Avalanche Forecast
Regions: Cascades - South West.
A noticeable change in the weather impacts the West-South region Thursday night and Friday developing new avalanche problems and increasing the avalanche danger. Areas near the Cascade volcanoes will see the greatest precipitation and most elevated avalanche hazard. Be leery of any slope greater than 35 degrees where you find more than 6 inches of new snow.
Discussion
Snow and Avalanche Discussion
After nearly two weeks of quite avalanche concerns, a series of storms will begin to impact the West-South region on Thursday night. The southerly flow associated with these systems should favor the volcanoes, with lesser amounts of precipitation near Crystal, Chinook Pass, and White Pass. As a result, the avalanche danger will be focused on the volcano locations. To make matters more complicated, the weather models diverge significantly regarding the strength and timing of each precipitation event. You’ll need to monitor conditions and recognize when the weather you’re experiencing doesn’t match the forecast. If your observations don’t line-up with what you expected, dial back your terrain use to lower angle and lower consequence slopes.
We expect areas near Paradise to accumulate nearly an inch of water by Friday afternoon with slightly lowering freezing levels. Winds during the same time period should begin to increase in all locations and could redistribute any new snow. Consequently, the avalanche danger should peak later in the day.Â
At lower elevations, more rain will continue to fuel the melting and shrinking of the snowpack. You should expect the snow coverage to look different than it did over the last few weeks. You may find open creeks, holes near rocks, snow falling from cliffs, and even bare ground. This brings with it a unique set of non-avalanche related hazards to backcountry travelers. Give these springtime issues a wide berth especially during periods of rain and warm temperatures.
We expect similar springtime hazards such as these glide cracks, exposed rocks, and openings seen on Mt Herman at 4300 ft. Â 04/03/19 Photo: Andrew Kiefer
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:
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Is there any recent snow accumulation that could cause avalanches? If so, what kind of avalanches could you trigger? And where?
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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?
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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
Storm slabs will grow deeper and easier to trigger as more snow accumulates during the day Friday. Be leery of slopes greater than 35 degrees when you find more than 6 inches of new snow or see evidence that the wind drifted the snow into slightly deeper deposits. Pay attention to the depth of new snow as you travel. Does it vary from location to location? Do you see more than 6 inches of snow? Has the new snow bonded to the old surface? These observations can help you highlight when you are dealing with new storm slabs.
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: Possible
Expected Size: 1 - 1
Loose Wet
During this first storm, the lower elevations will receive mostly rain. The snowpack at this elevation should handle the additional water well, but you may still find isolated areas of deep wet snow where you could trigger a loose wet avalanche. You’re most likely to see unstable wet snow at mid-elevation shaded slopes where the snowpack has not undergone a significant spring transition. If you find wet punchy snow, be leery of nearby steep slopes especially around creeks, gullies, and cliffs where even a small avalanche can harm you.
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.
Elevations: Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1