Avalanche Forecast

Issued: Jan 10th, 2017 10:16AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

New wind slab should build through Wednesday morning due to easterly winds depositing snow primarily on lee westerly aspects. However, also watch for cross-loaded slopes and lingering wind slab on a variety of aspects. Watch for firmer wind transported snow and avoid freshly loaded slopes in all elevation bands.  

Summary

Detailed Forecast

A low pressure system crossing Oregon Tuesday night will cause a sustained round of moderate easterly winds through Wednesday morning. Light snowfall should taper off in the morning for the south Washington Cascades with clearer conditions expected over the north Cascades. Winds will not be nearly as strong as the east wind event of last week, but there is low density snow available for transport and cold temperatures Wednesday will help preserve any new instabilities.

New wind slab should build through Wednesday morning due to easterly winds depositing snow primarily on lee westerly aspects. Transport winds will be more out of the northeast in the Mt. Baker area. However, also watch for cross-loaded slopes and lingering wind slab on a variety of aspects. Watch for firmer wind transported snow and avoid freshly loaded slopes in all elevation bands.  

Small loose dry avalanches are possible in steep wind sheltered terrain but will not be listed as an avalanche problem. 

Snowpack Discussion

Weather and Snowpack

A cold and dry Arctic air mass was over the Northwest last week. The main weather event during this cold snap was a day of strong east winds last Wednesday that created widespread variable snow surfaces by scouring windward slopes and re-distributing the snow to a variety of aspects.

Reports indicate the winds eliminated most of the faceted surface snow and surface hoar that formed in the near and above treeline during the cold weather. But these weak surface crystals were still reported below treeline prior to snowfall that fell over the weekend.

A pair of warm fronts moving through the PNW Sunday and again Sunday night left temperatures see-sawing near or above freezing for areas away from the Passes with milder Pacific air finally dislodging colder air in the Passes late Sunday night. 5-10 inches of snow fell through Monday morning with another 5 inches accumulating in the Mt. Baker area from showers through Monday afternoon.

A weak low pressure system tracking across southern Washington Tuesday morning produced another 1-3 inches of snow with light winds from Stevens Pass and south. East or Northeast winds were increasing Tuesday afternoon. 

Recent Observations

Wind slabs were touchy last week during the height of the east wind event with two serious skier triggered hard wind slabs reported in the Crystal backcountry on Wednesday. The larger wind slab avalanche resulted in the 2nd avalanche fatality of the season in Washington. See the excerpt above for more details. 

NWAC pro-observer Lee Lazzara was out in the Glacier Creek drainage west of Mt Baker on Saturday and noted surface hoar below treeline in wind sheltered locations. Alpental pro-patrol noted a similar setup of surface hoar below treeline. 

Mt. Baker pro-patrol checked in on Monday not only with the most snow from the latest storm but also with the most sensitive storm slab both in area and out. 12-14" inch storm slabs released easily and on most aspects in area during control work Monday morning. A natural avalanche cycle that likely occurred Sunday night produced widespread 12-18" slabs off the Shuskan Arm. 

On Tuesday, pro-patrol at Alpental found sensitive and widespread but shallow storm slabs of 3-4". There were no indication of slabs releasing to deeper layers in their area. 

NWAC observer Dallas Glass was in the Kendall Peak area below treeline Tuesday. Dallas found the new snow generally right side-up with good snow conditions. A freezing rain or rain crust was ski supportable and down 25 cm. The crust disappeared above 4800 feet. The only direct signs of instability noted were small loose dry sluffs on steeper slopes. Via the highway, east winds were beginning to transport snow on Granite Mountain by mid-afternoon. 

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

Likely

Expected Size

1 - 1

Valid until: Jan 11th, 2017 10:16AM