Avalanche Forecast

Issued: Jan 31st, 2019 10:14AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Heavy snowfall, rain, and wind will create dangerous avalanche conditions Friday. At mid and upper elevations, natural avalanches will be possible and human triggered avalanche will be likely. Avoid travel in and below avalanche terrain during periods of heavy precipitation.

Summary

Discussion

Snow and Avalanche Discussion

A week-long stretch of dry, spring-like weather is coming to an end. Beginning Thursday night, a wet and potent storm will bring a mix of snow, rain, and strong southwest winds. Uncertainty exists as to how much precipitation we will receive. The forecast is calling for at least 1.5in of water with 4500-5000ft snow levels over the next 24hrs. Several inches of snow are expected to accumulate above 5000ft, while below, wet snow and rain will fall. The bulk of the precipitation will come during the day Friday. Avalanche hazard will increase throughout the day and likely peak Friday afternoon. Avalanches will be likely at upper elevations where the most snow accumulates. Avoid avalanche path runout zones, even at lower elevations. 

The new snow is not expected to bond well to variable old snow surfaces. Over the past four days, observers in the Baker Lake, Heather Meadows, and Cascade River road areas found snow surfaces to be weak. Sugary facets formed near the surface - above and below a crust. This weak snow has developed on east, north, and west aspects near and above treeline. Slick melt-freeze crusts exist on south aspects. Mid and lower snowpack layers are strong. The snowpack below 3500ft is shallow, icy and firm. 

Snowpack Discussion

January, 31, 2019

As we turn the corner to February we're coming out of a week-long high pressure ridge and into unsettled weather. The snowpack survived extremely warm temperatures and sunny skies over the week with minimal new wet avalanche activity reported. This break in the weather allowed for avalanche danger to steadily decline in all regions. With stormy weather, the danger is once again elevated.

We’ve heard a variety of stories from backcountry travelers over the past week. There have been reports of extremely firm slopes creating slide-for-life conditions. Others reported perfect spring like snow. Some encountered difficult breakable crust. And, for a lucky few, softer, drier, mid-winter snow has been found. A common thread in most zones is where precipitation falls as snow, it likely will be landing on slick surfaces. It's time to pay attention to the new old interface formed by our most recent storm.

North-South:

While a high elevation rain event, around January 23rd, formed surface crust in many regions, it’s the constant melt-freeze cycles from the past week, that caused a divergence in the Northerly and Southerly snowpacks.

North: On shady slopes, things haven’t exactly been soft. The crust formed at the end of the last storm extends to high elevations (Mt. Hood 7000+ft, South Cascades 6500 ft, Passes and Central Cascades 6000 ft. and West-North 5500 ft.). Only areas in the East Cascades seemed to escape the wrath of this breakable crust. Without the help of the sun, shady slopes haven’t been softening even during this period of warm weather. Instead, the surface crust underwent some weakening. Observations found faceting on top of and below this crust. In some locations, this caused the crust to begin to degrade, becoming less supportive. Surface hoar has also been reported from the typical valley bottoms and sheltered terrain near water sources. At low elevations, rain may melt any weak snow on the surface. Slopes receiving significant dry snow should be suspect for a poor bond at eh interface buried on Thursday night.

South: On sunny aspects, the sun drove warming and melting of surface snow. Long, cool, winter nights allowed for the surface to freeze again. This repeating melt-freeze pattern created a thicker, firmer, and more supportable surface. On many days, weak surface snow, such as near surface facets or surface hoar, melted during the day limiting its development. On cooler days, very firm travel conditions were reported. Crusts may provide a poor bond for any snow falling on them.

 

Near surface facets developed on shaded snow surfaces in many areas. These facets are from near Mt Baker. Photo: Lee Lazzara

East-West:It’s not uncommon for our east-side forecast zones to experience lingering persistent weak layers (PWL’s). This season, we’ve also seen several different PWL’s in our western zones. This break in the weather gave the snowpack time to gain strength in all zones.

West: While you may find some weak snow in the upper few inches of the snowpack, the mid and lower snowpack has been found to be quite strong. Firm rounded grains, stout  crust, and strong frozen melt-forms make up the majority of the snowpack at this time.

East: The east-side snowpack continues to be highly variable. You may find deep strong snowpacks closer to the crest or you could encounter shallow weak snowpacks areas further east. While there are number of potentially weak interfaces, there are two more common layers we’ve got our eyes on.

  1. January 22nd surface hoar and small facets. You can find these just under the recent storm snow, about a foot down. .

  2. December 26th surface hoar. This layer can be found from 16” to 40” down and is still producing clean, planar shears with tests.

You are most likely to find these layers to be preserved on wind sheltered, shady, and open slopes above 5,500ft. You can find more defined weak layers where snowpack is less than 4 feet deep and variable especially east of Highway 97. Persistent weak layers have been “dormant” or unreactive during the week of high pressure. They may or may not become reactive with the next storm. As new snow accumulates, it will begin stressing these weak layers. We’ll keep tracking them to watch their progression..

The lower eastern slopes and the Columbia River. Snow exists at low elevations, but snowpack depths are shallow. Photo: Matt Primomo

 

Problems

Storm Slabs

An icon showing Storm Slabs

New snow and wind will form fresh slabs near and above treeline. Expect dangerous conditions to develop by the time 6in of new snow accumulates. Thicker and more reactive slabs will form in areas where the wind drifts new snow. The most dangerous conditions will exist at upper elevations where cooler temperatures will allow more snow to accumulate. Monitor how the new snow is bonding to itself and to old snow surfaces. Watch for shooting cracks and collapses within the storm snow as you travel. Look for sudden changes in snow surface texture and hardness to identify wind-drifted areas of snow. Give the new snow time to stabilize. Avoid avalanche terrain during periods of intense precipitation. Seek out lower-angled and supported terrain away from avalanche path runout zones.

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

Likely

Expected Size

1 - 1

Loose Wet

An icon showing Loose Wet

Loose wet avalanches may occur near and below treeline where a mix of wet heavy snow accumulates, and where rain falls on new snow. Slick and firm old snow surfaces may serve a productive bed surface for wet avalanches involving new snow to slide. These avalanches may entrain significant amounts of snow as they run downslope. They could be hazardous in confined terrain where debris from even a small avalanche could pile up. Avoid steep slopes where you find wet unconsolidated snow surfaces. Watch for roller balls and pinwheels as good indicators that wet avalanches may follow.

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

Possible

Expected Size

1 - 1

Valid until: Feb 1st, 2019 10:14AM