Avalanche Forecast
Regions: Cascades - North East.
Another warm day with sun will drive wet avalanches on steep sun-exposed slopes. Large avalanches may be possible in isolated areas where weak, old snow existed prior to the recent storm. Steer around thick pillowed slabs on steep slopes at middle and upper elevations.
Discussion
Expect warming temperatures and more sun on Friday ahead of the next storm which will move in Friday night.
On Thursday, observers reported a large avalanche in recent snow on a steep, unsupported and sunn-exposed rock slab. Most other recently reported avalanches likely ran on the 23rd or 24th in recent storm snow on wind-loaded, leeward slopes. Some observations suggest that the February 23rd interface may be a problem for human triggering a bit longer than normal. The recent storm snow is resting on a variety of interfaces including a crust, small facets, and surface hoar in isolated locations. Surface hoar most likely to be a problem in somewhat sheltered, shaded pockets and groves from 5,000ft to 6,500ft in elevation.
To the south, a notable natural avalanche was reported by Stevens Pass Patrol To have run either late on the 24th or early on the 25th on a southeast aspect at 5,700ft. The slope had previously been ski cut and bombed for avalanche mitigation on the morning of the 24th with no results. The avalanche was over 2 feet deep, and it ran on a crust-facet combination at the February 22nd interface.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. 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.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
Loose Wet
Friday will be another warm and sunny day. Watch for loose wet avalanches on steep sun-exposed slopes. Roller-balls and wet surface snow are indicators that you could see wet avalanches. Don't linger on or under slopes over 35 degrees if the snow is becoming wet and slushy. The sun will start warming east aspects early in the day and will move to west aspects by afternoon. Areas near exposed rocks may melt quicker than other locations. A good rule of thumb for sunny slopes, is to avoid them once wet snow is unsupportive enough to up to your boot tops.
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
Wind Slabs
Steer around features that are drifted, or hold deep pillows of snow. Strong winds formed stiff slabs on leeward slopes early in the week. The February 23rd interface, below the recent slabs, may keep it possible to trigger avalanches for a bit longer than usual.
On some slopes, the recent snow may be resting on weak, faceted grains and may not be well bonded. Look for signs of instability such as shooting cracks, whumphing collapses, and recent avalanche activity. Stiffer slabs could be triggered from lower on the slope than expected. Surface hoar is buried and preserved in isolated locations. It is most likely to be a problem in somewhat sheltered, shaded pockets from 4,500ft to 6,500ft in elevation.
Large cornices exist on many ridges. Avoid standing on or under cornices that loom over leeward slopes. Cornices may fall during this period of warm temperatures, especially where they receive direct sun.
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