Avalanche Forecast

Issued: Jan 29th, 2018 11:50AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Wind Slabs and Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Dangerous avalanche conditions continue in the northeast Cascades. Recent heavy snowfall and strong winds have created large storm and wind slabs. Watch for lower density newer snow actively forming wind slab Tuesday. Avoid steep, wind-loaded, or convex slopes and give recent storm snow instabilities time to heal on Tuesday. 

Summary

Detailed Forecast

Heavy rain and mid-to-high elevation snow will rapidly turn to light to moderate snow showers which will continue under moderate W-SW winds through the day on Tuesday.

Large wind slabs were created during a series of moist and windy storm systems from Saturday through Monday night. These wind slabs need time to heal and may be buried under lower-density snow. Consistent W-SW winds will continue to transport lower density snow and will build fresh wind slabs, particularly on NW-N-NE-E-SE slopes near and above treeline. These fresh wind slabs may become large in size by the end of the day Tuesday. Travel cautiously in wind-affected terrain, avoiding wind-loaded terrain near ridgelines and watch for signs of wind-transported snow, including snow that feels stiff or chalky.

Large storm slabs have likely developed in areas/elevations that stayed mostly snow during this storm cycle and also need to time heal. These areas may include near and above treeline terrain in the central and south, or any elevation band in the north. Travel cautiously and choose conservative terrain on Tuesday to mitigate recent storm related avalanche problems. 

Snowpack Discussion

A warm and moist system on Sunday and Monday brought higher density snowfall to mid-to-upper elevation sites on the east slopes of the Cascades, with the heaviest snowfall occurring in the Cascade East-North zone on Monday afternoon. Washington pass has seen moderate SW winds accompanying the snowfall on Monday.

Estimated recent snow totals at mid-elevation sites; 15-25" in the northeast zone through Monday afternoon with 5-15" in the central-east and southeast Cascades.

The weight of the recent snow is causing relatively rapid settlement  (7" in 24 hours at Lyman Lake Monday) of the underlying lower density snow which has accumulated since the 1/16 crust.

Winds throughout the active period since 1/16 have been redistributing snow, but the strongest winds were in the Central Cascades, especially Mission Ridge.

Some recent observations from around the east slopes central/north zones found buried surface hoar above the 1/16 crust. There is still quite a bit of uncertainty about the distribution of this layer in these zones. Extra caution should be taken when traveling in areas further east of the crest or in areas where less snow has been received. Snow profiles and snowpack tests are the only means to identify and locate this layer. A non-exhaustive list of locations where surface has been found on the 1/16 crust includes specific slopes in Icicle Creek and the Twisp R., and in isolated locations near the Barron Yurt and in the bottom of Liberty Bowl. 

Observations

North

On Sunday, NCMG in the Washington Pass area found the 1/16 crust about 3+ft (1 m) down on non-solar aspects. New storm snow was becoming heavier throughout the day. One natural wind slab was observed in steep terrain. 

On Friday, North Cascades Mountain Guides found the upper snowpack generally gaining strength. Some snowpack test failed below the 1/16 crust.

Central

On Sunday, NCMG was on Dirtyface Mountain and reported a deep and very upside-down upper snowpack that was not showing the anticipated reactivity given the structure. Buried wind slabs were present, but unreactive near ridge crest.

Mission Ridge Ski Patrol reported very high winds and active wind loading on N-NE slopes Saturday with very reactive wind slabs producing 4-30" crowns. In wind sheltered locations, soft surface snow was found. Surface hoar had previously been observed in the area on the 1/16 crust.

A public observation from Wednesday in the Blewett Pass area identified basal facets at the bottom of the snowpack and surface hoar above the most recent crust. Snow cover was still regionally low in this area.

South

No recent observations

Problems

Wind Slabs

An icon showing 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

Likely

Expected Size

1 - 1

Storm Slabs

An icon showing 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

Valid until: Jan 30th, 2018 11:50AM