Avalanche Forecast

Issued: Jan 27th, 2016 12:22PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Along the west slopes the main problem should continue to be loose-wet avalanches on Thursday. Watch for wet surface snow deeper than a few inches and pinwheels or roller balls that usually precede loose wet avalanches. The potential still exists of some isolated slab releases on the January persistent layers. It should also be generally awful snow conditions, so other activities might be in order. 

Summary

Detailed Forecast

A strong front will move across the region overnight Wednesday and early Thursday, causing moderate to heavy rain to high snow levels, about 7-8000 feet. Cooling and a change to snow showers is expected later Thursday with a change to light showers. It is hoped that the rain overnight Wednesday and early Thursday will trigger any remaining stubborn, persistent layers that may be lingering along the west slope areas. These layers can be tricky to forecast just how much load will be needed, but we will hopefully be hitting a reset button following the cooling later Thursday and beyond. We are continuing to list the early and mid January persistent slab problem as unlikely in the forecast until we get on the other side of this rain event and further observations confirm it is no longer an issue.

Previous wind slab in the near and above treeline may be loaded and weakened by wet surface snow. This avalanche problem may be getting unlikely, but watch for signs of previous firmer transported snow mainly on northwest to southeast aspects.

Also avoid slopes below buckling snow and glide cracks which can release at any time without warning.

Rain should change to snow gradually through the day Thursday and diminish. 

 

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 with freezing levels approaching 8000 feet with rain in the Mt Baker area has caused increasing wet snow conditions by late Wednesday afternoon.

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 unsupportive 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. 

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: All aspects.

Elevations: All elevations.

Likelihood

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: North, North East, East, South East, North West.

Elevations: Alpine.

Likelihood

Unlikely

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 28th, 2016 12:22PM