Avalanche Forecast

Issued: Dec 22nd, 2019 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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Human-triggered avalanches are likely on slopes steeper than 35 degrees. Travel with a partner, keep them in sight, and be conservative with your terrain choices. The snowpack is still adjusting to a large amount of new snow produced during the storm and needs time to stabilize.

Summary

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

Since December 18th, over 7 inches of water has fallen in the Baker area. Don’t overlook the huge volume of snow we just received. Human-triggered avalanches are likely near and above treeline, especially on convex slopes 35 degrees and steeper. Numerous natural, human, and explosive triggered avalanches up to size D3 were reported over the past few days, including a very large avalanche on Shuksan Arm Friday afternoon. Storm slabs have been failing between 1-3 feet deep, with a handful possibly stepping down to deeper layers in the snowpack. Seek out supported, lower-angled slopes away from overhead hazard when traveling near and above treeline. Below treeline has decreased danger due to wet snow locking up as a result of cooler temperatures.

In addition, there are areas of deep, unconsolidated snow, so travel with a partner and keep them in sight. With deeper snow, snow immersion related accidents are common. You can find out more about snow immersion and safety here.

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

Likely

Expected Size

1 - 1

Valid until: Dec 23rd, 2019 10:00AM