A spring storm is creating the potential for wind slab avalanches above treeline and loose wet avalanches at mid elevations. Heightened avalanche danger exists above treeline. Watch for localized drifts near ridges on slopes over 35 degrees.
New snow and moderate southwesterly winds could form small wind slabs above treeline. Drifts will be small and localized near ridges and on leeward sides of terrain features. Use small test slopes to check how reactive wind slab avalanches are and how well the new snow is bonded to old firm surfaces. Steer around drifts on slopes steeper than 35 degrees.
Wet avalanches may be most problematic where rain falls on slopes harboring previously dry layers in the upper snowpack, like steep, shaded slopes near treeline. The rain/snow line should be close to 6,000ft with 0.5" of rain possible on the upper end of the forecast. This could be just enough to cause some small slides but doesn't raise a huge concern for a major wet avalanche cycle.
On Wednesday, you may find moist or wet snow surfaces on most slopes. If rain falls on new snow at upper elevations, wet avalanches could occur there, as well. Watch for isolated wet avalanches and areas of unsupportive wet snow at low and mid elevations.
A moderate to weak spring storm is driving today's avalanches conditions. Snow, rain, and moderate winds will all contribute to avalanche problems. On Tuesday, an observer in the Henry Creek area reported no new avalanches and crusts on most low to mid-elevation slopes. Northerly aspects below 4,100ft were capped with a crust about 8-10" thick and meltwater had reached the whole snowpack. Crusts were generally supportive to boots, except near trees and where rocks had melted holes in the snow. Warm spring weather has taken a toll on the snowpack over the past couple weeks. The snowpack is shrinking, low elevations slopes are bare, and spring considerations are on our mind.
Be sure to consider all the mountain hazards. The snow cover is melting away from rocks and trees, and you may easily punch through into some of these holes in the snowpack. Creeks and lake inlets/ outlets are beginning to open up. "Slide for life" hazards exist on steep slopes. Be mindful of the potential to slide down or off consequential slopes when snow surfaces are still frozen. Check out the Regional Synopsis for a discussion on Peak Snowpack, Spring Strategy, and Scale of Loose Wet Avalanches.
March 29th, 2019
Peak snowpack, spring strategy, and the scale of loose wet avalanches
Peak Snowpack
Now that March is winding down, let's take have 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.
