Avalanche Forecast
Regions: Cascades - South West.
Storm slab, wind slab, cornices and deep unconsolidated snow are all key features to plan to avoid on Thursday.
Detailed Forecast
A cool upper trough will continue to sink south over the Pacific Northwest on Thursday. West winds should begin to decrease Thursday initially in the Olympics and north Cascades. Snow showers Thursday along the west slopes should bring another few inches to the north and another 5-10 inches to the central and south.
Dangerous avalanche conditions should persist Thursday especially in the near and above treeline zones where new or recent storm and wind slab should be likely. Storm and wind slab avalanches should remain within recent storm layers but could move fast. Watch for evidence of wind deposited snow or pillows. Test for inverted strong over weak storm snow and give cornices a wide margin.
Loose dry avalanches in non-wind affected terrain will be difficult to manage on steep slopes. Even a small loose dry avalanche could have unintended consequences such as knocking you off your feet and into a terrain trap.
The recent unconsolidated storm snow is now very deep in many places!. There was a snow immersion fatality in a tree well in bounds at Snoqualmie on Saturday so always travel with a partner.
Snowpack Discussion
The Christmas snow globe keeps giving here in the Pacific Northwest! It's gotten very deep out there. By Thursday morning sites along the west slopes will have received about 5-9 FEET of snow in less than a week.
Recent pro back country or pro patrol observations indicate extensive avalanches mainly confined to recent storm snow.
A natural avalanche cycle was reported early Tuesday morning at Alpental, likely a response to heavy showers in the waning convergence zone over Snoqualmie Pass Tuesday morning. These storm slab avalanches were extensive ranging mostly from 8-12" but did not propagate greatly.
In the Crystal backcountry on Tuesday NWAC pro observer Dallas Glass found some snow pack whumping and numerous triggered storm slab and some loose dry releases. The storm slab was releasing at about 25 and 45 cm where there may have been storm cycle density changes. The greatest wind loading and cornice formation was noted on generally east facing terrain near ridges.Â
Crystal backcountry near Bullion Basin, a shallow 10" skier triggered soft slab, SW aspect, 5800 feet. Released near noon Tuesday, December 22. Photo: NWAC pro observer Dallas Glass   .
The Alpental pro patrol Wednesday reported widespread large 12-14 inch storm slabs from explosives and ski cuts. Up in the north at Mt Baker where there was less new snow the Mt Baker pro patrol reported triggered small loose dry avalanches.
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: 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.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1