Avalanche Forecast

Issued: Mar 8th, 2016 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Significantly changing weather and snow conditions should be seen along the west slopes Wednesday afternoon. Travel in avalanche terrain above treeline is not recommended Wednesday afternoon. Dangerous conditions should also be seen in the near and below treeline.

Summary

Detailed Forecast

A weakening front should move over the Northwest Tuesday afternoon and night. This may cause some mostly light amounts of snow along the west slopes but probably not as much as earlier expected.

The bigger story is the incoming atmospheric river on Wednesday. A warm front should south to north over the Olympics and Cascades Wednesday morning to afternoon. This should bring stormy wet weather with rising snow levels initially to Mt Hood Wednesday morning and to the Olympics and Washington Cascades Wednesday afternoon. East winds should do little to lower the snow levels in the Cascade passes.

Little change may be seen by Wednesday morning along most of the Cascade west slopes. But by Wednesday afternoon watch for increasing alpine winds and increasing moderate to heavy rain or snow and rising snow levels.

Mainly in the above treeline new wind slab of increasing density due to warming is very likely to form on lee slopes. Watch for firmer wind transported snow as the storm develops Wednesday afternoon.

Also mainly in the above treeline new storm slab of increasing density is also very likely due to warming. Watch for snowfall that begins to accumulate Wednesday afternoon at more than an inch an hour.

Rain in the below and possibly in the near tree line starting Wednesday afternoon is also likely to create loose wet avalanche conditions. Watch for pinwheels and natural loose wet avalanches.

The rain where  heavy enough by Wednesday afternoon may activate previous layers and cause releases of previous wind slab layers or wet slab avalanches mainly in the near and below treeline.

Snowpack Discussion

Weather and Snowpack

Last week was wet and relatively mild with a series of active fronts transiting the region resulting in fluctuating freezing levels and periods of very strong winds. The heaviest snowfall and precipitation occurred late Sunday 2/28 and Tuesday 3/1. The latter half of the week featured more rain than snow at NWAC sites along the west slopes.

Mid and lower elevations late last week and over the weekend saw natural and triggered loose-wet avalanches most days due to increasing solar effects or during rain events.

A front Sunday and a cooler upper trough Monday brought some snow with 2 day storm totals of about 2-12 inches along the west slopes ending Tuesday morning.

The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

NWAC pro-observer Tom Curtis was at Stevens Pass on Friday found evidence of lots of large loose wet and some wet slab avalanches.

Locally heavy rain at the Mt. Baker area led to sensitive loose wet ski cuts by the Mt. Baker pro-patrol Saturday morning. A natural loose wet cycle was observed in the Bagley Lakes area and one glide avalanche in unsupported terrain released nearby. Also in the Baker area, a glide avalanche occurred Saturday night off Shuskan Arm entraining moist surface snow and becoming very large while running to the valley bottom.

A report on Turns All Year for the Crystal area for Saturday indicated wet snow conditions with pinwheels on north and south slopes.

A report via the NWAC Observations page for Sunday for Snoqualmie Pass indicated wet heavy snow conditions and previous loose wet avalanches.

The pro-patrol at Alpental on Tuesday found 2 day storm snow of 8-12 inches on  the upper mountain but had no results by ski cuts and reported that sun breaks were not enough to cause loose wet snow. Explosive control caused areas of shallow 4 inch storm slab releases.

 

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.

Elevations: Alpine, Treeline.

Likelihood

Very 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: Alpine, Treeline.

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

Valid until: Mar 9th, 2016 10:00AM