Avalanche Forecast

Issued: Jan 10th, 2018 11:05AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Large avalanches are very likely on Thursday. Travel in avalanche terrain is not recommended!

Summary

Detailed Forecast

Travel in avalanche terrain is not recommended on Thursday! 

On Thursday, a strong storm arrives in the early morning hours with rising snow levels and strong to very strong crest level winds with heavy rain and snow. 

Storm snowfall will be quite intense across both the west and east slopes of the Cascade with one foot or more of snow expected to accumulate during daylight hours. This snow will fall at increasing density, creating an upside down storm layers on top of the multitude of crusts and recent lower density snow already existing in the snowpack. This storm is very likely to produce a widespread natural avalanche cycle, with large avalanches in all elevations bands. 

Wind slabs forming Tuesday through Wednesday were already showing signs of significant reactivity in many areas on the crust surfaces present. On Thursday, Strong to very strong SSW crest level winds (turning westerly in the late afternoon) will quickly form very touch large to very large wind slabs on a variety of aspects, but most notably NW though SE.

Early season hazards still exist. Many creek beds have still not filled in for the winter and crusts in many areas are quite firm.

Snowpack Discussion

Since Sunday most east slope areas have received 6-12 inches of snow. The most recent snow has fallen with cooling temperatures but periods of very strong winds, especially Tuesday through early Wednesday. The recent storms have created sensitive wind slabs on a variety of aspects near and above treeline and on some exposed terrain below treeline. In wind sheltered terrain storm snow is low density and generally right side up. However, in some locations, storm slab formation on 1/5 and 1/9 crusts have been reactive. The supportive 1/9 crust formed from a widespread freezing rain event Thursday night and Friday in the central east slopes zone, extending to the Stevens and Snoqualmie Pass areas. 

In the northeast Cascades, no freezing rain crust exists in ski terrain along the Hwy 20 corridor nor has buried SH been discovered in avalanche terrain.

Snowdepth decreases substantially the further east of the Cascade crest one travels. In many areas below treeline, there is not enough snow to present an avalanche danger. 

Observations

North

On Monday, NCMG found  4-6 inches (10-15 cm) of lower density snowfall over a well consolidated base with little signs of recent wind transport in the Washington Pass area. No new avalanches have been observed in this area over the last two days. The 1/5 freezing rain crust found further south has not been found along the Hwy 20 corridor. 

Central

A public observation from Friday 1/5 in the Mission Ridge backcountry noted a 1/4" thick freezing rain crust up to 5000' making for treacherous travel conditions. 

Basal facets (2-3mm) observed on 1/2 above 5000' on NW-N-E aspects and failing in snowpit tests in the Blewett Pass area (and likely the Mission Ridge area) will need to be watched when enough snow returns to this area to create a potential avalanche problem.  

South

No recent observations

Problems

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

Very Likely

Expected Size

1 - 1

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: All elevations.

Likelihood

Very Likely

Expected Size

1 - 1

Valid until: Jan 11th, 2018 11:05AM