Continued warming and sunny skies will create dangerous avalanche conditions in the middle and second half of the day. Large and dangerous natural wet avalanches are possible. Plan your travel to move off of steep, sunny slopes before the surface snow gets weak and wet. In areas with less than 4 feet of snow on the ground, you could trigger the entire snowpack.
Summary
Discussion
The avalanche hazard will be minimal at daybreak but will increase to Considerable danger by afternoon. Warm temperatures and sun are the main drivers of our avalanche concerns. Keep typical spring concerns on your mind. Important considerations include wet snow, cornice falls, and timing your travel with progressively warming snow surfaces.
Observers reported wet loose avalanches the past few days on a variety of slopes, some big enough to bury or kill a person. A significant avalanche cycle occurred in Tumwater canyon with avalanches on almost all aspects with avalanches running as recently as Sunday and closing Highway 2. On Thursday, an observer reported a cycle of large slab avalanches (possibly occurring on the 12th) in Swakane Canyon on northerly aspects with shallow snow cover. These avalanches ran on the ground.
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
Avalanches may begin on east and southeasterly aspects early in the day and continue onto westerly 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. 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 loose wet avalanche cycle has been prolonged and slow to run its course. Many slopes have already produced wet avalanches. There's potential for some large avalanches, gouging into old snow layers or even loose avalanches to initiate slabs. With increased uncertainty around avalanche conditions, it's a good idea to choose conservative terrain. Use extra caution in the second half of the day. Take note of how much the sun is affecting slopes by aspect, elevation, and slope angle to help you assess 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: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Wet Slabs
As day-time temperatures warm, stay off of and out from under large sun-exposed avalanche start zones, unsupported slopes, and terrain over 30 degrees where meltwater is reaching buried weak layers.
Increased daytime warming is making each day the warmest we've seen since January. This is the second day in a row with a poor overnight freeze. As water melts into the snowpack, dangerous wet slab avalanches are becoming possible. These avalanches are notoriously difficult to predict. When they do happen, they can be large and destructive.
A number of interfaces exist in the upper snowpack including sun crusts on southerly aspects and facets and surface hoar on northerlies. Most concerning is a layer of old facets resting on a crust that was buried in early February. This interface is about 3 feet below the snow surface. 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. If you find yourself in an area that appears to have a shallow snowpack, stick to well-supported slopes and avoid areas of wet, unsupportive snow. You can check how deep water is traveling through the snowpack by digging and looking for moist layers of snow.
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 18th, 2019 11:00AM
