Stevens Pass Avalanche Forecast

Issued: Dec 23rd, 2013 10:00AM

Mon Dec 23rd Current Conditions
Alpine Considerable Treeline Moderate Below Treeline Moderate
Tue Dec 24th 2 Day Outlook
Alpine Moderate Treeline Moderate Below Treeline Low

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.

Stormy weather Monday and new sensitive wind slab and storm slabs will be slow to stabilize by Tuesday, give steep exposed terrain some more time to heal, especially in areas that have had the greatest recent loading.

Summary

Detailed Forecast

Tuesday should see the beginning of another extended dry period. Initially on Tuesday freeing levels should be very low with cool air in place following Monday's frontal passage. Winds should diminish to generally light on Tuesday.

This weather should allow for the beginning of recent unstable conditions to settle and slowly stabilize. However, this process my take some time in certain areas, such as higher terrain and where significant wind slab has formed below ridges. 

While the overall trend is for lowering danger, triggered avalanches should remain likely in the alpine elevations and possible in the near treeline band. 

In areas with light wind, where rain changes to snow, and with lower rates of loading, the cooling trend should help bond new snow to old snow and help build right side up conditions with lower density snow nearer the surface. This can mean some good skiing in those types of areas. Underlying wet snow should also begin to refreeze and strengthen.

Shallow snow cover and shallow new snow will continue to limit the avalanche danger at lower elevations.

Remember this is a regional forecast and you must make your own on site evaluations to see if they are different than forecast.

Snowpack Discussion

Warm wet weather the weekend of 14 and 15 December created another crust layer in most areas. This seems to have capped and stabilized underlying hoar frost and faceted snow and made those layers generally unreactive.

Cold weather, light amounts of low density snow and some surface hoar formation were generally seen mid-week last week.

This set the stage for warmer wet weather that arrived Friday and over the weekend, culminating with a cold frontal passage Monday afternoon. A warm front moved across the Northwest Friday. This generally caused strong southwest winds, increasing moderate to heavy snow and greatly warming temperatures. Another warm front moved across the Northwest on Saturday night and Sunday. This generally caused strong northwest winds and wet snow or rain at further warming temperatures. NWAC sites near and west of the crest for the 48 hours ending Sunday morning had ended up mostly with about 4-17 inches of very wet heavy snow.

Another round of wet heavy snow began overnight Sunday through Monday afternoon depositing another 4-10 inches as of Monday afternoon. The most recent heavy wet snow has also been accompanied by strong west to northwest crest level winds. NWAC Weather stations at the top of Crystal Mountain and Mission Ridge recorded peak gusts to 79 mph and 99 mph respectively Monday afternoon! 

The warming, dense or wet new snow and strong winds have combined to build both unstable storm slab layers as well as sensitive wind slabs by Monday afternoon, especially in the alpine elevations.

The most pertinent report Monday was from the pro patrol at Mt Baker Ski area where new sensitive wind slabs were releasing full paths with many natural slides reported including a rather large release from Table Mountain running to the Bagley Lakes valley floor. Some slides were releasing sympathetically as well with most wind slab crowns ranging from 30-60 cm (1-2 ft).

There was also a report of a wet slab release Sunday midday in the Alpental Valley below Chair Peak that ran a significant distance. Poor visibility precluded the view of starting zone elevation but a crown depth was estimated over 30 cm. As of Monday, additional heavy dense loading has occurred in that area and should maintain a heightened interest for anyone thinking about travelling in steeper terrain for a few days.   

Other recent observations: An NWAC observer just east of Stevens Pass on Saturday reported that the recent snow gave some cracking and small wet loose avalanches. The Mt Hood Meadows ski patrol Saturday and Sunday reports a stable freezing rain crust at least to 7300 ft but it is unknown if this condition extends very far around the mountain.

Shallow snow cover has been limiting the avalanche danger at the lowest elevations.

 

Problems

Wind Slabs

An icon showing Wind Slabs

Likelihood

possible

Expected Size

1 - 1

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.

Storm Slabs

An icon showing Storm Slabs

Likelihood

possible

Expected Size

1 - 1

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.

Valid until: Dec 24th, 2013 10:00AM

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