Avalanche Forecast Stevens Pass

Date Issued: Valid Until:

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Avalanche Forecast

Sat Apr. 13th · 11:00AM

Alpine

Danger Ratings Considerable

Treeline

Danger Ratings Considerable

Below Treeline

Danger Ratings Moderate
Wind Slabs Wind Slabs
Loose Wet Loose Wet

Alpine

Danger Ratings Below Threshold

Treeline

Danger Ratings Below Threshold

Below Treeline

Danger Ratings Below Threshold

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

Another round of snow and westerly wind will favor the Stevens Pass zone. You can trigger fresh wind slab avalanches near ridges at mid and upper elevations. By midday, it will be easy to trigger loose wet avalanches on east through south through west aspects.

Discussion

Snow and Avalanche Discussion

NWAC Spring 2019 Forecast Schedule:

Daily avalanche forecasts will end today, Sunday, April 14th. Look for regional Avalanche and Snowpack Summaries every Friday at noon through May 24th. Avalanche warnings will be issued as needed throughout the Spring if unusually dangerous avalanche conditions develop. 

The pattern of cool, gusty spring storms continues. The concern for avalanches is in new and recently fallen snow near the surface. On Saturday, Stevens Pass Patrol reported shallow drifts at ridges, but no new avalanches. You’ll find a big difference in the upper snowpack between slopes at low to mid-elevations and those above about 5,000 feet. At lower elevations, recent storms haven’t added significant layers to the snowpack. Above 5,000ft, up to 2 feet of relatively dry snow rests on layers that have fully transitioned to melt forms. Just below this interface (April 5th), you can find very weak, unfrozen snow. While the wet snow hasn’t produced avalanches, it is something to keep an eye on as warming melts the snowpack in the future weeks.

Moving further into spring be prepared to evaluate changing conditions as you move through the mountains. Plan to give yourself a good margin for error as hard to predict events like cornice fall, glide avalanches, icefall, and rockfall may occur sporadically. 

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

Wind Slabs

Wind Slabs

Pay attention to quickly changing conditions in the alpine, and watch for freshly formed wind slabs from westerly wind. It may be easy to trigger these avalanches and they could be large at upper elevations. Use small test slopes and low-angle wind features to evaluate the bonding of any cohesive new snow. Steer around obvious drifts and slopes over 35 degrees and on unsupported or convex features. Cornices continue to grow, so be sure to give them plenty of distance.

Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

likely

Expected Size

1 - 1

Loose Wet

Loose Wet

Wet snow and/or rain, along with daytime warming and radiation coming through the clouds may weaken snow surfaces once again. Watch for changing conditions throughout the day. You can easily trigger wet loose avalanches on slopes over 35 degrees as the surface snow warms. Even without clear skies, the strong April sun will warm the snow through the clouds on sun-exposed slopes. While these avalanches are typically easy to predict, it’s important to be aware of the consequences when you are in avalanche terrain. Wet loose avalanches can be forceful and can injure you or push you into terrain traps like rocks or trees. Be observant and look out for any triggered avalanches as you travel on suspect slopes. Stop and regroup out from under slopes that others are traveling on.

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: Treeline, Below Treeline.

Likelihood

possible

Expected Size

1 - 1