Avalanche Forecast

Issued: Jan 21st, 2016 10:33AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Loose wet avalanches should be the main avalanche problem Friday with plenty of wet snow in the upper snowpack. Natural wet loose avalanches are possible but human triggered wet loose avalanches should be likely on steeper slopes. Be especially wary near terrain traps, where even a small avalanche could have unintended consequences. Travel conservatively Friday until more information is known about lingering persistent weak layers across the west slopes and Passes.

Summary

Detailed Forecast

Elevated snow levels and light to moderate rain, except heavy at Mt. Baker, should continue for areas away from the Cascade Passes. The Passes may continue to flirt with light snow, rain or mixed precipitation Thursday night. 

Steadier precipitation should transition to light to moderate showers with snow levels falling to 4500 feet by Friday afternoon. Light amounts of new snow should bond well to the wet and refreezing surface on Friday. 

Persistent weak layers received a good test over the last 48 hours. We will continue to list persistent slab as a problem until more information becomes available, but it's likely that PWLs have been eliminated from the volcanoes due to warmer and wetter conditions, but may still linger in the Cascade Passes although more difficult for a human to trigger. 

Loose wet avalanches should be the main avalanche problem Friday with plenty of wet snow in the upper snowpack. Natural wet loose avalanches are possible but human triggered wet loose avalanches should be likely on steeper slopes. Be especially wary near terrain traps, where even a small avalanche could have unintended consequences.

Storm slab avalanches are possible in the Cascade Passes and higher elevations that only saw a brief period of rain. It's difficult to predict the likelihood of storm slabs in the Passes given the potential for different precipitation types and crust layers. New wind slab may begin to build on lee easterly slopes above treeline Friday but should be fairly shallow. 

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 January 3rd and 11th. One or both of these layers have been seen in snow pits by NWAC pro observers and avalanche professionals the past few days ranging from Mt Baker to the Crystal Mountain area. These persistent weak layers are most likely to be found on non-solar (northerly) aspects in the below and near-treeline band. However, we cannot rule out the above treeline elevation bands so the problem is listed for all three bands. We will have to see how the PWLs survived the most recent storm cycle. 

A progressive pattern brought a series of storms to the Northwest with several feet of new accumulating along the west slopes. The most recent frontal system brought up to a foot of snow late Wednesday night through early Thursday morning. 

Warmer air brought a quick change to rain for areas away from the Passes like Mt. Baker, Paradise and Crystal. 2 to 4 inches of water has fallen over the west slopes in the last 48 hours through 5 pm Thursday, with the heaviest rainfall in the Mt. Baker area. The Passes were slower to warm on Thursday. Freezing rain was reported up to 5000 feet by Alpental pro-patrol by 9 am Thursday morning. 

Snow and Avalanche Observations

NWAC Pro observer Dallas Glass found the buried surface hoar layer well intact 45-50 cm down on a west aspect of Kendall Peak Saturday. Backcountry rangers at Paradise reported buried surface hoar in snow pits at 5900 feet last weekend. Earlier in the week, Alpental pro-patrol identified the two buried weak layers down 70 and 100 cm between 3000-4000' but have not seen any recent avalanches release to these layers. 

NWAC pro observer Ian Nicholson at Yodelin on Monday found both the 1/3 and 1/11 surface hoar layers buried about 40 and 75 cm respectively on north aspects, within the below treeline elevation band. Tests indicated propagation was likely, with the deeper layer slightly more reactive.

NWAC observer Lee Lazzara was in the Canyon Creek area of Mt. Baker and found buried surface hoar 55 cm down (~2 ft) around the 5500 ft level and likely to propagate in column tests. Lee also observed a snowmobile triggered slide from a distance that broke 500' feet across the slope near treeline. This large slab avalanche likely released on the buried surface hoar layer. Luckily no one was caught. 

Observations were limited on Thursday due to the storm and poor visibility, but Stevens Pass pro-patrol reported widespread and sensitive 14-16" storm slabs easily triggered by ski cutting or explosives Thursday AM. Alpental pro-patrol reported similar sensitivity with 8-10" storm slabs Thursday morning. Below 4000' at Alpental, the snow became moist and loose wet avalanches entrained surface snow. At Heather Meadows (Mt. Baker), steady rain had percolated several feet down into the snowpack at 4200' by mid-day Thursday. 

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

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

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

Possible

Expected Size

1 - 1

Valid until: Jan 22nd, 2016 10:33AM