Avalanche Forecast
Issued: Dec 19th, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Heavy snow, strong winds, and rising snow levels drive very dangerous avalanche conditions. Avoid all avalanche terrain Thursday night and Friday. Be prepared for the very dangerous conditions to remain with us into Saturday.
Summary
Discussion
A potent atmospheric river event aimed at Washington State quickly raises the avalanche danger in the West North zone on Thursday night. On Friday, avoid all slopes where avalanches may start, stop, or run.
A handful of avalanches over the past week on north and east aspects above 5000ft, on rocky, frequently unsupported/convex slopes steeper than 35 degrees. A skier triggered an avalanche in Mazama Bowl on Sunday and was carried downslope, partially buried, but uninjured. One observer in the Cascade River Road area reported evidence of a large natural avalanche that likely occurred late last week (observation).
This weaker layer (potentially down 3-4 ft) on Friday may be the culprit for the largest avalanches on Friday.
Snowpack Discussion
December 19th, 2019 (The regional synopsis is updated every Thursday @ 6 pm)
Let’s take a moment to recap what happened over the past week:
A potent winter storm arrived on the 12th, adding to the very shallow snowpack throughout the region. Anywhere from 6-36” of snow fell between the 11th and 15th. Places like Mt Baker, Paradise, Mt Hood, and White Pass were the winners regarding snowfall, with quite a bit less for areas further east of the crest like Mission Ridge, Blewett Pass, and Washington Pass.
The most notable avalanche activity occurred in the Mt. Baker backcountry where numerous human triggered storm slab avalanches occurred (with several big enough to injure, bury, or kill a person). The majority of these occurred on upper elevation, north and east facing terrain. Besides the danger posed by the size of the avalanche, many slides quickly revealed rocks and other obstacles barely hidden below the snow surface.
The storm layer took time bond with the underlying snow surface, and we saw the likelihood of triggering slab avalanches slowly decrease over a handful of days. The avalanche danger was at Considerable for many places on Saturday the 14th, then tapered to Moderate on Sunday, and eventually reached Low in many places by Tuesday and Wednesday.
A large, human triggered avalanche on the old snow interface. East aspect of Table Mountain (West-North Zone) at 5,500ft. December 15,2019. Photo by Brooks Broom.
Backcountry travelers have noted generally thin and shallow snowpacks, with a range of structures across the region. Many folks were skeptical of the layering they found. People were able to dig down and observe weak layers in many areas. These were buried in late November and early December. In some places they presented as weak snow over a crust, in others, a layer of feather-like surface hoar. Cold temperatures have likely preserved many of these layers, and will be worth considering as the snow piles up.
Below is a list of commonly visited locations and their snowpack depths in inches (as of December 19th at 4am). Check the weather station hourly data feed and watch as the subtropical javelin of moisture (also known as a strong atmospheric river) changes conditions dramatically over the next few days.
Total Snow Depth in Inches as of December 19th at 04:00 AM.
-Matt Primomo
Problems
Storm Slabs
As mother nature continues to unload on the West North area, storm slabs will become large or even very large. Natural avalanches will become likely and human-triggered avalanches very likely, making avalanche terrain unsafe for travel. We are primarily concerned with instabilities forming within the new snow from Thursday night into Friday, at interfaces with 5” of colder snow that fell Wednesday night, or on a smooth crust or wind-stripped surface pre-dating Wednesday night’s snowfall.
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: All elevations.
Likelihood
Expected Size
Loose Wet
As warmer air moves into the West North zone overnight and into Friday morning, expect a cycle of natural wet snow avalanches and with the amount of new snow, these avalanches could be large. Don’t assume that rain means it’s safe to travel in the backcountry.
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
Expected Size
Valid until: Dec 20th, 2019 10:00AM
