Avalanche Forecast

Issued: Jan 18th, 2018 12:31PM

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

Human-triggered avalanches are likely on Friday. New snow and moderate SW winds will combine to create sensitive wind slabs near and above treeline. The danger increase will be storm snow dependent. Both storm and wind slab will increase in size throughout the day. 

Summary

Detailed Forecast

Cool temperatures, moderate snow showers and periods of moderate S-SW winds will continue to deposit new snow and further build existing wind and storm slab layers.

Building wind and storm slabs should maintain dangerous avalanche conditions near and above treeline. Storm slabs may form quickly and become sensitive to human triggering, especially in those areas receiving the greatest amounts of new snow (more than about 8" of new snow accumulated) and higher snowfall rates (extended periods of more than about 2" per hour). 

Conservative terrain choices will be essential in higher and wind exposed terrain. Best to travel in wind sheltered, lower elevation terrain well away from steep open slopes until this new storm snow stabilizes. 

Variations in new storm snow amounts throughout the zone will lead to subsequent variation in local danger. Regardless, the current snowpack structure should not support significant snow loads well.

Where buried surface hoar exists over a smooth crust layer, expect very sensitive avalanches that may run long distances, especially in areas receiving the greatest storm snow amounts. This layer was recently observed in the Icicle Ck drainage between 4000-6800 ft. 

Snowpack Discussion

New storm snow amounts varied by late Thursday, ranging from about 6-12 inches.

Above treeline, greater new snow amounts and periods of very strong winds have caused dangerous avalanche conditions. New wind and storm slabs in higher exposed terrain will require careful terrain choices and conservative decision making.  

Near and below treeline the new snow is fairly well bonded to a melt-freeze crust of varying thicknesses. This has caused less dangerous conditions with new snow forming good bonding. However, heavy snowfall and moderate to strong winds have built increasing wind and storm slabs at progressively lower elevations. There are still heightened avalanche conditions with new and increasingly deep wind and storm slabs of 1 ft or more as of late Thursday.

Buried surface hoar has been observed in the Cascade East - Central zone as of Wednesday 1/17. This layer is likely to exist in the north zone as well, but lack of direct observations leads to lower confidence of its extent and distribution. 

Last week’s storms brought 8-14 inches of snow to the east slopes of the Cascades which has been consolidated by warm temperatures.

A supportive crust (1/5) formed from a widespread freezing rain event in the central-east zone, extending to the Stevens and Snoqualmie Pass areas. This layer is not present in the northeast from Washington Pass to Holden.

Snowdepth still decreases substantially east of the Cascade crest. In many areas below treeline, there has not been enough snow to present an avalanche danger.

Observations

North

On Thursday snow professionals in the Washington Pass area found good bonding of new snow to the old surfaces with favorable snow profiles and no avalanche activity, even traveling on steep slopes. No buried surface hoar was found in this zone.

Central

On Wednesday, 1/17, NWAC observers Jeremy Allyn and Jeff Ward traveled in the Icicle Creek drainage to 6800 ft. Of particular note was buried surface hoar found over a melt-freeze crust, buried about 3-4 inches on 1/16. The buried surface hoar was not limited to valley bottoms, but found to all elevations traveled, near 7000 feet. In areas void of the buried surface hoar layer, 3-4 inches of low density snow overlies a recent melt-freeze crust which also extended to about 7000 feet.   

A snow professional reported from the Mission Ridge area on Sunday where a 4-5' deep snowpack on a north aspect at 6400 feet contained two potential weak layers of concern. (1) A layer of 2 mm facets 45 cm above the ground lies beneath the 12/15 crust. This layer was reactive in column tests and propagation tests (see this report) . (2) Basal facets just above the ground were present but rounding and were not the weak layer of interest in numerous tests. These layers will need to be monitored as they could develop into potentially dangerous weak layers in the future under loading.

South

No recent observations

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, North West.

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 19th, 2018 12:31PM