Very dangerous avalanche conditions exist. Rising freezing levels, strong winds, and precipitation are a recipe for avalanches. This weather continues into Friday. Avoid travel in avalanche terrain.
Very strong winds and wet, heavy snow will build dense wind slabs above treeline. As temperatures rise Thursday night and Friday, large natural and human triggered avalanches are very likely.
Avoid travel on or below wind loaded slopes.
Heavy snow is turning to mixed snow and rain, or even just rain, below treeline on Thursday evening. Rain and mixed precipitation will continue Friday making wet avalanches likely. Avoid steep unsupported slopes, convex rollovers, and slopes above cliffs or leading into gullies.
It will be easy for you to trigger a wet avalanche. Even a small avalanche can be dangerous. Many obstacles such as rocks and stumps will be hidden by the new snow.
Around 8 inches of snow fell in the last 24 hours accompanied by strong winds. Freezing levels continue to rise and avalanche danger will peak Thursday night and continue into Friday. Strong winds, especially above treeline, will continue to build large and dangerous wind slabs. Rain and mixed precipitation below treeline will produce natural and human triggered wet avalanches.
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
