Register
Get forecast notifications
Create an account to receive email notifications when forecasts are published.
Login
Archived

Avalanche Forecast

Dec 23rd, 2016–Dec 24th, 2016
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Olympics.

Don't be caught surprised by the unusual PWL at Hurricane on Saturday. Limit your travel to low angle slopes until we are sure this layer is stabilized.

Detailed Forecast

An upper trough will exit the Northwest on Saturday and high pressure will begin to build offshore. Light snow showers mainly along the west slopes Saturday morning should give way to partial clearing Saturday afternoon with light west to northwest winds and cool temperatures.

Don't be caught surprised by the unusual PWL at Hurricane on Saturday. Limit your travel to low angle slopes until we are sure this layer is stabilized.

Wind slab should also be possible on Saturday due to recent south winds. Watch for firmer wind transported snow mainly on northwest to southeast slopes.

Although cornices won't be listed as an avalanche problem avoid ridges that may have a cornice and slopes below cornices.

The same danger levels will be indicated in all the terrain bands in the outlook for Sunday due to the cool weather and until there is known favorable change in the PWL layer.

Snowpack Discussion

Weather and Snowpack

A front crossed the Northwest on Thursday followed by an upper trough on Friday. This has caused about 8-10 inches of snow at Hurricane. South winds Thursday became light on Friday a cooling trend Friday.

Recent Observations

NWAC pro-observer was at Hurricane Ridge on Friday and gave an important report so pay attention. He was triggering collapsing and whoomping in every open area that he visited on Friday. In 2 snowpits on slopes less than 30 degrees he found the December 17th PWL and preserved surface hoar and preserved faceted snow at about 46-50 cm below the surface with moderate Extended Column tests indicating propagation. While the ski conditions were excellent he was unable to access higher open terrain safely.

Matt also reported that cornices were growing on the lee northeast sides of ridges on Friday.

Avalanche Problems

Persistent Slabs

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Likely

Expected Size: 1 - 1

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: Possible

Expected Size: 1 - 1