Avalanche Forecast
Regions: Cascades - East.
Expect very dangerous avalanche conditions near the Cascade Crest and western portion of the zone after a potent storm delivered a rapid load of heavy snow and rain. You can trigger avalanches big enough to kill you. Avoid avalanche terrain at upper elevations and use extra caution in any area where avalanches could come from above.
Discussion
Observers reported a cycle of large, natural avalanches resulting from heavy precipitation Thursday night into Friday. The Holden Mine reported avalanches that could easily bury or kill a person (up to size D2.5). The avalanche activity and snowfall were more significant in the western half of the zone and closer to the Cascade Crest. Storm totals so far: 10†snow with 2.1†water equivalent at Sasse Ridge Snotel by Friday AM, 32.5†in less than 24 hours at Holden Village (2nd highest on record since 1962) by Friday AM, 47†of snow with 5.2†water equivalent at Trinity Mtn Snotel as of 5p Friday, and 4†at Mission Ridge.
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
Storm Slabs
Avoid avalanche terrain above treeline and be especially cautious in the runout or track of avalanche paths at all elevations. Avalanches could run into lower elevation terrain from above. Use caution on all slopes 35 degrees and steeper. Watch out for steep convexities and unsupported slopes where you could trigger avalanches more easily.
The recent snow still needs time to bond and stabilize. You could trigger avalanches big enough to kill you within the storm snow or even on weak layers deeper into the old snow. Expect more snow, bigger avalanches, and possibly wind drifting at upper elevations. Lower and middle elevation slopes saw rain and likely have some wet snow under the most recent accumulation.
There’s still a lot of uncertainty about how this very rapid load has affected the snowpack. Take time to make your own 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: Very Likely
Expected Size: 1 - 1