Avalanche Forecast
Regions: Cascades - South West.
Carefully evaluate steep slopes still showing signs of firmer wind transported snow from early this week. Snowfall should be light by the end of the daylight hours on Friday and not cause a great change in conditions. But use more caution if snowfall is greater than expected.
Detailed Forecast
A weak cold front will move northwest to southeast over the Olympics and Cascades Friday afternoon and night. This should cause increasing northwest winds Friday in most areas. Light rain if any should quickly change to light snow due to a rapid cooling trend. The main light snow should be seen in the Olympics and north Cascades Friday afternoon spreading to the south Cascades Friday evening with little making it to Mt Hood Friday night.
The light amounts of snow in the Olympics and Washington Cascades should not greatly change snow conditions. Any new storm slab should be mainly in the Olympics and in the north Cascades, small and shallow, and will be rated as unlikely.
Watch for lingering wind slab from early in the week where the northeast winds redistributed snow mainly above and near treeline.
Sun crusts are also likely on steep south slopes where sun warmed snow from Thursday refreezes on Friday.
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.The least fell at Snoqualmie Pass due to initial very wet snowfall. Rapid loading caused storm slab during and after this storm which is expected to have since stabilized.
Strong northeast winds hit at the tail end of the storm Monday and Tuesday. Regionally this transported snow and formed some local wind slab on exposed slopes and near ridges but this layer in most areas bonded well and lacked 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 tilted inland yesterday and today producing sunny weather with warming at higher elevations. But Arctic air at the surface east of the crest and offshore surface flow is still producing cold temperatures at lower elevations in the Cascade passes and east of the crest Thursday.
Wednesday and Thursday right side up powder is providing great conditions on sheltered shaded slopes. The warming and sunshine has been causing some snowballing and small wet loose avalanches on steep slopes facing the sun.
A generally strong mid and lower snowpack is expected to be preventing deeper instabilities. 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 west of the crest.
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.
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