Avalanche Forecast

Issued: Jan 28th, 2017 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

A mixed bag of conditions is possible in the Olympics and Cascades on Sunday. New mostly shallow wind slab has been emphasized Sunday afternoon in the northwest Cascades. Possible lingering loose wet avalanche conditions have been emphasized Sunday morning in the central west and southwest Cascades.

Summary

Detailed Forecast

A weakening front will move over the upper ridge and cause increasing alpine winds on Sunday. It should still be mild or rather warm through the morning hours. Then light to moderate rain or snow should spread to the Olympics and northwest Cascades with a cooling trend beginning in all areas by Sunday afternoon.

In the northwest Cascades the main avalanche problem should be new mostly shallow wind slab Sunday afternoon. This should be mainly on northwest to southeast slopes near and above treeline but watch for new or older firm wind transported snow on all aspects in the upper terrain bands.

In the central west and southwest Cascades the main avalanche problem should be the possible lingering loose wet avalanche conditions Sunday morning. Increasing winds should help keep surface snow firmer and limit loose wet avalanche activity above treeline. Watch for natural releases of any size and wet surface snow deeper than a few inches. Old wind slab may also linger mainly on specific, steep northwest to southeast terrain features above treeline.

Snowpack Discussion

Weather and Snowpack

An arctic air mass settled over the Northwest with fair, very cold weather the second week of January.

An atmospheric river arrived over the Northwest Tuesday 1/17 with highly variable weather seen throughout the Cascades during this event. Heavy rain was seen up to about 6000 feet or higher in the North Cascades and up to about 7000 feet or higher in the south with snow at higher elevations. Three day precipitation totals through Thursday 1/19 were roughly 8 inches at Mt Baker, 5 inches at Crystal and 1-2 inches at the Passes. Sleet and freezing rain Tuesday 1/17 at the Passes changed to snow at higher elevations at Stevens and Snoqualmie on Wednesday 1/18.  At Stevens and Snoqualmie Pass the freezing rain crust is about 1 inch thick.

The most recent storm cycle from Thursday 1/19 through Sunday 1/22, deposited 1-2 feet of snow along the west slopes of the Cascades with the greatest totals seen at Mt. Baker. Strong S-SE winds also built unstable wind slabs on lee slopes primarily in the central and south Washington Cascades.

Clouds and cool weather with little to no new snow accumulation occurred Tuesday and Wednesday.

A large upper ridge and warm air mass built over the Northwest Thursday to Saturday. By Saturday temperatures have reached the 40's at Hurricane and at higher elevations especially along the west slopes. Cooler temperatures are seen Saturday at Stevens and Snoqualmie and at lower elevations along the east slopes.

Recent Observations

North

NWAC observer Lee Lazzara was in the Mt. Baker backcountry on Wednesday and observed a generally stable snowpack for the terrain he traveled through. Winds slabs were present, but generally unreactive near and above treeline. Wind slabs were found on the more traditional NW-E-SE aspects. The most recent storm snow was well bonded to the 1/17 rain crust in the Mt. Baker area. Several glide cracks from the rain event are still visible and at least one natural glide avalanche occurred last weekend.

Lee was out in the Mt Baker area on Grouse Ridge Thursday. In this area, there were less recent wind effects with the last storm cycle and therefore, wind slabs were not encountered in terrain up to about 5800 feet. The rain crust from 1/17 was generally buried about 1 foot and showed good bonding to the settled storm snow. No avalanches were seen or triggered, with nice settled powder conditions still providing good quality skiing. Lee noted widespread small surface hoar formed over the past few days and if buried by a light snowfall in showers may become a layer to watch out for with future loading.  Glide cracks were also seen on some slopes in this area below treeline.

Central

NWAC pro-observer Jeff Ward was on north slopes on Rock Mountain near Stevens Pass on Monday and found 50-65 cm of storm snow on the 1/17 crust with good stability and no recent avalanche activity.

A skier via the NWAC Observations page for the Source Lake-Bryant area on Monday reported 15-60 cm of storm snow on the 1/17 crust. Previous wind transport was evident at higher elevations. A skier triggered a large storm slab on a N-NE slope at about 4000 feet, which ran on the 1/17 crust. Natural and ski triggered, loose wet avalanches were also seen on solar slopes.

NWAC's Dennis D'Amico and Matt Schonwald were on Roaring Ridge in the Snoqualmie Pass area Tuesday. On N through E aspects at 4400 feet they found about 30 cm of right side up storm snow well bonded to the uneven and thick 1/17 freezing rain crust.

Dallas Glass was back in the Snoqualmie Zone Thursday in the Denny Mountain area. Older wind slabs responsible for many triggered avalanches early this week, had now stabilized significantly and were no longer deemed an avalanche problem in this terrain. This observation is supported by the numerous aggressively skied terrain features throughout the Alpental Valley over the past few days with little to no avalanche activity. In snowpit tests, Dallas did find well formed facets above the 1/17 crust and this layer produced reactive snowpit tests, especially on solar aspects. These preliminary observations are limited in understanding the distribution or sensitivity of this layer, so more investigation is planned.

A report on Turns All Year for the Snoqualmie Pass area indicated some good conditions still to be found on some north slopes.

South

The Crystal pro-patrol reported extremely sensitive new wind slabs on NW aspects Sunday morning 1/22. About 4-6 inches of new storm snow were transported by very strong S-SE winds overnight and early Sunday morning. This built very sensitive wind slabs on lee slopes by early Sunday. Wind slabs, mostly in the 6-8 inch range, released sympathetically and were running long distances. Shooting cracks were extending over 100 feet from skis.

By Monday and Tuesday the Crystal pro-patrol reported that previous reactive wind deposited snow had bonded and greatly stabilized with no new avalanches.

On Friday the Crystal pro-patrol had not observed any avalanche activity for several days. But sun breaks were greatly affecting snow on solar aspects.

Problems

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.

Aspects: South East, South, South West.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

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, Treeline.

Likelihood

Unlikely

Expected Size

1 - 1

Valid until: Jan 29th, 2017 10:00AM