Avalanche Forecast
Regions: Stevens Pass.
There should be a big difference in the weather and avalanche conditions between the north and south Cascades on Monday. The most dangerous conditions should be seen in the central to south Cascades.
Detailed Forecast
A strong low pressure system will move west to east across Washington on Monday. There should be a big difference in the weather and avalanche conditions between the north and south Cascades with the strongest winds and heaviest snowfall in the south Cascades. Right now it looks like the main effects of the low may reach as far north as the central Cascades. But a different track of the low could bring much different than expected weather and avalanche conditions on Monday so the forecast is somewhat uncertain.
Light to moderate winds and snow are expected in the north Cascades on Monday. Stronger shifting winds, moderate to heavy snow and rapid loading are expected in the central to south Cascades. New storm slab and wind slab potential will be greatest in the central to south Cascades. Back country travel is not recommended above treeline in the central and south Cascades on Monday. Dangerous avalanche conditions are expected in the other areas along the west slopes.
There was a snow immersion fatality in a tree well in bounds at Snoqualmie on Saturday so always ski with a partner. Terrain anchors are still causing some anchoring at the lowest elevations. Use caution near creeks which are still open in some areas at the lowest elevations.
Snowpack Discussion
A series of cool storms has deposited increasing storm snow at relatively low freezing levels since early December. Some rain made it up to about 4500 feet in the north and about 5500 feet in the south along the west slopes on about December 17th. There has been about 1.5-2.5 feet of snowfall at sites along the west slopes the past 2-4 days.
Widespread sensitive storm slab was reported by the Baker, Stevens and Alpental pro patrols on Friday. Slab were mostly in the 1 foot range with some to 2 feet at Alpental when the wind shifted to west and there was warming there.
Recent storm slab layers stabilized somewhat on Saturday. NWAC pro observer Lee Lazzara was at Mt Baker and found some density inversions in the recent storm snow mainly at about 30-40 cm but tests showed little propensity for propagation. He noted a couple recent small storm slab avalanches. NWAC pro observer Ian Nicholson was at Paradise on Saturday and he found progressive collapses at about 13 cm below the rain crust from December 17th and 18th also showing little propensity for propagation. He also noted a couple recent small to large storm slab avalanches.
A back country skier also triggered a 15 inch storm slab that ran about 500 feet on Skyline Ridge at Stevens Pass on Saturday on a northwest slope at about 5400 feet.
The pro patrols at Stevens Pass and Alpental reported reported sensitive small storm slab and loose dry avalanches on Sunday.
Avalanche Problems
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Very Likely
Expected Size: 1 - 1
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, North West.
Elevations: Alpine, Treeline.
Likelihood: Very Likely
Expected Size: 1 - 1