Avalanche Forecast

Issued: Jan 5th, 2017 10:51AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Moderate to locally strong E-NE winds Tuesday night and Wednesday have loaded unusual aspects and built wind slab in all elevation bands, especially in the south and central Cascades. Allow recent wind slabs time to stabilize and avoid terrain where even a small slab avalanche could have unintended consequences. 

Summary

Detailed Forecast

Becoming cloudy overnight Thursday through Friday with occasional very light snow, mainly in the north part. Very light amounts of new snow are expected through Friday, if any. Winds should be mostly westerly and light to moderate. 

Moderate to locally strong E-NE winds Tuesday night and Wednesday have loaded unusual aspects and built wind slab in all elevation bands, especially in the south and central Cascades affecting locations such as Snoqualmie Pass, Crystal Mountain, Paradise and White Pass. 

Allow fresh wind slabs time to stabilize and avoid terrain where even a small slab avalanche could have unintended consequences. Avoid travelling on any terrain if you encounter firmer wind transported snow, which may be on all slope aspects or cross loaded slopes, especially in areas with varied terrain and modified wind directions. 

Snowpack Discussion

Weather and Snowpack

A low pressure system moved south over western Washington on Saturday night, followed by cold Arctic air. NWAC stations along the west slopes had W-SW winds Saturday and 5-13 inches of snowfall by New Year's Day.

An unusual moist reverse orographic east flow caused snow mainly in the central to south Cascades Sunday afternoon and night. NWAC stations along the west slopes had a shift to NE winds and another 0-10 inches of snow on Monday morning with the most snow in the central to south part and further cooling.

Fair cold weather with east winds were seen on Monday and Tuesday. Fair and cold weather continued Wednesday with easterlies moderate at Pass level and strong near ridgetops and above treeline. The strongest recorded winds were at Crystal mountain which saw gusts above 100 mph Wednesday morning.

Sunny weather continued Thursday with slightly warmer temperatures. Crest level winds shifted from moderate easterly Thursday morning to light to moderate westerly by Thursday evening.   

Recent Observations

Consistent observations were received on New Year's Day from three NWAC pro-observer's out enjoying the after champagne, champagne powder! Their reports from the Mt Baker area, Stevens Pass and the Alpental Valley all indicated deep, low density surface conditions, right side profiles and good skiing. Crusts layers were buried fairly deeply and unreactive in tests (Solstice crust at Snoqualmie buried 90 cm on average).

East winds began to affect some areas by Monday. The NPS ranger at Paradise on Monday reported low visibility with significant snow transport to SW-W slopes on Monday. Shallow wind slabs were easily ski triggered on west aspects near treeline at White Pass Monday afternoon. 

A couple reports are available via the NWAC Observations tab. A report from the Artist Point/Table Mountain area near Mt Baker indicated continued good stability with some wind slab starting to form on Monday. A skier on Mt Roosevelt at Snoqualmie Pass triggered a 10-20 inch wind slab and was carried but not injured in a WNW couloir on Monday.

A report on the NWAC observation page reported widely visible and ongoing transport occurring in the Stevens Pass area Wednesday due to east winds. 

We received a report of a skier triggered wind slab avalanche in the Crystal backcountry around midday Wednesday. A 15 cm (6") wind slab avalanche of pencil hardness broke 40-50 ft across and carried the skier 200 ft on a NW aspect in the upper portion of the below treeline band. The skier was not seriously injured, but lost equipment. NWAC is compiling more information on this incident and will be posting a report soon.

Additional field observations ranging from the Mt Baker area to Mt Rainier Thursday, highlighted the effects the recent strong E-NE winds have had on the snowpack. In general, the surface conditions in exposed terrain are a mixture and a mess, ranging from shallow to deep wind slabs of mostly hard firmness, sastrugi, scoured and wind sculpted surfaces and some soft snow.

In wind sheltered terrain and treed slopes, settling and stable powder conditions persist with some excellent skiing still available.

NWAC received tragic news late Thursday of an avalanche fatality in the Crystal Mountain south backcountry. The Crystal ski patrol came across the slide and debris and found the victim late Thursday morning. The avalanche likely released Wednesday. The victim was traveling solo. Members of the NWAC team will be on-site Friday to assist with the investigation.  

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: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 6th, 2017 10:51AM