Avalanche Forecast

Issued: Jan 25th, 2016 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

A variety of avalanche problems seems possible on Tuesday. Take time to be familiar with the forecast for the area you wish to visit.

Summary

Detailed Forecast

Light to moderate rain or snow with pretty high snow levels should seen in most of the Olympics and Cascades Tuesday.

A variety of problems seems possible along the east slopes on Tuesday.

In the northeast zone where it may stay cool enough for new snow on Tuesday new shallow wind slab and new shallow storm slab may be the most likely avalanche problems. Watch for firmer wind transported snow that is a sign of wind slab and rapidly accumulating snow deeper than a few inches that is a sign of storm slab.

In the central east and southeast zones the main problem may be loose wet avalanches on Tuesday. Watch for wet surface snow deeper than a few inches and pinwheels or roller balls that usually precede loose wet avalanches. This may be most likely on non solar slopes holding loose snow but will be indicated on all aspects.

Previous wind slab mainly in the near and above treeline may also be loaded and weakened by rain or wet snow. This should be mainly on northwest to southeast aspects.

We will continue to list the January 3rd and 11th persistent slab problem as unlikely in the forecast until further observations show we can confidently put the problem behind us.

Snowpack Discussion

Weather

Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th and were reported throughout the Cascade east slopes. There were many reports of triggering of these layers along the east slopes through last week.

A warm front last Thursday caused light or moderate amounts of snow which changed to freezing rain or rain east of the crest.

This was followed by cooler weather and some snow. NWAC stations along the east slopes indicate up to about 4 inches for the 2 days ending this morning.

Snow and Avalanche Observations

A report here on Turns All Year for the Clara Lake area near Mission Ridge on January 17th is dramatic.

See the NWAC YouTube page here for videos from Blewett Pass and Icicle Creek for January 18th.

NCMG guides travelling adjacent to Delancey Ridge Friday saw widespread evidence of the recent natural cycle with numerous crowns visible throughout the region. Many crowns were estimated to be about 1 meter.

NWAC pro-observer Tom Curtis was on Iron Mountain near Blewett Pass Sunday and found the January 11th layer 35-70 cm down on NW-SE aspects in the below and near treeline bands. He found that the buried surface hoar crystals are rounding and test gave low quality results and did not indicate propagation.

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: Alpine, Treeline.

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, Treeline.

Likelihood

Likely

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

Possible

Expected Size

1 - 2

Valid until: Jan 26th, 2016 10:00AM