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Northwest Avalanche Center NWAC, Northwest Avalanche Center

Stevens Pass Avalanche Forecast

Apr 12th, 2019 12:30AM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs and Loose Wet.

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Update 0700 Saturday 4/13: Another strong spring storm will impact the area on Saturday. Avalanche danger will rise into the afternoon hours. Expect to encounter reactive wind slabs in upper elevation terrain that may become large and dangerous towards the end of the day.  Loose wet avalanches remain possible on steep slopes at mid elevations with wet snow or rain, and daytime warming. 

Summary

Discussion

NWAC Spring 2019 Forecast Schedule:

Daily avalanche forecasts will end 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. 

Snow and Avalanche Discussion

An incoming storm will arrive in the morning with gusty winds. Expect the danger to increase as the day progresses, both for wet snow instabilities at mid/low elevations, and fresh wind slabs at upper elevations. The danger will be directly related to the amount of new snow. By the very end of the day the danger may be in the Considerable range for above treeline terrain in the area. Recently, a series of spring storms gave us over a foot of new snow on the pass since Wednesday. On both Thursday and Friday, a widespread loose wet avalanche cycle occurred on most slopes during the day, the exception being high elevation northwest to northeast facing terrain. The recent snow is sitting on crusts with underlying weak, wet snow. Loose wet avalanches entrained the entire 12" of storm snow, shaking trees, and left quite a number of large debris piles in their wake. Control at SPMR pulled out a number of wind slabs as well, some large enough to injure, bury, or kill a person. These were on a variety of aspects, but above 5,000ft. 

An ongoing "shed cycle" continues...each time we get more snow, some of it comes down. Be prepared to evaluate changing conditions on the fly, and 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. 

Large (D1.5) loose wet avalanches ran naturally off the southeast aspects of Lichtenberg Mountain on Thursday. Matt Primomo photo.

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

An icon showing Wind Slabs

Pay attention to quickly changing conditions in the alpine, and expect wind slabs to be forming during the day. These may be fairly reactive, especially during times of heavy snowfall. Wind slabs may grow to be large in the upper elevation terrain by the end of the day. Carefully evaluate the bonding of any cohesive slabs. 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

possible

Expected Size

1 - 1

Loose Wet

An icon showing Loose Wet

Wet snow and/or rain, along with daytime warming and radiation coming through the clouds may weaken snow surfaces once again. When snow surfaces melt and become wet and sticky, loose wet avalanches can run on steep slopes. Rollerballs and pinwheels are good signs that loose wet avalanches may follow. These slides may run far, entraining the recent storm snow, and where enough new snow exists, they may pack a punch. Don't put yourself into a situation with high consequences, such as risk of being carried into trees, gullies, and over cliffs. Be careful of going underneath any steep slopes, as they may hit you from above. 

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

Valid until: Apr 13th, 2019 12:30AM