Avalanche Forecast

Issued: Mar 7th, 2019 10:26PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

UPDATE: Forecast changes were made to reflect overnight weather and other new information.

The West-North zone picked up a respectable amount of snow during this storm. Expect avalanche conditions to be more dangerous Friday, especially at higher elevation where more snow accumulated. This is a good time simply avoid open slopes greater than 35 degrees and allow this new snow time to bond. As the sun comes out, look out for loose avalanches naturally occurring on steep sunny slopes. 

Summary

Discussion

Snow and Avalanche Discussion

The West-North zone rung-out around a foot of new snow with this weather system. Observations from Thursday found the new snow loose and unconsolidated until late afternoon when increasing winds and snowfall rates created subtle slabs in some locations. Avalanches were limited to very steep convex slopes where the wind drifted the snow into slightly firmer slabs.

This storm system largely wraps up for the West North area Thursday evening, but you could find yourself underneath a passing shower. This leads to some uncertainty with the avalanche forecast. If you experience more snow than the weather forecast suggests, you may be dealing with locally hazardous avalanche conditions. The new snow buried a variety of old snow surfaces. Including weak facets, settled powder, and firm crust. If you find a location where the new snow rest over facets and a crust, avalanches may act in unusual ways. You could see wide propagation and/or thin crowns.

A shovel tilt-test highlights the interface between the old and new snow. You may find old weak facets at this interface in some locations. Photo: Andrew Kiefer.

Snowpack Discussion

March 4, 2019

Here we are at the beginning of March. The days are noticeably longer, the sun feels warmer, but the winter is far from over. This may be a good time to take stock of February’s events, where we are now, and what we might expect in the future.

A February to Remember

Many longtime Cascade travelers continue to remark about the long span of high-quality travel conditions in February. Indeed, we’ve experienced three weeks of very cold and snowy conditions. Backcountry travelers frequently reported, “best conditions in years”. February also saw a few rounds of natural and human triggered avalanches. These avalanches were different from our more frequent Cascade storm slabs; here today and gone tomorrow. Most avalanches failed on buried weak persistent layers. As such, most zones spent much of February with Persistent or Deep Persistent Slabs in their forecasts.

Natural avalanche in Icicle Creek Feb 13, 2019. Photo: Matt Primomo

With the notable absence of high elevation rain events, snowpacks around the northwest grew remarkably. On the west slope, most weather stations added 2-4 ft of snow depth. East-side locations added 1-2 ft. Mt Hood locations added 4-5 ft. Even with these impressive snowpack growths, many Cascade Snotel sites are recording near or just below average snow depths for the winter.  

During most of February, snow conditions were soft and powdery making for fun recreating in many areas. Photo Matt Primomo.

Where We Are Now

Lack of avalanche activity on the layer of buried facets from Feb 8 allows us to gain confidence in many areas. In many areas, there has been a lack of avalanche activity and snowpack tests have been less reactive. These two pieces combined have allowed us to drop persistent slabs from some forecasts and shift others to “unlikely.” The exception is the East Cascades where a shallow snowpack has preserved buried facets, and persistent slabs will remain possible until the sensitivity drops further. So, we’re out to the woods, right? Well, maybe. The same cold and stormy weather that brought us excellent snow quality, also allowed us to bury new weak layers. So far we haven’t confirmed any avalanches on these shallow weak layers, but they have our attention and we’re tracking them.

Recently our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow. Several wind events, predominantly from the east, placed wind slabs on a variety of aspect. With the colder than normal temperatures, wind slabs may not heal as quickly as normal.

A small natural wind slab avalanche in the Crystal Mt backcountry. Photo; Dallas Glass

What Might We Expect

As we move into March, it’s anyone’s guess what specific weather patterns lay in store of us. However, there are two items that stick out in our minds.

  • The strength of the March sun: You can already feel it just walking around town. As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character. 

  • High elevation travel: Frequently March begins to usher in a period where we push higher and deeper into the mountains. Remember, we don’t have as much information about these far-flung locations. If you use the longer days of March to travel to bigger objectives, keep your eyes open. When observations don’t line-up with the avalanche advisory, you may be experiencing a different snowpack. It’s times like these we need to stop and reevaluate.

Mt Baker on a nice day in early February. Photo: Lee Lazzara

February was amazing! but March is here… there’s still plenty of winter left. Stay safe out there.

Problems

Storm Slabs

An icon showing Storm Slabs

The West-North zone received enough new snow during this storm to really catch our attention. You'll most likely find storm slabs in locations that received more than 8-10 inches of snow, or where affected by the wind. Look for signs of storm slabs as you travel. Do you see cracking in the snow as you break trail? When you dig in the surface snow with your hand, do you find stronger snow over weak snow? Do you see signs of wind affected snow? When you answer yes, avoid open slopes greater than 35 degrees. 

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: All elevations.

Likelihood

Likely

Expected Size

1 - 1

Loose Wet

An icon showing Loose Wet

When the sun comes out expect the snow to change quickly. Natural loose wet avalanche may occur on steep rocky sunny slopes. Don't linger under southerly aspects, and be leery of traveling in areas where avalanches may burry you deeply. With all the new snow some of these avalanches may grow large. 

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

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 8th, 2019 10:26PM