Avalanche Forecast
Regions: Stevens Pass.
Expect likely new wind and storm slab layers on Thursday. Wet loose avalanches should also be possible in areas that receive significant new snow.
Detailed Forecast
Decreasing west northwest winds, with increasing light snow showers at cooler temperatures and low snow levels should be seen on Thursday.
New wind slab should be most likely on lee north to east slopes near and above treeline. Watch for cracking or firmer wind transported snow.
New shorter lived storm slab should also be likely on a wider variety of slopes mainly near and above treeline. This will be most likely in areas that receive an inch of snow of more an hour for several hours.
The equinox is near and the sun is gaining power. Despite the cool temperatures and low snow levels wet loose avalanches seem possible in all the elevations bands. Watch for wet surface snow deeper than a few inches, pinwheels, and natural wet loose releases that usually precede more major wet loose snow avalanches.
The cooling trend should also build favorable stable new snow layers in many sheltered shaded areas that receive light amounts of new snowfall.
Snowpack Discussion
Past Weather and Avalanches
A cycle of heavy snow and rain and locally deep avalanches was seen in early March. Deep natural and explosively released wet slab avalanches were reported throughout the Cascades near and west of the crest. The High Campbell chair was destroyed by an explosively released deep wet slab avalanche at Crystal Mountain on 10 March.
Deep wet slab release from early last week, photo from 11 March near The Tooth in the Snoqualmie backcountry by Andy Hill.Â
Recent Weather and Avalanches
We are shifting avalanche concerns to more recent storm related layers and are moving away from deep snowpack concerns at least for the short term.
The last stalled and moist frontal boundary draped mainly over the Olympics and north Cascades Saturday afternoon finally sagged south on Sunday, with rain generally changing to snow in a north to south fashion across the area.  A favorable temperature trend with gradual cooling occurred with this storm. NWAC sites along the west slopes picked up 5-20 inches of snow late Saturday through Monday with more likely at higher elevations that cooled sooner. Some storm and wind slab avalanches were seen following the last front for example at Mt Baker.
A natural storm slab on a west aspect near treeline at Mt Baker on 17 March by NWAC observer Jeff Hambelton.
But areas of without extensive instability were also seen due to the cooling trend, favorable profiles and short lived storm layers. The new snow was generally reported to be over wet snow in the upper snow pack.
Little avalanche activity was reported in the Cascades on Tuesday, mainly small wet loose avalanches such as from NWAC observer Dallas Glass at Stevens and the Alpental ski patrol.
The next cold front is crossing the Northwest on Wednesday. This is causing increasing winds, rain and snow and a cooling trend on Wednesday. It looks like sites near and west of the crest should have a few inches to over a foot of new snow on Thursday morning.
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.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Loose Wet
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. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. 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.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: South East, South, South West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1