Avalanche Forecast
Regions: Stevens Pass.
Detailed Forecast
The strong frontal system should continue to shift southward Thursday night spreading moderate to briefly heavy rain or snow at gradually cooling temperatures to most areas. Very strong crest level WSW winds should shift to WNW overnight and couple with snowfall at higher elevations to build unstable wind slab layers on lee slopes, mainly from North through Southeast facing slopes in the alpine elevations, causing an increasing avalanche danger Thursday night.
Rain and snow should turn to snow showers by early Friday with showers ending along with diminishing winds. This should begin refreezing old wet snow surfaces and also provide good bonding of new snow to reforming crust layers. This should lead to a gradually decreasing danger. However, previously formed wind slab should persist below ridges on steeper open lee slopes where dangerous avalanche conditions should remain.
Snowpack Discussion
The existing snowpack consists of a mix of crust layers and consolidated old snow layers. Â Recent, mostly dry periods as well as warm temperatures and periods of sunshine have all combined to form a mostly stable old snowpack void of distinctive weak layers per numerous recent snow pit and field observations.Â
Surface crusts range from firm and supportable on sun exposed slopes to breakable on shaded terrain.
Snowdepths remain well below normal, generally ranging from 1-2Â meters in the alpine regions to less than 1 meter near and below treeline with significantly lower snowdepths below about 3000 feet.Â
A strong frontal system began affecting the region Thursday causing moderate to heavy rain at relatively high freezing levels in the north and Olympics. This weather should be producing wet snow conditions near and below tree line and beginning to build wind slab on lee slopes above tree line.
The avalanche danger will continue to be low at the lower elevations due to the low snowpack.
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.
Likelihood: Possible
Expected Size: 1 - 1