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Avalanche Forecast

Archived

Mar 8th, 2020–Mar 9th, 2020

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.

Regions

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Stong solar radiation on Monday will create wet loose avalanches on steep, sun-exposed slopes. If you notice snow surfaces becoming wet and weak, move to shaded or lower angled terrain. Use caution around open slopes greater than 35 degrees at upper elevations where you may still be able to trigger an avalanche within the recent storm snow.

Discussion

Several slab avalanches were triggered on Saturday and Sunday in the West North zone (observation, observation). Most avalanches failed on density breaks within recent storm snow, and a couple broke deeper on the old/new snow interface buried Thursday. The majority of avalanches were triggered on convex slopes steeper than 35 degrees in northerly, upper elevation terrain. Several loose wet avalanches also ran naturally late in the day on south and west aspects. 

Monday will bring the first full day of intense sunshine following several days of stormy weather and mostly cloudy skies. Temperatures should remain cool, but snow surfaces will rapidly lose strength on sunny slopes, and widespread wet loose avalanche activity is expected. Large cornices also loom on most high ridges. Use caution to not travel on or underneath overhung cornices.

Snowboarder triggered avalanche (D1.5) on Table Mountain (NE, 5400ft). 03/08/20. Photo: Zack McGill

Snowpack Discussion

March 5, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

Ridgetop wind transport near Washington Pass. 3/4/20 Photo: Josh Hirshberg

 

The beginning of March brought the first spring-like weather to the region. The days are getting longer and the sun (when it shines) has a noticeable effect on the snow. Alternating sun breaks and snowstorms is the theme of the week and the avalanche danger fluctuated just as fast as the weather. The southern zones and the east slopes have seen periods of light snowfall and significant sunshine while the northern zones and west slopes have had significant snowfall and occasional sun breaks. Above freezing temperatures were common at lower elevations throughout the region. 

Last week in review:  Temperatures were above freezing at most trailhead locations and concerns about the previous week’s buried surface hoar and weak layers were fading. Saturday temperatures plummeted as strong winds and new snow built wind slabs in most areas and drove the avalanche danger to considerable in almost every zone. Sunday’s calm weather and clearing in most areas helped stabilize wind slabs, but our attention turned to the strength of the sun and daytime warming. Lingering wind slabs and loose wet avalanches were forecast in every zone. For the remainder of the week, the avalanche problems in every zone were confined to the recent snow and each brief storm cycle had us thinking about wind slabs at upper elevations and loose wet avalanches from daytime warming and sunshine.

Spring isn’t actually here -- another round of cold temps and snow showers are on the way this weekend, but this past week’s oscillating weather is a good reminder to begin to shift our mindset. The sun is gaining strength, the days are getting longer, and we begin daylight savings this week. Even a brief period of sunshine can dramatically alter fresh, new snow. Check the mountain weather and avalanche forecasts for the most up to date info, enjoy the backcountry and let us know what you see out there!

--Peter Moore

Wet loose avalanche debris near Snow Lake. 3/5/20 Photo: Dallas Glass

Fresh cornice development from strong winds in the Stevens Pass backcountry. 3/4/20 Photo: Tom Whipple

Problems

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.