Avalanche Forecast
Issued: Jan 24th, 2018 1:17PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions will persist Thursday, especially in terrain receiving wind transported snow. Avoid steep open slopes. Storm snow is very deep! Tree-well and snow immersion hazards are very dangerous, so keep communication with your partner at all times. Persistent weak layers may be found in isolated areas in the northeast and central-east zones, where uncertainty exists. The greater uncertainty, the greater margins of safety are required, especially considering consequential terrain.
Summary
Detailed Forecast
Moderate snow late Wednesday should taper to light to moderate showers with a cooling trend overnight Wednesday and Thursday.
Increasing crest level winds are expected Wednesday night and Thursday.
The avalanche danger will lower slightly Thursday, but it will take time for these deep storm slab layers to stabilize, mainly at lower elevations with little wind exposure. Near and above treeline, strong winds should continue to build wind slabs along lee slopes below ridges and exposed open terrain.
Dangerous avalanche conditions will persist due to the likelihood for triggering a large avalanche. Recent storm and wind slabs will require time to heal, so be patient and match your terrain selection appropriately, namely very conservative.
Significantly transformed snow conditions have taken place during this extended storm cycle. Tree-well and snow immersion hazards are very dangerous, so keep communication with your partner at all times.
Persistent weak layers may be found in isolated areas in the northeast and central-east zones. If entering a new area, you need to determine if a persistent slab problem exists before entering consequential terrain.
Snowpack Discussion
The extended storm cycle continued Wednesday, sharing equally with the east slope locations! Total storm snow of 2-3 ft is typical in these zones now overlying a melt-freeze crust buried Jan,16th. The bonding continues to be favorable to this crust from most recent observations.
Very little to no avalanche activity has been seen in the north area over the past few days, however travel has been limited to mainly sheltered terrain below treeline where storm snow has little cohesion.
In areas near Mission Ridge Wednesday, storm snow was highly reactive with denser 1 ft storm snow over over the 1/16 melt-freeze layer.
Observations above treeline have been limited leading to a higher level of uncertainty in this terrain.
Buried surface hoar was observed on the 1/16 crust in the Cascade East - Central zone on Wednesday, January 17th. The layer was also found on the crust in the Twisp River drainage on Monday, January 22nd, but faceted grains below the crust were even more concerning. The crust layer interfaces were the suspect culprit for a large avalanche on Sunday in the Cascade East-North zone. This layer will need to be watched during the upcoming stormy period as the extent and distribution of these weak layers remain highly uncertain and likely confined to isolated areas.
Snow depth still decreases substantially east of the Cascade crest. In many areas below treeline, there has not been enough snow to present an avalanche danger.
Note - The NWAC Washington Pass station is back online - nice work DOT! Precipitation and other sensor repairs to be planned...
Observations
North
Wednesday, Jan, 24th, NCMG observations on Delancy Ridge found nearly 3 ft of low cohesion storm snow in sheltered terrain below treeline with no avalanches noted. No reactivity was found at the 1/16 interface with the melt-freeze crust. Travel was avoided in higher elevation, more likely wind affected terrain, where there remains more uncertainty.
On Monday, snowpit tests by a snow professional on a SE aspect Near Treeline in the Twisp River Valley gave sudden collapses within faceted grains below the 1/16 crust. Test results on a NE aspect yielded consistent sudden planar results above the 1/16 crust with partially decomposed surface hoar on the interface.
NWAC received a second-hand report of a large skier-triggered avalanche on west-facing slope (likely near treeline) on Abernathy Peak in the upper Twisp River drainage. The avalanche propagated widely and was suspected to have failed on the 1/16 crust.
Central
On Saturday an avalanche professional on Dirty Face Mountain near Lake Wenatchee observed a well bonded upper snowpack near and below treeline. No buried surface hoar was found and no avalanches were observed.
On Wednesday, 1/17, NWAC observers traveled in the Icicle Creek drainage up to 6800 ft. Buried surface hoar was found over a melt-freeze crust. The buried surface hoar was not limited to valley bottoms, but found to all elevations up to 6800 feet.
South
No recent observations
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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
Expected Size
Valid until: Jan 25th, 2018 1:17PM
