Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - North West Avalanche Forecast

Dec 21st, 2019 10:02AM

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

The storm is ending, but human-triggered avalanches remain likely. Give the snowpack time to adjust to the massive load of new snow. Choose conservative terrain, and avoid open slopes steeper than 35 degrees.

Summary

Discussion

Since December 18th, the Mt. Baker area received over 7in of water, translating to 36-48in+ of new snow above 4000ft. This potent storm brought impressive snowfall rates (2in/hour) and doubled the height of snow, which now ranges from 60-80in between 4000-5000ft. 

Numerous natural, human, and explosive triggered avalanches up to size D3 (large enough to destroy a house) were reported on Friday and Saturday. Most notable was a very large natural avalanche on Shuksan Arm that likely occurred on Friday (photo below). The majority of reported avalanches occurred near and above treeline on convex slopes steeper than 35 degrees, and failed 1-2ft deep within new snow layers. Some failed 2-3ft deep on the old/new snow interface, and a handful may have stepped down on deeper layers in the snowpack. These recent avalanches are the clearest sign unstable snow exists, and that you can still trigger a very large and dangerous avalanche. 

Very large natural avalanche (D3) on a north aspect at 5400ft on Shuksan Arm. 12/21/19. Photo: Adam U

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

An icon showing Storm Slabs

Likelihood

likely

Expected Size

1 - 2

Human-triggered avalanches are likely near and above treeline, especially on convex slopes 35 degrees and steeper. Although natural avalanche activity likely peaked already, fresh storm slabs are still primed for human-triggers. Avalanches could grow very large, and even step down on older snow layers. Don’t overlook the huge volume of snow we just received. Major storm events like this should give us pause, especially when they occur on a thin early season snowpack. The storm snow needs time to stabilize. Seek out supported, lower-angled slopes away from overhead hazard.

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.

Valid until: Dec 22nd, 2019 10:02AM