Avalanche Forecast
Regions: Cascades - North West.
Watch mainly for lingering local wind slab from early in the week.
Detailed Forecast
An upper ridge will reassert itself over the US coastal waters on Saturday. Northwest flow and moisture should decrease a bit over the Olympics and Cascades. Clouds should decrease a bit during the day and lead to some partly sunny weather especially in the south and east of the crest.
Watch for lingering wind slab from early in the week where the northeast winds redistributed snow mainly near and above treeline.
Sun crusts, while not an avalanche danger on Saturday, are likely on steep south slopes where the sun warmed the snow on Wednesday and Thursday.
Right side up stable powder may also still be found on shaded sheltered slopes on Saturday.
Heads up in advance for Sunday when moderate to heavy snow with warming should bring a significantly increasing avalanche danger.
Snowpack Discussion
A storm system hit the Northwest last weekend with strong west to northwest winds. NWAC sites west of the crest picked up about 6-22 inches of storm snow with a good cooling trend. Strong northeast winds hit at the tail end of the storm Monday and Tuesday. This transported snow and formed some local wind slab on exposed slopes and near ridges. But in most areas this layer bonded well and lacked underlying weak layers.
Wind affected surface snow on Wednesday on a NWAC/WSDOT field trip to the weather station at Chinook Pass by Dennis D'Amico.
An upper ridge over the coastal waters tilted inland Wednesday and Thursday producing sunny weather and warm temperatures at higher elevations. This caused some snowballing and wet loose avalanches on steep slopes facing the sun.
A weak front is crossing the Northwest on Friday. This may cause some light snow and will bring cooling to most areas. Little if any snow will make it to the south such as Mt Hood. Sites in the lower Cascade passes warmed today when cold east winds changed to westerly. The minor new snow if any Friday should not cause significant new layers.
A generally strong mid and lower snowpack is expected west of the crest. There was one report of a persistent weak layer near an older crust at Stevens Pass a few days ago. But no known avalanche activity has occurred on this layer.
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: South East, South, South West, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Possible
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: Unlikely
Expected Size: 1 - 1