Avalanche Forecast

Issued: Jan 19th, 2018 10:35PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Deep areas of wind and storm slabs should make triggering an avalanche in steep terrain possible or even likely. Avoid travel on steep slopes receiving wind deposited snow, where fresh wind slab deposits will be sensitive to human triggering. Expect dangerous avalanche conditions at higher elevations.  Choose conservative terrain, by travelling in wind sheltered terrain well away from avalanche paths.

Summary

Detailed Forecast

A band of enhanced snow showers should arrive Friday night with diminishing showers Saturday. This should freshen storm snow with winds continuing to redistribute new cold snow to lee slopes by Saturday. 

This weather should cause widely variable snow conditions with some wind exposed slopes scoured to recent crusts and lee slopes building deeper wind drifts and slabs. Watch for changes in snow conditions, particularly stiffer more dense snow, likely a wind or storm slab. Avoid travel on slopes if you encounter wind stiffened surface snow, especially on steeper exposed terrain and steep roll-overs.  

The temperatures should remain cool, depositing new low density snow that will be easy for winds to re-distribute on exposed terrain.  

Wind slabs near and above treeline remain the primary avalanche problem in this area, so remain conservative in your terrain choices especially as you venture into exposed terrain.

The best and safest snow will remain in sheltered terrain away from ridges. 

Snowpack Discussion

New storm snow amounts since late Wednesday vary, ranging from about 10-20 inches.

Above treeline, greater new snow amounts and periods of very strong winds have caused dangerous avalanche conditions. New wind 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 1-2 ft in favored areas.

Buried surface hoar has been observed in the Cascade East - Central zone as of Wednesday 1/17. This layer has not bee found recently in the north zone or in the area above Lake Wenatchee, however the layer cannot be ruled out entirely as its extent and distribution is still uncertain. 

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 was 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 Friday, NWAC observer Jeff Ward traveled near Mt Poe near Lake Weneatchee. No buried surface hoar was found over the Jan 16 crust layer in this terrain. Wind re-distribution has evident, especially in higher exposed terrain.. Recent storm snow ranged from 16-20 inches in this area with no recent avalanches observed. 

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 20th, 2018 10:35PM