Avalanche Forecast
Regions: Cascades - North East.
Dangerous avalanche conditions are lingering after a major winter storm added 3 feet of snow and stressed weak, older snow. Use caution on and near slopes 30 degrees and steeper and make conservative terrain choices. Give the recent snow time to gain strength before venturing into steeper terrain.
Discussion
Washington Pass picked up over 3 feet of snow and 3.2" of water equivalent since Thursday afternoon. Observers have reported large natural avalanches in Silver Star creek, the Delancy Ridge area, on a northwest aspect of Silver Star Mountain, and a northeast aspect of Hinkhouse peak. Observers witnessed an avalanche run on Saturday around noon on a north aspect of Peak 6460ft near Silver Star creek. On Sunday, an observer near Vasiliki ridge reported collapses that covered areas as wide as 30 feet.
Snowpack Discussion
December 19th, 2019Â (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.Â
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.Â
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.Â
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.Â
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.Â
Total Snow Depth in Inches as of December 19th at 04:00 AM.
-Matt Primomo
Avalanche Problems
Persistent Slabs
You can trigger dangerous and surprising avalanches even from below or on adjacent slopes. Any triggered avalanche could be big enough to kill you and could break widely across the terrain. Use caution on or near slopes 30 degrees and steeper. Put a large buffer of space between you and any avalanche terrain. Watch out for rocky terrain, steep convexities, and unsupported slopes where you could trigger avalanches more easily.
Be observant by looking and listening for signs of avalanche danger. Recent avalanches, whumpfing collapses, and cracks shooting through the snow are signs that you could trigger a persistent slab avalanche. Take time to dig in the snow and look for older, weak layers. You can find a layer of facets that formed in November, often under a thin crust about 2.5-3 feet below the snow surface. You can use snowpack tests to help identify this layer.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1