Avalanche Forecast
Issued: Dec 21st, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions are lingering after a major winter storm hit the region Thursday night into Saturday. Use caution on slopes 35 degrees and steeper and make conservative terrain choices. The western half of the zone, near the Cascade Crest, holds the most dangerous conditions
Summary
Discussion
Observers reported a cycle of large, natural avalanches on Friday and Saturday. The Holden Mine reported avalanches that could easily bury or kill a person (up to size D2.5), including avalanches that may have been triggered remotely. The avalanche activity and snowfall were more significant in the western half of the zone and closer to the Cascade Crest. Avalanche observations have been limited, so far. Storm totals are: Sasse Ridge Snotel- 3” water equivalent, Holden Village- 32.5” in 24hrs Thursday to Friday (2nd highest on record since 1962), Trinity Mtn Snotel- 47” snow with 6.7” water equivalent, and Mission Ridge- 5” snow. Rain fell on Friday and Saturday at many locations.
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
Use caution on slopes 35 degrees and steeper. Watch out for steep convexities and unsupported slopes where you could trigger avalanches more easily. Avalanches in the recent snow could be big enough to kill you. Expect higher storm totals and possibly bigger avalanches at upper elevations. Lower and middle elevation slopes saw rain and have moist, heavy snow near the surface. This "upside-down" snowpack structure is a perfect recipe for triggering avalanches.
The recent snow still needs time to bond and stabilize. There’s uncertainty about how this large and rapid load has affected the snowpack. This also concerns for a few older weak layers deeper into the old snowpack. Take time to make observations. Use snowpack tests and small test slopes to asses the bonding of the snow within storm layers, as well as, older layers.
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 22nd, 2019 10:00AM
