Avalanche Forecast
Regions: Stevens Pass.
Expect continued warming and partly sunny skies for Saturday. You can trigger small loose wet avalanches on sun-exposed slopes as temperatures warm through the day. Give yourself travel options to move off of steep, sunny slopes before the surface snow gets weak and wet.
Discussion
Snow and Avalanche Discussion:
Warm temperatures and sun following a storm on the 11-13th are the main drivers of our avalanche concerns. With increased daytime warming, high temperatures each day will be the warmest we've seen since January. Though the snowpack hasn't made the transition to spring, keep typical spring concerns on your mind. Wet snow, cornice falls, and timing your travel with progressively warming surfaces are all important considerations.Â
A storm starting on the 11th buried a mix of sun crusts on southerly aspects and weak, faceted snow on northerlies. Stevens Pass received 8-12" of snow. On the 13th-15th, observers reported many small natural wet loose avalanches on the southeast face of Lichtenberg Mtn and southwest aspects near the Skyline area and Windy Mtn. On the 12th, avalanche workers reported numerous dry loose avalanches (up to D1.5), and WSDOT triggered a large (D2) wind slab on a northwest aspect in wind exposed terrain above the highway.
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, 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.
Avalanche Problems
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 northwest aspects. Look for signs of loose wet instability such as wet, heavy surface snow, rollerballs, and fan-shaped avalanches. If you notice these clues, avoid going underneath steep slopes where loose wet slides may run naturally.
The cloud cover has been ongoing and challenging 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, North West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1