Avalanche Forecast
Regions: Olympics.
Coming out of a big storm cycle with limited observations, there's high uncertainty about how quickly avalanche conditions will improve. Make your own observations, watching for new or recent avalanche activity and instability markers like cracking in the snow as signs that dangerous avalanche conditions are still present. Give the snow time to stabilize and ease into the terrain.
Discussion
The snow finally tapered off Saturday morning over the Olympics but not before dumping another round of significant snow Friday night. Hurricane Ridge recorded a whopping 33†of snow over the last 2 days with 5.00†of water since the beginning of the event. Above 4000-4500’ this has filled in what had been a meager snowpack. Below 4000’ a thin snow pack still exists.Â
At least 2 natural storm slab avalanches released along the Hurricane Ridge Road early Saturday morning on E-SE aspects respectively and another slab released naturally during the day Saturday. NPS observations were limited due to the heavy snowfall and road clearing efforts. Notable layers we were tracking Thursday before the storm cycle:
(1) On northerly aspects, a layer of 3-4 mm well-preserved surface hoar
(2) On southerly aspects, a strong 5 cm crustÂ
While the storm snow weaknesses stabilize and the snowpack strengthens, keep in mind we don’t know how well the new/old snow interfaces have bonded. A small avalanche could potentially step down to a deeper layer for a much larger and more dangerous avalanche.Â
Below treeline, low snow means that rocks, stumps, creeks and other hazards are still present, so travel carefully as you drop down in elevation.Â
12-21-19: Natural slab avalanche that released during the Saturday at 3700'. Great example of a slab failing on a steep unsupported slope. Photo NPSÂ
Forecast Schedule
For the 2019-20 winter season, avalanche danger ratings will be issued for the Olympics every Friday through Sunday and during mid-week holidays. During the week, No Rating will be issued but forecasts will include expected conditions and relevant travel advice. If you are out in the Olympics, share your backcountry observations with us and the greater community.
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
Nearly 3 feet of snow fell over the last two days and we have a limited number of direct observations. After a storm cycle like this, you need to buffer in a wider margin of safety to account for the uncertainty.
Avoid unsupported steep slopes where you are more likely to trigger a storm slab. New or recent avalanches or cracking in the new snow are clear signs of lingering instability.
Dig down to the pre-storm interfaces (listed above) to see if they are still present. You can use small test slopes before working your way into bigger terrain.
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: Likely
Expected Size: 1 - 1