Avalanche Forecast

Issued: Jan 9th, 2017 10:31AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Shifting and increasing E or NE winds Tuesday afternoon will begin to build new wind slabs on unusual westerly aspects especially near and above treeline. Also avoid slopes above terrain traps in the below treeline band where recently formed weak layers may have been buried by new snow.

Summary

Detailed Forecast

A weak low pressure system crossing Oregon on Tuesday should produce light snowfall for the south Washington Cascades through early afternoon while little to no precipitation and less cloud cover should be seen closer to the Canadian border.

However, a deeper low pressure system approaching the Oregon Coast later Tuesday should cause shifting and increasing E or NE winds throughout the Cascade range by Tuesday afternoon that will begin to build new wind slabs on unusual westerly aspects especially near and above treeline. Previous wind slab may linger on other slope aspects also in the near and above treeline bands. Watch for firmer wind transported snow on a variety of aspects if you push higher in the terrain Tuesday.

Generally shallow storm slabs should be less sensitive on Tuesday except in areas where they overlie recently buried weak layers below treeline. Remember that recently formed surface hoar and surface faceted snow was observed in the below treeline band heading into the last storm cycle. Until more information is received, avoid slopes above terrain traps in the below treeline band where these weak layers may have been buried by the most recent snowfall. While storm slabs should generally be shallow in depth, slightly larger storm slabs are still possible in the Mt. Baker area.  

Snowpack Discussion

Weather and Snowpack

A cold and dry Arctic air mass was over the Northwest last week. The main weather event last week was a day or more of very strong east winds centered on Wednesday that went on a powder wrecking rampage, scouring windward slopes and re-distributing the snow to lee slopes while building sastrugi in other areas.

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. 

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. One 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 waiting to buried by Sunday night's snowfall. 

NWAC pro-observer Ian Nicholson was out at Paradise on Saturday and found that stiff 10-15 cm thick wind slab on N-W-S facing slopes was not as reactive as last week but still a travel concern.

Mt. Baker pro-patrol checked in on Monday not only with the most snow from the lastest storm but also with the most senstive 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. 

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

Likely

Expected Size

1 - 1

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

Possible

Expected Size

1 - 1

Valid until: Jan 10th, 2017 10:31AM