lower to 3500ft by Tuesday. The wet loose hazard should be the greatest overnight and in the early morning hours, before tapering off during the day. Rollerballs and pinwheels are good indicators of weak snow surfaces and that loose wet avalanches could follow. 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. Don’t linger on or under slopes 35 degrees and steeper if you find moist and sticky snow surfaces.
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.
Elevations: Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1