Avalanche Forecast Snoqualmie Pass

Date Issued: Valid Until:

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Avalanche Forecast

Thu Apr. 11th · 11:00AM

Alpine

Danger Ratings Considerable

Treeline

Danger Ratings Considerable

Below Treeline

Danger Ratings Moderate
Loose Wet Loose Wet
Storm Slabs Storm Slabs

Alpine

Danger Ratings Moderate

Treeline

Danger Ratings Moderate

Below Treeline

Danger Ratings Moderate

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Warmer temperatures and some filtered sunshine should work quickly Friday affecting the recent snow. Give this pattern a significant amount of respect. Steer away from open slopes greater than 35 degrees, and don’t get surprised by natural avalanches descending from steep slopes where the recent snow became warm and wet.

Discussion

Snow and Avalanche Discussion

A Puget Sound convergence zone brought a healthy dose of precipitation to Snoqualmie Pass Wednesday night and Thursday. As of Thursday afternoon, this zone received over 3 inches of water during this series of storms. How much snow does that translate to? Well, that depends on elevation. At Pass level, we’ve only seen a few inches of wet heavy snow, while at the highest elevations you could be looking at close to 2 feet. Expect quite a change in avalanche conditions as you go up in elevation.

All this recent snow just means there’s plenty to avalanche on Friday as the weather changes. During this transition, avalanches may blur the lines between wet and dry, and loose and slab. On Friday, we’re expecting filtered sunshine, warming air temperatures, and a few passing showers to impact the area. This time of year, you don’t need clear skies for the sun to impact the snow. If you can feel the warmth of the sun, then it’s influencing the snowpack.

Recent changes in snow level, precipitation type, and wind created a wide variety of snow surface conditions in the backcountry. Be prepared for wet heavy snow, breakable crust, firm surfaces, old powder, and even bare ground as you travel. Springtime hazards such as opening creeks, widening glide cracks, sagging cornices, and holes near rocks should be on your mind. Use caution when you travel near or under these features.

Snowpack Discussion

April 9th, 2019

Spring Conditions

The snowpack and weather have shifted solidly to spring-like conditions. A major warm-up started in mid-March with a prolonged period of strong sun and warm temperatures. This created a major difference between the snowpack on sunny slopes and that on shaded aspects. More recently, warm, wet, and sometimes weak spring storms have brought more rain than snow. The bulk of the precipitation with these storms focused on the southern forecast zones. Even so, mid-elevation rain established a dramatic snow line (about 4-4,500ft) below which the snowpack is minimal to non-existent in most zones. Going into the second week in April, intense snow and wind drove a prolonged period of High danger at Mt Hood.

A crown of a very large avalanche (D3+) above Mt. Hood Meadows resort. 04/08/2019. Photo Credit: Peter Moore.

Challenging Weather Forecasts

The Cascades have been experiencing unsettled spring weather with rain to many low and mid-elevation slopes and snow at upper elevations. Spring weather forecasts in the Cascades are notoriously challenging. With these storms, the weather models have been inconsistent and the accuracy has been limited to 12-24 hours, at best. A trend has been significant precipitation amounts for the Mount Hood area and other south-central Cascade volcanoes. 

Very bare southeast aspects of Rock Mtn/Nason Ridge. April 2nd. Photo: Josh Hirshberg

Shrinking Snowpack

From the peak height of snow in mid to late February through early April, mountain weather stations in the 4,000-5,000ft range showed an average of 27% decrease in height of snow. The percentage decrease ranged from 22-29%. This year's spring snowmelt is much earlier than normal. If you’re 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. On upper elevation shaded slopes there’s still potential for large wet slab avalanches with prolonged warm temperatures or high elevation rain events.

A natural loose wet avalanche (D1), Lichtenberg Mtn, N, 4,850ft. 4/7/2019. Photo: Will Govus

Spring avalanche considerations

As you head into the mountains there are a few questions to ask yourself common to spring avalanche conditions:

  1. Can you trigger avalanches due to new snow?

    1. If so, would they be storm slabs or wind slabs? And where?

  2. Can you trigger avalanches due to warming or rain?

    1. Will recent snow be warmed enough to result in loose wet avalanches?

    2. Will these avalanches be predictable point releases or more destructive wet slabs or gouging loose wet avalanches?

    3. What are the recent high and low temperatures and the forecasted temperatures during the time you’ll be in the mountains?

    4. 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?

Debris from a natural loose wet avalanche (D2), Lichtenberg Mtn, SW, 5,000ft. 4/7/2019. Photo: Josh Hirshberg

Other Considerations

In addition to daily avalanche hazard, the early snowmelt has created 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.

The last daily avalanche forecast for all zones will be issued for April 14th. Statewide mountain weather forecast and weekly avalanche condition advisories will continue through May. The weather station data is available year round. Keep checking the advisories and help us out by submitting observations when you are in the mountains.

Glide avalanches and holes opening up in rocky terrain on an east aspect of Mount Herman. 4/3/19 Photo: Andrew Kiefer

Problems

Loose Wet

Loose Wet

The snow changes quickly this time of year, and Friday should be a great example. Wet, sticky snow and new rollerballs may be your first clues that loose wet avalanches are becoming likely, and it's time to steer clear of slopes greater than 35 degrees. Just because you aren’t on the steep slope doesn’t mean your safe. Pay attention to the slopes above you. Don’t get surprised by natural avalanches that may occur during this warming. Some of these avalanches may entrain significant amounts of new snow, grow large, and cross common travel routes.

Several layers exist within the new snow. You could see loose wet avalanches stepping down into these layers and causing larger slab avalanches. If you see this occur, dial back your terrain travel and give yourself wide buffers between you and avalanche terrain.

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

Storm Slabs

Storm Slabs

You’re most likely to find lingering dry snow during the morning hours, at higher elevations, and on more shaded slopes. If you encounter dry snow, approach this terrain with suspicion. The snow levels fluctuate during these storms potentially creating several layers within the recent snow. Ease into the terrain and avoid features such as convex rollovers, unsupported slopes, and very steep terrain where you may be more likely to trigger a slab avalanche.

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