Avalanche Forecast

Amidst a very major atmospheric river event, Mt. Baker ski area (in the adjacent zone) reported natural, human, and explosive triggered avalanches up to size D2 on Friday. Visibility was limited due to stormy conditions but many avalanches were reported. The largest of these avalanches failed at the base of the new snow. Recent avalanches are the clearest sign of unstable snow and that you can trigger an avalanche. 

Large natural avalanche in Marthas Ladder in the Baker backcountry. NE Aspect, 4900’ Photo: Andrew Kiefer

The snow line hovered around 3500 ft for the new snow at Mt. Baker. We think that snow line should be in the 4000-5000 ft range further south but will be lowering Friday night and Saturday, moving the storm snow instabilities down the hill and reducing wet avalanche concerns. The largest avalanches are likely to occur above the rain-snow line. Due to the historic amounts of precipitation received during this storm - 4.5" for Mt. Baker and 5" for both Stevens and Snoqualmie Passes, there is a slight chance of very large avalanches, so travel in the runout of any large avalanche path is not recommended on Saturday, at any elevation.

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