Avalanche Forecast

Issued: Feb 29th, 2020 10:00AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Loose Wet and Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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A big change in the weather Sunday could create dangerous avalanche conditions, especially in areas that received the most snow in the recent storm. Watch for changing conditions on slopes receiving direct sunshine. You could see large natural avalanches as the snow sheds from steep sunny slopes

Summary

Discussion

It's March and the sun is strong. Mostly clear skies should allow the sun to quickly impact the snow surface. Don’t expect slopes you cross early in the morning to feel and react the same later in the day. For most locations in the West South, the sun will be a big change to the cold dry storm snow. A few areas Saturday saw brief sun breaks. This will soften the blow of the sunny for these locations. 

Storm totals as of Saturday afternoon range from 8-18”, with the highest accumulations on the volcanoes. The new snow fell on a variety of snow surfaces including firm crust, settled powder, and weak facets/surface hoar. Our primary avalanche concerns involve the new snow and how it’s bonding to this old surface.

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. 

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. 

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. 

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. 

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. 

Problems

Loose Wet

An icon showing Loose Wet

Sunday will be a battle between cool temperatures and a strong sun. We think the sun will win and cause the snow surface to become wet and unstable. If the sun is out, expect snow to shed from rocks and trees causing new rollerballs and loose wet avalanches. That will be your clue to steer around similar terrain. In areas that received more snow, you could see some of these natural avalanches grow large as they run on a recent crust and even trigger slabs at the base of the new snow. Don’t forget about the slopes above you. If they are in the sun, they could avalanche. 

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

Likely

Expected Size

1 - 1

Storm Slabs

An icon showing Storm Slabs

Even though the sun should be out, the storm just ended. Give the new snow time to heal by steering around areas where you are more likely to trigger a storm slab avalanche. Avoid convex rollovers, very steep rocky slopes, and areas of wind affected snow. You can use small slopes and hand pits to see how the snow may react and how it is bonding to the old interface. Surface hoar and facets were reported prior to the storm on NW-N-E aspects in the near treeline band. If an avalanche fails on these grains it could break widely or trigger remotely.

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

Valid until: Mar 1st, 2020 10:00AM