Avalanche Forecast
Regions: Cascades - South West.
A combination of warming temperatures and additional precipitation should tip the snowpack over the edge Sunday causing large natural avalanches in some locations. Steer away from areas where avalanches can start, run, and stop. Changes in the weather may rapidly change the conditions you experience in the mountains.
Discussion
Snow and Avalanche Discussion
Several winter storms continue to impact the West-South zone. As of Saturday afternoon, 6-12†of snow has accumulated with the highest totals near Crystal Mountain and Paradise. Crystal Mountain Ski Patrol reported an active avalanche cycle Saturday with natural and skier triggered avalanches over 2 feet deep.
The stormy pattern continues Sunday in the West-South. Even though the models decreased their expected precipitation amounts for Sunday, the combination of warming temperatures and additional precipitation should create very dangerous avalanche conditions. The bulk of the moisture appears to focus on Oregon and the Mt Hood area. The southern volcanoes like Mt St Helens and Mt Adams stand to receive more water than areas farther north. If you are traveling in the southern portion of this zone, be prepared for large natural avalanches to occur in many locations. Areas that receive less snow, should see lower avalanche danger.
Since the weather models are in such poor agreement about the incoming weather, we have a lower level of confidence in this forecast. Resist the urge to step into bigger terrain if you don’t find unstable snow early in the day. The weather could change quickly, and bring with it, rapid changes in the avalanche conditions. Monitor the weather, and recognize how the conditions you’re experiencing differ from the forecast.
Even though the recent weather felt like winter, there are several springtime hazards in the mountains. Creeks, particularly at low elevations, opened wide during the recent warm weather. Glide cracks continue to grow and a few glide avalanches have been found. Holes appeared near many trees and rock. Cornices continue to sag overhead. Use caution when you travel near any of these spring hazards that could be hidden by a few inches of new snow.
Â
Snow bridges over many creeks are beginning to sag, crack, and fail. Choose your creek crossings carefully. Photo: Dallas Glass
Â
Snowpack Discussion
April 3rd, 2019
Spring snowmelt
The snowpack in much of the Cascades has changed dramatically in the past two weeks. The weather has shifted solidly to spring-like patterns. The spring warm-up started in mid-March with a prolonged period of relatively clear skies and warm temperatures. Moving into April, we’re seeing periods of unsettled spring weather bringing rain to many low and mid-elevation slopes and snow to upper elevations.
Very bare southeast aspects of Rock Mtn/Nason Ridge. April 2nd. Photo: Josh Hirshberg
Since the peak height of snow in mid to late February, mountain weather stations in the 4,000-5,000ft range show an average of 27% decrease in height of snow. Looking at weather stations in nearly every zone, the percentage decrease ranged from 22-29%. This year's spring snowmelt is much earlier than normal. Traveling in the mountains the loss of snow coverage is most noticeable on southerly, sun-exposed slopes and below 4,000ft. On northerly aspects and slopes above 5,500ft, the snowpack has seen less dramatic changes and has even maintained some dry layers.
NWAC climatological snow depth data from April 1st. You can view it on our website here.
Spring avalanche considerations
As you head into the mountains there are a few questions to ask yourself common to spring avalanche conditions:
-
Is there any recent snow accumulation that could cause avalanches? If so, what kind of avalanches could you trigger? And where?
-
What are the high and low temperatures of the past 24 hours as well as the forecasted temperatures during the time you’ll be in the mountains? Could these create weak, wet snow surfaces?
-
How is the cloud cover contributing to the melting or freezing of surface snow? Did clear skies allow for a sufficient overnight freeze? Will the sun be strong enough to weaken surface layers?
Glide avalanches and holes opening up in rocky terrain near Mount Herman. Photo: Andrew Kiefer
Other considerations
In addition to daily avalanche hazard, the early snowmelt is creating some other travel considerations. Some roads and lower elevation slopes may not have enough continuous snow coverage for travel on snow machines. Holes melted around rocks, trees, and creeks could create a fall hazard. When nighttime temperatures and cloud cover allow for surface freezes, bring appropriate equipment to mitigate slip and fall hazard on steep slopes.
We are approaching the end of our daily avalanche forecasting season. The mountain weather forecast will continue into the spring, and the weather station data is available year round. Keep checking the forecast for conditions updates on the end of season information.
Avalanche Problems
Storm Slabs
More precipitation and warming temperatures should cause a classic up-side-down unstable snowpack structure to form in some areas. We think this will result in a natural avalanche cycle especially in areas that received more than 8 inches of snow during this stormy period. At higher elevations avoid traveling in locations where avalanches can start, run, or stop. This snowpack will need time to adjust to these changes in the weather.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Very Likely
Expected Size: 1 - 1
Loose Wet
Two factors could influence loose wet avalanche activity, rain, and sun. With rising snow levels, we’re expecting rain to impact the storm snow at lower and mid elevations. There’s also a possibility for sun breaks in the afternoon. During these times, the strong April sun will quickly affect the snow at all elevations. Whether it’s rain or sun, the solution is simple. Expect natural loose wet avalanches from steep terrain. These avalanches may descend downhill and cross normal travel corridors. At all elevations, limit your exposure to steep slopes, and avoid traveling on any slope greater than 35 degrees.
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: Likely
Expected Size: 1 - 1