Avalanche Forecast

Issued: Jan 28th, 2016 11:45PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The main problem Friday is expected to be wind slab in the above and near treeline zones. Wet snow conditions may be slow to refreeze and strengthen at lower elevations.  

Summary

Detailed Forecast

Another front should arrive later Friday, bringing additional moderate snowfall at lower snow levels. Winds should be strong, especially above treeline. The main concern Friday will be newly forming wind slab on lee exposed slopes, especially NW-N-SE facing near and above treeline. 

With cooling temperatures, the new snow should have a favorable profile, but may mask older unstable storm slabs.

 

Snowpack Discussion

Weather/Snowpack

Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on January 3rd and 11th.

A warm front last Thursday caused heavy rain on the Washington volcanoes. Heavy snow changed to lighter freezing rain and rain in the passes. This caused an avalanche cycle and consolidation along the west slopes.

This was followed by cooler weather and some snow. NWAC stations along the west slopes indicate about 3-6 inches for the 2 days ending Sunday morning.

Mild weather, freezing levels approaching 8000 feet and rain in the Mt Baker area caused increasing wet snow conditions. Heavy rain Thursday changed to snow adding additional load.

Snow levels dropped through the day Thursday as temperatures lowered some 10 degrees or more through the day. This has changed rain to snow gradually, likely allowing a favorable bond to form between new snow and the old wet snow surface.

Recent Observations

NWAC pro-observer Lee Lazzara was near the Mt Baker Ski Area on Sunday and found that the rain late last week had percolated to at least 1 meter. He did not find signs of the January 3rd or 11th persistent layers and did not get significant stability test results. He noted glide cracks on some slopes.

NWAC pro-observer Ian Nicholson was at Skyline Ridge at Stevens Pass on Sunday and at 5000 feet, he found that the upper snowpack crystals were rounding and stabilizing. He found the January 3rd and 11th layers at 85 and 115 cm are surviving, but slowly breaking down and were not reactive.

NWAC pro-observer Dallas Glass was at Paradise on Sunday and saw evidence of wind transport and solar effects which were limited by clouds in the afternoon.

A private report and a report via the NWAC Observations page indicate a loose wet avalanche cycle occurred in the Alpental Valley on Sunday. Another report via the NWAC Observations page noted loose wet activity at Stevens on Monday.

NWAC pro-observer Dallas Glass was out again in the Alpental Valley Monday. He reports large roller balls and more loose-wet and some wet slab avalanches on W, solar and E aspects. He also noted buckling snow and widening glide cracks, but without releases. He identified the January 3rd and 11th layers on a north slope at 4700 feet, but the layers did not indicate reactive test results.

Large glide or climax avalanches continue to be a concern, especially with unsupported terrain features, such as large rock faces. One such release occurred Wednesday afternoon, Jan. 27th, in the Stevens Pass area, a size 2.5 climax slide released from an unsupported rock face at about 5600 feet on a NE aspect. Fortunately, the terrain was closed and no one was on the slope below. 

With the heavy rain falling Thursday, no new observations were reported.

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, Treeline.

Likelihood

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: Below Treeline.

Likelihood

Possible

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 - 2

Valid until: Jan 29th, 2016 11:45PM