Avalanche Forecast Cascades - West

Date Issued: Valid Until:

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Avalanche Forecast

Thu Apr. 11th · 12:11AM

Alpine

Danger Ratings Considerable

Treeline

Danger Ratings Considerable

Below Treeline

Danger Ratings Moderate
Storm Slabs Storm Slabs
Loose Wet Loose Wet

Alpine

Danger Ratings Considerable

Treeline

Danger Ratings Moderate

Below Treeline

Danger Ratings Moderate

Help us improve the avalanche forecast!  link to: https://avbulletin.avalancheresearch.ca

Expect the avalanche problem to change gradually by elevation Thursday as you move from rain to snow. At higher elevations, be suspicious of any steep slopes where the find the wind affected the new snow. At lower elevations, rain will keep the upper snowpack wet and heavy. This may produce natural loose wet avalanches especially in locations where snow changes to rain.

Discussion

Snow and Avalanche Discussion

The highway 90 and highway 2 corridors should see a solid shot of precipitation Wednesday night and Thursday, with slightly less expected in the Mountain Loop. Snow levels should fluctuate within the below treeline band. You may find that avalanche issue you encounter blur the lines between wet and dry, and loose and slab. The biggest source of uncertainty in this forecast revolves around weak convergence zone activity. This bands of focussed precipitation could impact the area bringing periods of heavier rain and snow.

Fluctuations in precipitations, temperatures, and winds over the past several days created a wide variety of snow surface conditions around nearby Snoqualmie Pass. Be prepared for deep wet snow, breakable crusts, firm surfaces, and even soft dry snow. Recent warming wreaked havoc on the lower elevation snowpack creating open creeks, holes near rocks, widening glide cracks, and even bare ground. Travel cautiously and anticipate these springtime hazards.

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

Storm Slabs

Storm Slabs

Changes in snow level, precipitation intensity, and wind should create a stronger slab over weaker storm snow at mid and upper elevations. Avoid all open slopes greater than 35 degrees in areas where you find more than 6 inches of new snow. Storm slabs may grow larger throughout the day as more snow accumulates. You can use small slope tests, hand pits, and shovel tilt tests to confirm the upside-down nature of the new snow. Be especially leery of areas where the wind drifts the new snow into deeper and potentially more reactive slabs. Give wind drifted slopes a wide berth.

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

likely

Expected Size

1 - 1
Loose Wet

Loose Wet

The lower elevation snowpack has been through a lot. It should handle the incoming rain well, but if the West Central zone receives the higher end of the forecasted water amounts, that’s quite a bit of new weight added to the snowpack. You’ll be most likely to see loose wet avalanches in locations where the storm begins as snow then transitions to rain later in the day Thursday. Keep an eye out for new rollerballs, wet surface snow, and rain on drier snow to indicate that loose wet avalanches may occur. Use caution if traveling on or under any slope where you suspect wet surface snow.

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