Avalanche Forecast

Issued: Mar 16th, 2019 11:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Warming temperatures and sunny skies will continue for Sunday. Expect loose wet avalanches at low elevations and on sun-exposed slopes as temperatures warm through the day. In areas with less than 4 feet of snow on the ground, you could trigger the entire snowpack.

Summary

Discussion

Snow and Avalanche Discussion:

Avalanche concerns will be minimal first thing in the morning but will increase by afternoon. Warm temperatures and sun are the main drivers of our avalanche concerns. Increased daytime warming is making each day the warmest we've seen since January. Though the snowpack hasn't made the transition to spring, keep typical spring concerns on your mind.  Important considerations include wet snow, cornice falls, and timing your travel with progressively warming snow surfaces. 

The snow buried a mix of sun crusts on southerly aspects and facets and surface hoar on northerlies. Observers reported wet loose avalanches the past few days on a number of sun-exposed slopes, some big enough to bury or kill a person. On Saturday, WSDOT forecasters reported small wet avalanches on a north aspect at 2500ft in Tumwater Canyon on. On Thursday, an observer reported a cycle of large avalanches (possibly occurring on the 12th) in Swakane Canyon on northerly aspects, running on the ground. 

Blewett Pass only picked up an inch of new snow. The wind formed reactive slabs much deeper than that with new and old snow. 3/13, Photo: Matt Primomo

Snowpack Discussion

March 14, 2019

It’s starting to feel a lot like spring. The sun is stronger. Temperatures are warmer. The days are longer. Each year March brings a period of transition for the snowpack. Despite the feel of spring, this transition could take quite a while. Right now, its still winter in the mountains and most of the snowpack is cold and dry.

A northeast through southeast facing bowl in Tunnel Creek, Stevens Pass. The red line shows the transition from northeast to east aspects. Crusts were on the surface of east aspects (right of the line). Surface snow on northeast was soft and dry. March 11th, 2019. Photo: Josh Hirshberg

Aspect dependent snowpacks

A common trend this time of year is the distinct difference between snowpacks on shaded slopes and those on sunny slopes. We might refer to this as aspect dependent snowpacks. The strong March sun can create crusts and send melt-water through the snowpack on sunny slopes.

That process is beginning, right now. As of the 14th, sun-exposed aspects have a crust at the surface. Otherwise, these slopes hold mostly dry layers in the upper 3 feet of snow. The sun now affects more aspects than in the earlier half of the winter. You may find sun affected and melt-freeze crust on any slope that received direct or indirect sunshine. This could include some northwest or even northeast aspects.

Shaded slopes are still holding onto cold winter like snowpacks. That doesn’t mean you won’t find any melt-forms in these areas, but the snowpack looks, feels, and acts more like mid-winter. Avalanche problems can follow a similar trend by aspect. You may see wet avalanche problems on sunny aspects and lingering dry slab problems on shaded slopes.

Two photos taken on March 10th in the Snow Lake area. The left is N facing terrain on Chair Peak with cold, dry surface snow. The right photo shows loose wet avalanches on E facing slopes of Mount Roosevelt. Photo: Ian Nicholson

Balancing problems

So, how do we balance these sometimes opposing snowpacks? In a word, observations. Transitional periods like this require diligence and planning. It seems obvious, but observations you make on a south-facing slope aren’t applicable to N facing terrain. Consider this as you plan your day. Secondly, you’ll want to make observations to verify the avalanche forecast. As we’ve discussed, snowpack structures can diverge during these transitional periods. When your observations line-up with the information you find in the daily avalanche forecast, the forecast is likely valid. If you see observations that don’t match, it’s time to stop, reevaluate, and possibly choose more conservative terrain options.

Making snowpack observations to verify the forecast. Photo: Pete Durr

Less elevated danger and changing trends

You may not find low avalanche danger on every avalanche forecast, but spring is a time of year when less significant hazards can allow backcountry travelers to explore deeper into the mountains. The longer days and often lower avalanche hazard can support stepping into bigger terrain. Remember that low danger doesn’t mean "no danger." Additionally, avalanche conditions can change quickly during spring due to rapid warming over the course of a day or snowfall from potent storms.

We forecast for very large regions. You travel on individual slopes. You may encounter anomalies in the landscape where conditions don’t match the forecast. As with any time you travel in snowy mountains, you can stay safe by using standard travel protocols to minimize your exposure to avalanche terrain and the hazards of the day. Keep your eyes open and be ready to adapt to changing conditions.

What's next?

Winter is far from over. Keep checking the forecast, tracking conditions, making observations, and telling us what you find.

Problems

Loose Wet

An icon showing Loose Wet

Avalanches may begin on east and southeasterly aspects early in the day and continue onto southwest aspects by afternoon. Early in the warming trend, wet avalanches occurred first on the steepest, most directly solar aspects (SE-SW). With each warm day, you may see avalanches continue at higher elevations and on east, west, and even onto northerly aspects at low elevations. Look for indicators such as wet, heavy surface snow, rollerballs, and fan-shaped avalanches. If you notice these clues, avoid going on or under steep sunny slopes.

The cloud cover has been an ongoing challenge to predict this week. The loose wet avalanche cycle has been prolonged and slow to run its course. Many slopes have already produced wet avalanches. Take note of how much the sun is affecting slopes by aspect, elevation, and slope angle and which slopes may be most problematic on a given day.

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

Persistent Slabs

An icon showing Persistent Slabs

At lower elevations, where the snowpack is shallow, a poor structure can be found with facets underneath a crust from early February. The shallow snowpack is weak, especially on shaded aspects. In the foothills, facets can be found beneath February's storm snow, near the ground. These deeper slabs are most concerning on isolated, shaded slopes where stronger snow may rest over facets. At upper elevations, these layers are generally not a concern anymore. 

If you find yourself in an area that appears to have a shallow snowpack, stick to well-supported slopes and avoid steep convex rolls. Travel one at a time from well defined safe zones when moving through steeper terrain. If you find signs of instability such as cracking and collapsing, avoid traveling on or near slopes steeper than 35 degrees. 

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 - 1

Valid until: Mar 17th, 2019 11:00AM