Rain, snow and wind will slightly raise the avalanche danger on Wednesday. Think about new snow avalanche problems at higher elevations and wet loose avalanche potential near the snow/rain line and on northerly aspects that still harbor dry snow. If the storm ramps up more than expected Wednesday, dial back your terrain choices.
Fresh wind slabs may form above treeline Wednesday. Approach steep unsupported slopes with wind-drifted snow cautiously, feeling for firm or hollow sounding snow as a sign that wind slabs may be present. You can stay safe by traveling on ridges, wind-scoured areas and any slope less than 35 degrees.
Rain will increase the likelihood for small wet loose avalanches Wednesday, especially on steep slopes on northerly aspects that still have dry snow and have not previously avalanched or in elevations that oscillate between the rain/snow line. As snow levels fall during the day, the wet loose potential will decrease at higher elevations as snow surfaces firm up.
Monitor the surface snow throughout the day and change your aspect and move to lower angled terrain if you find wet surface snow deeper than 6”. Be especially cognizant of the consequences of even a small avalanche around terrain traps like gullies or cliff bands.
Snow and Avalanche Discussion
A storm system will bring wind, rain and some new snow to the Mt. Hood area Tuesday night and Wednesday. After our prolonged March warm-up, the snowpack should be well suited to handle the rain but wet loose avalanches are still possible in isolated areas. Shallow wind slabs may form on lee slopes at higher elevations if snow levels lower during the day as forecast. Make the mental transition from the recent dry and mild weather to think about new snow concerns if traveling at higher elevations Wednesday.
Snow cover below 4000' is disappearing quickly and south facing terrain near ridgelines are melting out as well. The mountains are still experiencing their spring transition. As a result, you may find a wide variety of snow surfaces, including breakable crusts, firm icy surfaces, and wet heavy snow. During this transition remember to keep a lookout for potential springtime hazards such as opening creeks and glide cracks. Use caution if you travel near these features.
March 29nd, 2019
Peak snowpack, spring strategy, and the scale of loose wet avalanches
Peak Snowpack
Now that March is winding down, let's take a look at the snowpack on a seasonal scale. One good question to help put things into perspective is: “Have we hit peak snowpack depths?” This question is easily answered by, “It depends”. Have we hit peak snowpack for lowland areas? Yes, that occurred in the last month at some point. Have we hit peak snowpack depths for mountain locations at around 6,000ft? Most likely, but that could still change. You can see in the graph below that we have a range in the date of peak snowpack depth by location. This is most likely due to a variation in storm tracks, and elevation differences.
Mount Baker, Heather Meadows (4,210ft): February 13 with 154”
Washington Pass, Base (5,450ft): February 13 with 76”
Stevens Pass, Schmidt Haus (3950ft): February 23 with 92”
Paradise at Mount Rainier National Park (5,400ft): Tied for February 20 and March 13 with 159”
Snowpack depth in inches over time from early December 2018 through March 29, 2019.
You can see that the last substantial winter storm to hit the state was in mid-March, since then we’ve been on the general decline in snowpack depths. Initially, this timeframe started with very warm temperatures and strong sun for over a week. The snowpack began to warm up, and at lower elevations, it melted. This brought on a prolonged cycle of wet loose avalanching, with many slides gouging deep, into older snow. Since then, temperatures have moderated, and the melt has slowed, but not stopped. This early-spring interlude looks to continue for the short term, but April may bring a shift in the weather pattern.
Spring Strategy
With mostly stable weather and a mostly stable snowpack, many folks have shifted their backcountry travel strategy to what may be referred to as the “Spring Diurnal Mindset”. Check the link to the paper by Roger Atkins on the most excellent “Strategic Mindset”. During a Spring Diurnal mindset, professional ski guide operations typically check to make sure an adequate overnight refreeze has occurred. Once a slope thaws, avalanche terrain is avoided on that aspect, and the group shifts to a different aspect or calls it a day. This is because the trend of avalanche danger, along with other mountain hazards (such as cornice fall) during this type of pattern tends to fluctuate. At night, the sun drops below the horizon, the air temperatures cool, and the snowpack refreezes. In the morning, the sun rises and hits the slopes, and the crust that has been thickened over a period of similar days begins to melt and become weaker. Prime recreating on sun softened slopes can occur when you find that balance of melted snow on top, but still some frozen crust underneath. The “corn harvest” occurs when you have a supportable crust beneath this sun-softened snow. When that crust is fully melted, and the snowpack underneath contains wet, saturated snow, the snowpack becomes weak and potentially dangerous. Wet loose avalanches may be initiated, along with wet slabs if there are weak layers lingering in the pack.
The sun’s energy (shown here as Watts per meter squared) is much stronger than it was earlier in the season, as such, it’s influence on the snowpack is substantial. The increasing daily would trend look a bit more consistent if it were not for cloud cover. From the Paradise Wind Site at 5,380ft in Mount Rainier National Park.
The Scale of Loose Wet Avalanches
As mentioned, the initial pronounced warmup of the season changed our snowpack from a cold, dry, winter pack to a spring-like one. During this time (March 15-22) the mountains went through a prolonged period of wet loose avalanching. Many “gouging” wet loose avalanches were observed. These got to be large and destructive on some big terrain features. Eventually, the temperatures cooled, and luckily, most of the damage was done.
Large, gouging wet loose avalanches ran in a large south facing gully near Mount Cashmere. March 20, 2019.
Chunky, old debris from a wet loose avalanche that gouged into older snow above Eightmile Lake. This slide ran over a 1,000ft vertical and broke some small trees. March 27, 2019.
Most recently, we have been experiencing a daily refreeze of the surface, which has put a freeze (get it? hahaha) on the wet loose cycle. With this thicker and more stout surface crust, anytime we receive more than a few inches of snowfall and it lands on top, it will be time to start thinking “wet loose avalanche” all over again. The strong sun of spring won’t take long to begin changing the snow on the surface. These “superficial” wet loose avalanches tend to be fairly predictable in nature, and direct evidence such as rollerballs, moist snow surfaces, and small loose wet slides alert us to reactive conditions.
With both flavors of wet loose avalanches (gouging wet loose, and superficial wet loose) the scale of the slope is important. The larger the slope, the larger these slides may become.
Small wet loose avalanches that ran on the recently buried crust, on a fairly small slope. March 26, 2019.
Wet loose slide from the same day. A larger slope allowed a larger avalanche to run, even though it ran on top of the same crust. March 26, 2019.
The High North
It is still winter on the high elevation, north facing slopes. Here, cold, dry snow can be found. In some areas above 6,000ft faceted snow may even be found on the surface. This is something to keep in mind when this lovely spring “interlude” breaks, and the storm track inevitably shifts back to our direction.
