Avalanche Forecast

Issued: Feb 4th, 2015 11:03AM

The alpine rating is high, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Storm Slabs, Loose Wet and Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Dangerous avalanche conditions are expected east of the crest on Thursday especially above treeline. Back country travel in avalanche terrain above treeline in the northeast zone is not recommended on Thursday.

Summary

Detailed Forecast

Southwest flow will carry the first in a parade of mild wet fronts across the Northwest on Thursday. This will cause increasing winds and increasing moderate to heavy rain or snow Thursday morning with rising snow levels. Snow levels east of the crest should be lower than west of the crest some or much of Thursday.

New storm slab and wind slab should build east of the crest above treeline in the morning hours. The trend of rising temperature will help build upside down higher density snow over lower density snow above the snow level. New storm or wind slab of up to several inches seems possible above the snow level. Any surviving buried surface hoar from late January could act as a weak layer.

Increasing loose wet snow avalanches involving snow from so far in February should be seen mainly near and below treeline. The late January crust should make a good bed surface on many slopes. Small natural or triggered loose wet avalanches should be likely in many areas. This problem may work its way into the above treeline by the end of the day due to rising snow levels.

Dangerous avalanche conditions are expected on Thursday especially above treeline. Back country travel in avalanche terrain above treeline is not recommended on Thursday.

Snowpack Discussion

Mild weather with sunny days or minor rain or snow was seen in the Cascades from about January 26th to about January 31st. East of the crest this should have caused some consolidation and stabilizing. A surface crust will have formed on solar slopes but less so on shaded slopes or at higher elevations. Surface hoar formed on shaded and north slopes in some areas in fair weather during this time.

Slightly wet weather from about February 1 through today produced about 2-5 inches of snow at NWAC sites east of the crest with more at higher elevations in the northeast and central east zones.

NWAC pro-obs and North Cascade Guides reports last week in the northeast and central east zones have the January 15th facet/crust layers at 40 cm-1 m below the surface with the facets starting to turn to rounded grains and not reactive or not releasing in pit tests. So the January 15th facet/crust layers appear to be strengthening and stabilizing by late January. This problem will be removed from the central east zone but retained in the northeast zone until more information becomes available there.

NWAC pro-observer Tom Curtis was at 7500 ft on Mt Cashmere in the Wenatchee Mountains on January 28th and found a local facet/crust combo buried on a lee slope next to a ridge crest under 35 cm of thick, surface crust and 1F-P snow. This sort of slab could be very hard for a skier to trigger, but could be possible with a large sudden load, such as a snowmobile.

Other private groups in the Washington Pass area on February 2 and 3 report some upside down densities in the recent snow and natural and ski cut avalanches although the types of avalanches were not noted. 

 

Problems

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.

Likelihood

Very Likely

Expected Size

1 - 1

Loose Wet

An icon showing Loose Wet

Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

 

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

 

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Elevations: Treeline, Below Treeline.

Likelihood

Likely

Expected Size

1 - 1

Persistent Slabs

An icon showing 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: North, North East, East, West, North West.

Elevations: All elevations.

Likelihood

Unlikely

Expected Size

1 - 1

Valid until: Feb 5th, 2015 11:03AM