Avalanche Forecast

Issued: Feb 8th, 2017 11:26AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

A mixed bag of avalanche problems should be expected on Thursday. Back country travel in avalanche terrain is not recommended through Thursday morning.

Summary

Detailed Forecast

A strong wet warm front will lift north over the Olympics and Cascades on Wednesday night to Thursday morning. Plan on rapid warming Wednesday night and heavy rain or snow with higher snow levels through Thursday morning. This should cause an avalanche cycle in the Olympics and Cascades which should test the snowpack. Small avalanches may step to deeper layers during the cycle.

A mixed bag of avalanche problems should expected on Thursday with wind slab and storm slab very likely at times in the above treeline band and loose wet avalanches very likely in the below treeline band with some over lap in between in the near treeline band. Back country travel in avalanche terrain is not recommended through Thursday morning.

Wet slab avalanches and cornice failures should also be likely in areas with heavy rain Wednesday night to Thursday morning.

The cold front should cross the Cascades Thursday late morning or midday. This should cause a change to showers and lowering snow levels. The avalanche danger should begin to decrease following the cold front Thursday afternoon.

  

Snowpack Discussion

Weather and Snowpack

A strong storm cycle was seen Friday through Monday with about 3-5 feet of snow recorded along the Cascade west slopes.

A slow warming trend affected all areas Saturday 2/4 with rain reaching 4000-4500 feet in the south Cascades. Cool easterly flow kept temperatures locally cool at Pass level at Stevens and Snoqualmie Pass through the day Saturday, with a warm-up Saturday midday following a switch to westerly flow. This created a weak crust layer that has now been found both at Stevens and Snoqualmie down about 3 feet. 

A nice day was seen Tuesday with light wind and sun or filtered sun and high clouds allowed for snowpack settlement and stabilization of the deep recent storm snow. Most NWAC snowdepth gages settled about 6 inches or more during the day Tuesday.

Increasing winds, snowfall and warming were generally seen on Wednesday as the next storm approached.

Recent Observations

North

On Tuesday, the Mt Baker pro-patrol reported ski triggered soft slab avalanches of 8-10 inches on average, though they were not propagating widely as the older deep storm snow was showing signs of settlement and stabilization. There was evidence of a widespread natural cycle, likely Monday night on Table Mountain, with many crowns estimated in the 2-4 foot range.

NWAC pro-observer Lee Lazzara was in the Glacier Creek area Tuesday, reporting numerous natural and ski triggered storm snow soft slabs, on average 40 cm (16") on steep test slopes on both southerly and northerly facing slopes. Another ski triggered slide was evident from a nearby party that may have been a bit deeper, about 2 feet. 

Central

Reports from both Snoqualmie and Stevens passes Tuesday indicated continued storm snow settlement and stabilization, in the near and below treeline terrain. The thin melt freeze crust formed midday Saturday, 2/4 produced positive snowpit results but no avalanches were produced. The storm snow is gaining strength, showing increasing resistance with depth and providing some excellent conditions.

NWAC pro-observer Jeremy Allyn was in the Henry Creek drainage below Jim Hill on Wednesday and reported that wind transport was underway due to the incoming storm. Quick tests of the near surface snow did not show significant results and ECT and PST tests on the 2/4 interface at 75 cm down did not indicate propagation.

NWAC pro-observers Jeff Ward at Skyline at Stevens and Ian Nicholson near Alpental on Wednesday did find significant signs of instability.

Reports via the NWAC Observations page for Mt Snoqualmie for Tuesday and Wednesday indicate several recent D1-2 slab avalanches on steep terrain features.

South

NWAC pro-observer Dallas Glass was in the Crystal backcountry in Bullion Basin Sunday and found a reactive storm layer 35-40 cm down in snowpit tests on all aspects below and near treeline. Shallower storm slabs could be triggered on test slopes. Storm snow from this cycle had settled to about 60-65 cm. No natural avalanches were observed and recent wind transport was localized to near the ridgeline.  

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.

Elevations: Treeline, Below Treeline.

Likelihood

Very Likely

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: All aspects.

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

Valid until: Feb 9th, 2017 11:26AM