Avalanche Forecast

Issued: Jan 8th, 2018 10:47AM

The alpine rating is moderate, the treeline rating is low, and the below treeline rating is low. Known problems include Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Shallow wind slab may be found on lee slopes above treeline. Watch for changing conditions and continue to plan for challenging terrain travel at lower elevations.

Summary

Detailed Forecast

Light ain and snow should arrive on the Cascade East slopes in the late night and morning hours with a cooling trend and showery precipitation in the afternoon.

Wind slabs have been trending toward unreactive in the Washington Pass area, but with limited field observations and some wind slab noted on the Cascade West slopes, we are leaving wind slabs in the forecast.

Shallow wind slab may be found on lee slopes above treeline. Approach lee slopes with caution near on Tuesday, watching for signs of recent or active wind transport.  

In the Cascade East-South and Cascade East-Central zones, watch for signs of roller ball activity if snow changes to rain in areas where snow has accumulated. In this event, avoid steeper slopes.

Early season hazards still exist. Many creek beds have still not filled in for the winter and multiple crust surfaces may may travel at lower elevations more difficult than anticipated.

Snowpack Discussion

On Sunday, light snow fell along the east slopes of the Cascades with a slow warming trend. Some light freezing rain may mix with light snowfall Sunday night. 

In the northeast Cascades, very light amounts of new snowfall have been noted at Snotels since late last week.  No freezing rain crust exists in ski terrain along the Hwy 20 corridor nor has buried SH been discovered in avalanche terrain.

Extensive freezing rain accumulated at mid and lower elevations along the central-east slopes of the Cascades as well as in the Stevens and Snoqualmie Pass areas Thursday night and Friday. This significant freezing rain event has likely destroyed or capped a variety of existing snow surface forms. The newly formed crust is limiting the current avalanche potential and formed over a relatively strong older snowpack. 

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

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.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 9th, 2018 10:47AM