Avalanche Forecast
Regions: Snoqualmie Pass.
Plan for changing conditions as a strong March sun causes new snow to shed from trees and rocks creating rollerballs and natural loose wet avalanches. If you travel to higher elevations or onto shaded aspects, don’t forget the Pass just picked up new snow. Steer around from convex rollovers, unsupported slopes, and terrain below cornices.
Discussion
A strong March sun could create quite an impact on the snowpack Sunday causing conditions to change quickly. A few rays of sunshine on Saturday at the Pass should soften the impact of this big weather change, but you’ll still want to monitor conditions. Expect slopes you cross in the morning to feel and possibly react differently later in the day. As of Saturday afternoon, storm totals near the Pass ranged from 6-10â€. This snow accumulated on a variety of old snow surfaces including firm crusts, old debris, settled powder, and weak surface hoar. Our primary avalanche concerns deal with this new snow and how it’s bonding to the old surfaces.
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
Loose Wet
If the sun is out, it shouldn’t take long for the snow surface to become wet and unstable. You’ll see snow shedding from rocks and trees causing rollerballs and loose wet avalanches. That’s your clue to give similar slopes a wide berth and seek out shaded aspects. These avalanches may run a firm crust formed this past week, allowing them to travel far and fast. Most of these avalanches should be small, however, in bigger, more complex terrain, you could see a few isolated large avalanches. Loose slides could even trigger slabs failing at the base of the new snow. Keep in mind what slopes are above your head. They could be experiencing the sun, even if you are traveling in the shade.
Just as the sun can cause an unstable snow surface, it could also create unstable cornices. Don’t linger under these looming beasts during periods of sunny weather and warming temperatures.
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
Storm Slabs
As you transition on to more shaded terrain, shift your attention to the recent snow and how it’s bonding to the old surface. Small slope tests and hand-pits can give you insight into how the new snow may react. Give the storm snow time to heal by steering around features in the terrain where you are more likely to trigger an avalanche. Avoid convex rollovers, very steep rocky slopes, and areas where the wind drifted into firmer slabs. In some locations, the new snow may sit over facets and/or surfaces hoar allowing avalanches to fracture widely or trigger remotely. If you see any evidence of these persistent weak snow grains seek out lower angled 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.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1