Avalanche Forecast
Regions: Cascades - North East.
Colder temperatures will help preserve wind slab instabilities near and above treeline. Small loose dry avalanches are likely on steeper slopes. Despite a cooling trend, recently formed storm slabs may still be sensitive Sunday. Enjoy the new snow but choose conservative terrain and allow storm related instabilities time to heal.
Detailed Forecast
More snow is on the way Saturday night and Sunday with a cooling trend forecast. W-SW transport winds should also decrease during the day. Â
Colder temperatures will help preserve wind slab instabilities near and above treeline. We've highlighted more traditional lee easterly aspects on the elevation/aspect diagram, but be aware of cross-loaded slopes and that easterly winds earlier in the week loaded westerly aspects. Feel for firmer wind transported snow as you climb higher in the terrain.Â
Despite a cooling trend, recently formed storm slabs may still be sensitive Sunday. While storm slabs are most likely to release within the most recent storm layers, sun crusts on solar aspects may be more sensitive to triggering. If precipitation rates become more intense than predicted in the southeast and central-east Cascades Sunday, be prepared for the possibility for new storm slab instabilities.Â
Natural or human triggered small loose dry avalanches are possible in steeper terrain. Avoid terrain traps where a small but fast running small loose avalanche could have unintended consequences.Â
Continue to dig snowpits identifying and testing for PWLs in the snowpack, especially in areas further east of the crest with a shallower snowpack and on northerly aspects in the Washington Pass zone below the 1/17 crust.Â
Enjoy the new snow but choose conservative terrain and allow storm related instabilities time to heal. Â
Snowpack Discussion
Weather and Snowpack
An atmospheric river arrived over the Northwest 1/17 forming a crust that is still referenced in observations regarding the persistent slab problem.Â
Strong NE-E winds have been seen Wednesday and Thursday with very cold temperatures. Fresh, touchy wind slabs formed with large plumes of snow visibly transporting loose snow from exposed terrain.Â
A storm cycle began Friday with generally 6-12Â inches of snow accumulating along the east slopes closer to crest in the central-east and northeast zones, with lesser amounts at lower elevations and in the Mission Ridge/Blewett Area through 6 pm Saturday. A slight warming trend occurred Saturday, most pronounced mid and upper slopes.Â
Recent Observations
North
Persistent slab has been re-listed in the northeast zone highlighting a 5 cm layer of facets roughly 10 cm below the 1/17 crust in the Washington Pass zone and in areas further east. This reactive PWL has only been found in isolated locations, in cold non-wind affected north facing terrain. No avalanche activity has been observed on this layer.
NWAC pro-observer Jeff Ward was in the Varden Creek drainage Saturday and found 15-20 cm of storm snow well bonded to a variety of snow surfaces with the new snow generally lacking a slab structure. Small storm slab avalanches were only observed on steep solar aspects, likely failing on the most recent sun crust. Wind slabs on lee northerly aspects were the main avalanche problem with skier triggered avalanches possible near and above treeline. Â
Central
Jeff Ward was at Cannon Mountain off of Icicle Creek Monday 1/30. He found a variable and shallow snowpack that ranged from 1 to 2 m, depending on elevation and wind affect. Many windward slopes had been scoured to the 1/17 crust with thin wind slab present on lee slopes. The 1/17 crust was down 20-40 cm but no avalanche activity was observed on this layer. Large surface hoar was found at all elevations in non-wind affected terrain.
Both Tom Curtis and Jeff Ward traveled independently in terrain east of Stevens Pass Wednesday 2/1, covering the areas of Rock Mountain, Jove and Union Peaks. Both reported rapidly forming, very touchy wind slabs by mid-morning Wednesday. Wind slabs were building much further downslope due to the strength of the winds. Wind slabs up to 12 inches were seen and noted forming well below treeline.Â
South -Â No observationsÂ
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, North West.
Elevations: Alpine, Treeline.
Likelihood: Likely
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: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 1