Avalanche Forecast

Issued: Jan 31st, 2017 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Northeast or east winds will begin to redistribute new snow to lee westerly aspects Wednesday. Shallow new wind slab may become particularly touchy in areas where it bonds poorly to an underlying crust. Wind slab should primarily be found near and above treeline but look for firmer wind transported snow down into the upper portion of the below treeline band. All aspects are listed to cover older wind slab formed over the weekend.  

Summary

Detailed Forecast

Northeast or east winds will begin to redistribute new snow to lee westerly aspects Wednesday. Shallow new wind slab may become particularly touchy in areas where it bonds poorly to an underlying crust. Wind slab should primarily be found near and above treeline but look for firmer wind transported snow down into the upper portion of the below treeline band. All aspects are listed to cover older wind slab formed over the weekend.

Older wind slab should be stubborn to trigger at this point, but in the Mt. Baker area be aware of a higher likelihood of triggering wind slab where it overlies recently buried surface hoar. Field observations suggest this setup exists in isolated areas on northerly aspects near treeline in the Mt. Baker backcountry. 

Snowpack Discussion

Weather and Snowpack

An atmospheric river arrived over the Northwest on 1/17 forming a rain crust (freezing rain in the Passes) in most locations below 6000-7000 feet. A storm cycle from Thursday 1/19 through Sunday 1/22 deposited 1-2 feet of snow along the west slopes of the Cascades with the greatest totals seen at Mt. Baker.

A large upper ridge and warm air mass built over the Northwest last Thursday to Saturday with temperatures reaching the 40s in many places along the west slopes away from the lower parts of Stevens and Snoqualmie. Strong southwest winds were seen Saturday in the Northwest Cascades, with moderate southwest winds in the central west Cascades and light winds in the southwest Cascades. East winds were seen at Stevens and Snoqualmie. Sun crusts formed on most solar aspects during this period.

A weak front crossed the PNW Sunday afternoon and night, initially starting as rain below 6000 feet before depositing 1-4 inches of snow with the most at Mt. Baker. A generally ski supportable crust was reported in the Stevens and Snoqualmie Pass areas up to 5500 feet Monday and Tuesday. Light snow fell during a cooling trend with light winds Monday night, with 2-6 inches of snow accumulating along the central and south Cascades, with the most at Stevens Pass.   

Recent Observations

North

NWAC received an observation Saturday evening describing 2 groups of skiers who triggered 3 separate wind slab avalanches with depths averaging about 2 feet in the Mt. Baker backcountry. The wind slab avalanches were triggered on N through E slopes between 5000-5800 feet on Mt Herman with one slide burying a skier waist deep. Fortunately the skier involved was not injured. NWAC pro-observer Lee Lazzara was in the Bagley Lakes area on Saturday and reported wind slab formation. He also reported a large cornice failure on Saturday night onto a north slope of Table Mountain near Mt Baker at about 5000 feet. 

Lee was back out in the Mt. Baker backcountry Monday and through a fair number of column tests found wind slab reactive in isolated pockets near treeline where buried surface hoar sat below recently formed hard wind slab and above the old snow surface from 1/28. Wind slab was found on a variety of aspects above treeline, but did not show signs of likely propagation. About 10-15 cm of new snow was not well bonded to the previous snow surface but was too shallow to be a significant avalanche problem.  

Central

NWAC pro-observer Ian Nicholson was at Stevens Pass on Sunday and observed small loose wet activity in the morning before cloud cover and cooling quickly diminished the loose wet potential. A layer of small faceted crystals above the 1/17 crust are becoming rounded at about 40 cm down and were not reactive in column tests or on test slopes.

Ian was back out on Tuesday in the Alpental Valley and found 8-10 cm of new snow poorly bonded to 1/29 rain crust. The crust was thicker on solar aspects but ski supportable on most aspects up to about 6000 feet. 

South

NWAC pro-observer Dallas Glass was at Paradise on Sunday and traveled up to about 6400 ft. He found a surface melt-freeze crust from Saturday over about 15 inches of settled recent snow on the thick 1/17 crust on solar aspects. True northerly aspects still held some settled powder. During the day some wind transport was seen at higher elevations above treeline. Evidence of numerous small loose wet and size 1-2 wet slabs from Saturday were observed on solar slopes.

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

Possible

Expected Size

1 - 1

Valid until: Feb 1st, 2017 10:00AM