Avalanche Forecast
Regions: Olympics.
The Bottom Line: Dangerous snow conditions are expected in parts of the Washington Cascades on Friday.
Detailed Forecast
Light snow showers should end Friday morning. Partial clearing should be seen Friday afternoon and night with continued cool temperatures.
There should be little change in snow conditions on Friday.
The top concern at Stevens and Snoqualmie and east of the crest will remain the persistent buried hoar frost slab layer. This layer will be found anywhere from a couple inches to a couple feet under the recent storm snow. Shooting cracks and whoomping are the strongest red flag observations for this persistent layer. Remember that this is also expected on small open slopes below treeline in the Stevens and Snoqualmie areas and east of the crest.
New wind slab is likely to remain on lee slopes above tree line on Friday. Watch for signs of wind transported snow above treeline. Temporary storm slab may survive on Friday on sheltered slopes near and below tree line on Friday. These are the more likely concerns in the Olympics, west of the crest and on the volcanoes.
Snowpack Discussion
Heads up folks because the avalanche picture has greatly changed in the Washington Cascades from the past couple weeks.
Dangerous persistent large hoar frost that formed during the latter part of the recent 2 week fair weather period has been buried in the Washington Cascades mainly at Stevens and Snoqualmie and east of the crest. By the end of Thursday this persistent layer will be found in these areas under anywhere from a couple inches to a couple feet of the recent storm snow.
Surface hoar layer with thin rimed crust before it was buried near Rainy Pass, about 4600 feet, 28 January, Photo by NWAC Observers Jeff Ward and Dallas Glass.
NWAC observers at Stevens and Snoqualmie today report snowpack whoomping, easy sudden collapse pit tests and Extended Column tests indicating propagation due to the persistent buried hoar frost layer including on your favorite go to slopes for such conditions - open low angle slopes below treeline. Here is a link to a video made today near Stevens Pass of tests of the buried surface hoar layer.
https://www.youtube.com/watch?v=IsYF79R6Rio
However a buried hoar frost layer is considered unlikely in the Olympics, west of the crest and on the volcanoes due to rain in those areas at the start of the recent storm. But it will be listed in these areas as an unlikely concern until there is confirmation one way or the other.
The recent storm is also expected to have built new wind slab on lee slopes above tree line. NWAC observers today also report new temporary storm slab on sheltered slopes near and below tree line in the Stevens and Snoqualmie areas. These should be the most important concerns in the Olympics, west of the crest and on the volcanoes.
At least the mid and lower snowpack consists mainly of crusts and melt form crystal layers. The low snow amounts on many south slopes and at low elevations and ample terrain and vegetation anchors are limiting the avalanche danger in those areas.
Avalanche Problems
Wind Slabs
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: North, North East, East, South East.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Storm Slabs
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.
Elevations: Treeline, Below Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Elevations: Treeline, Below Treeline.
Likelihood: Unlikely
Expected Size: 1 - 1