Avalanche Forecast

Issued: Jan 4th, 2016 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Watch for isolated areas of wind slab primarily on north through southeast aspects above treeline. New storm slab layers could be reactive where new accumulations become more than a few inches over surface hoar and near surface faceted snow.

Summary

Detailed Forecast

A couple more weak short waves and weak bands of precipitation should move out of a large weak trough and over the Northwest on Tuesday. This will cause more periods of mostly light snow in the Olympics and Cascades on Tuesday. A slight warming trend should be seen on Tuesday.

This weather will continue to allow older wind slab to slowly settle and stabilize. Wind slab will most likely be found on north to southeast aspects near and above tree line. East winds were much lighter east of the crest since Thursday and will likely not have built new wind slab on westerly aspects. Watch and test for inverted strong over weak storm snow.

New snowfall may start to bury or further bury possible surface hoar and near surface faceted snow from around New Years. This new snow may continue to be somewhat upside down with denser snow near the surface which contributes to instability. These layers could be reactive where new accumulations become more than a few inches. This should be most likely in the central east and southeast parts. 

Snowpack Discussion

Fair weather was seen the last couple days of December through the weekend. This weather caused lots of consolidation and stabilizing of the snow from December. It also caused new widespread surface hoar and near surface faceted snow along the east slopes.

Tom Curtis was west of Blewett Pass around Iron Mountain New Years Day and found the December 13 buried surface hoar layer 60 cm down that showed sudden collapses in several column tests, including PST with a firm slab overhead. In this area, the layer was prevalent on NE-E aspects around 5300 ft near treeline. This layer was also noted in a recent observation posted to NWAC.  There were no recent skier or natural triggered avalanches on this layer, but it may be a layer of concern to track or look for moving forward in this specific area. 

PST End down 60cm on 20151213 buried surface hoar layer. West of Blewett Pass, January 1, 2016. ENE aspect NTL @ 5330ft. Photo: T. Curtis

The North Cascades Mountain Guides on January 1st and 2nd at Washington Pass generally report good stable conditions and a lack of results in snow pit tests. Minor wind effects have been noted near ridges.

The southeast zone has a shallower snowpack versus areas further north, but after a snowy few weeks, has more than enough snow to warrant an avalanche risk. No snowpack observations have been received from this zone.

A change Sunday night and Monday has been some snow and a warming trend. Snowfall has ranged from about 1-8 inches along the east slopes mainly in the central to south parts.

This new snow will be upside down with denser snow near the surface and may have accumulated on surface hoar and near surface faceted snow from around New Years.

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: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood

Possible

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: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 5th, 2016 10:00AM