Avalanche Forecast

A wide range of snowfall amounts accumulated across the area on Tuesday through early Wednesday. The main question, however, is what did the winds do to the upper elevations? Plenty of low density, cold snow was available for transport prior to Tuesday, as well as the recent new snow. Very strong winds through Tuesday likely built reactive wind slabs at the higher and more exposed areas. 

The recent storm buried a mix of sun crusts on southerly aspects and weak, faceted snow on northerlies. Higher avalanche hazard may be found where thick wind slabs overlie weak, faceted snow.

The past several days, the March sunshine is beginning to transition surface snow conditions. Areas that picked up more than a few inches of recent snowfall may expect Loose Wet avalanche activity another day and there is the potential for recently formed wind slabs to be triggered by Loose Wet avalanches or become more reactive during periods of snow surface warming.

As you travel, note signs of rapidly changing snow conditions, roller balls, fan-shaped avalanche debris or wind-affected snow surfaces. These are all signs to step back and re-evaluate your plan. Adjusting your slope angle to mitigate wet avalanche problems and choose lower elevation and wind-protected terrain if you start noticing signs of wind effect or strong over weak snow layers.

In shallower snowpack areas, you are more likely to find and potentially trigger a persistent slab avalanche within the top 1 to 2 feet of the snowpack. A public observation of a human-triggered slab avalanche came from a north aspect at 3200' just north of the Columbia River (Stacker Butte). We don't know what layer the slab avalanche failed on and the report is now several days old, but it's a clear reminder of weaknesses that remain in the upper snowpack. If you find signs of instability such as cracking and collapsing in shallower snowpack areas, dial back your terrain selection to lower-angled and well-supported slopes.

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, the transition to a spring snowpack could take quite a while. Right now, it’s still winter in the mountains.

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.