Avalanche Forecast
Regions: Stevens Pass.
Previous wind and storm slab layers from the weekend should be the main concerns on Wednesday. Greater caution would be necessary if the weak front Tuesday night and Wednesday morning brings more snow than expected.
Detailed Forecast
A weak cold front will cross the Northwest Tuesday night and Wednesday morning. This may bring some very light amounts of new snow but should not greatly change conditions. Some fair weather should be seen by Wednesday afternoon. Light or moderate northwest winds and cool temperatures should also continue on Wednesday.
Previous wind slab layers from the weekend will possibly linger on previous lee mainly north to east slopes near ridges and summits on Wednesday. Storm slab from the weekend should become unlikely by Wednesday but would be found in what were calmer areas if it still exists.
Use more caution and watch for new small wind slab or small storm slab if the weak front Tuesday night and Wednesday morning brings more snow than expected. This is mainly possible in the Snoqualmie Pass area.
Terrain anchors are still causing significant anchoring at the lowest elevations. Use caution near creeks which are still open in some areas.
Snowpack Discussion
An atmospheric river caused warm very wet weather last week. About 5-8.5 inches of rain was seen along the west slopes. This caused an avalanche cycle, consolidation and stabilizing of the older part of the snowpack along the west slopes and is expected to have eliminated persistent weak layers at Stevens and Snoqualmie.
A stormy pattern with cooling was seen late last week and over the weekend with about 1.5-4 feet of snowfall along the west slopes.
Reports varied on Sunday. The Mt Baker and Alpental pro patrols reported widespread avalanches released from explosives ranging from 1-2 feet and running good distances, releasing on storm snow interfaces. The Stevens Pass pro patrol reported morning control produced widespread soft slab results from both explosive and ski cuts with best results on NW-SE facing terrain with slides releasing on the most recent storm interface with Friday's storm snow. Across the street at Skyline Ridge at Stevens Pass NWAC pro observers found about 30-60 cm of mostly stable, mostly right side up storm snow, well bonded to Tuesday's rain crust. One 30-40 cm storm slab was triggered on a 40 degree slope which ran on the crust from early last week.
Cool weather with little new snow has been seen Monday and Tuesday. This will have mostly caused some consolidation and further stabilizing.
We do have one more report via the NWAC Observations page of a 25 cm slab that was triggered on a steep north-northeast facing slope at Stevens Pass on Monday. The NWAC pro observers had a field training Monday and Tuesday at Stevens Pass and found a graupel or stellar layer at 35-40 cm but tests did not indicate propagation. Rain from last week had percolated well down into the snowpack.
Â
Â
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: 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