Avalanche Forecast

Issued: Mar 17th, 2017 12:29PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Saturday's avalanche forecast will depend on the storm overnight. Heavy snow and strong winds will create very dangerous avalanche conditions above treeline. Below treeline, slowly refreezing wet snow and shallow new snow will create a significantly lower danger.  Local conditions may vary greatly, so be prepared to adjust plans accordingly.

Summary

Detailed Forecast

A low pressure system is tracking towards the central Washington coast from the SW, expected to move inland across the Olympics Friday night. This system should spread increasing precipitation, strong winds and rising snow levels, late Friday through early Saturday. The peak in winds, snow levels and precipitation are expected Friday night into early Saturday morning. Significant cooling, diminishing showers and winds are expected through the day Saturday.

The biggest factor affecting Saturday's avalanche forecast confidence will be how much precipitation is received overnight and to what elevation the rain/snow line reached.

New wind slabs are likely to have built on NW to SE aspects, mainly above treeline. But watch for firmer wind transported snow on other aspects, especially in areas of complex terrain.

At lower elevations, rain Friday night should leave a wet and slowly re-freezing snowpack with shallow new snow accumulation expected Saturday. Small loose wet avalanche conditions are most likely to persist at lower elevations, especially if the cooling is delayed early Saturday. Watch for surface wet snow deeper than a few inches, rollerballs or increasing small natural releases.

Although the likelihood of wet slab or glides avalanches has greatly decreased with the recent re-freeze, these avalanche problems may reemerge following the warm and wet weather Friday night. Continue to avoid unsupported slopes where you know there is a smooth underlying surface, or slopes with glide cracks.

It is always a good plan to travel well back from ridges, suspected of cornice formation, or on steep slopes below cornices.

Remember to re-evaluate or change plans if local conditions vary from forecast conditions.

 

Snowpack Discussion

Weather and Snowpack 

The first week or so of March was very cool and snowy. NWAC stations along the west slopes of the Cascades piled up about 3 to 8 ft of snow with the most at Mt Baker.

A strong frontal system brought increasing precipitation, winds and warming to the Cascades on Thursday 3/9. On Thursday night rain pushed up to about 5000 feet in the northwest Cascades and about 6500 feet in the southwest Cascades. This caused region-wide avalanches with many large to very large wet avalanches. 

Moist W-SW flow and a stalled frontal boundary produced periods of moderate to occasionally heavy rainfall at most locations throughout the west slopes of the Cascades on Monday and Wednesday. Three day precipitation amounts along the west slopes of the Washington Cascades ranged from 3 to over 7 in of water, most all in the form of rain. This caused more region-wide avalanches with many large to very large wet avalanches. About 1-2 feet of snowpack consolidation was also seen the past week along the Cascade west slopes.

Cooling and a little snow was seen on Wednesday night. Most NWAC stations along the Cascade west slopes only had 0-2 inches by Thursday morning but Mt Baker had 9 inches.

The past week of weather has done a reset of the snowpack along the Cascade west slopes. Most areas at the end of Thursday are light amounts of new snow over a firm, consolidated, stabilized snowpack.

Recent Observations

North

The Mt Baker ski patrol on Thursday 3/16 had no ski triggered avalanches and only 1 small explosively triggered 6-8 in wind slab. 

NWAC observer Lee Lazarra was in the Mt Baker backcountry Friday and reported well settled old rain soaked snowpack with a stout crust. The recent snow ranged from 4 to 8 inches of recent dry snow depending upon wind distribution. There were some shallow wind slabs noted on lee slopes in higher terrain. The avalanche issues going forward relate to the upcoming storm, Friday night and into Saturday. 

Central

NWAC observer Dallas Glass was in the Alpental Valley on Wednesday 3/15 and reported water draining well from the upper snowpack with wet snow in the top 45 cm. He was not able to trigger loose wet avalanches on steep slopes. The upper snowpack was a mix of small rounded crystal and melt layers. Tests of the Valentine's crust did not produce results.

South

NWAC  observer Dallas Glass was above Paradise, on Mt Rainier on Thursday 3/16 and found about 1 inch of new snow on a 6-8 inch thick solid crust and no avalanche problems. There was wind loading on the NE slopes up to 4 inches deep, but he found it unreactive. His main concern was an uncontrolled fall on steep icy slopes.

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.

Likelihood

Very Likely

Expected Size

1 - 2

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

Possible

Expected Size

1 - 1

Valid until: Mar 18th, 2017 12:29PM