Avalanche Forecast
Issued: Dec 20th, 2019 11:31PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs.
NWAC,
Northwest Avalanche Center
Updated 0730 to Warning: Very dangerous avalanche conditions should continue Saturday in the mountains. Avoid traveling in any location where an avalanche could start, run, or stop. Avalanches can become large and travel to lower elevations.
Summary
Discussion
Natural, human, and explosive triggered avalanches up to size D2 were reported on Friday from the Mt Baker backcountry and ski area. Visibility was limited due to stormy conditions but despite this limitation, several avalanches were still observed. The largest of these events failed at the base of the new snow. Recent avalanches are the clearest sign unstable snow exists and that you can trigger an avalanche.
Large natural avalanche in Marthas Ladder in the Baker backcountry. NE 4900’ Photo: Andrew Kiefer
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
Updated to HIGH all elevation bands 0730 for heavier snowfall Friday night and early Saturday morning.
Recent substantial snowfall, heavy precipitation rates, strong winds, and fluctuating freezing levels create the perfect recipe for avalanches Saturday. Avoid traveling on or under avalanche terrain. Be aware of steep slopes above you. Avalanches could grow large, step down into older snow layers, and travel to lower elevations. The storm should wane during the day Saturday, but the avalanche danger will remain high. Don’t overlook the massive volume of snow that impacted the mountains. Major events like this should give us pause, especially when they occur on a thin early season snowpack.
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.
Likelihood
Expected Size
Valid until: Dec 21st, 2019 11:31PM
