Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - East Avalanche Forecast

Apr 12th, 2019 12:30AM

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

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Update 07:00 Saturday 4/13- Another strong spring storm will impact the area on Saturday. Expect to encounter dangerous fresh wind slabs, in the alpine by the afternoon. Avalanche danger will rise into 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 from the west in the alpine. Expect increasing danger as the day progresses for both for wet snow instabilities at mid elevations, and wind slabs at upper elevations. The danger will be directly related to the amount of new snow. Places near the crest that have, and will continue to receive substantially more accumulation are more dangerous. On Friday, an observer near Hawkins Mountain reported a widespread loose wet avalanche cycle, and some long shooting cracks. These were found in wind drifted slabs on a northeast aspect at 6,000ft. In the upper Salmon la Sac/Teanaway drainages the avalanche danger may be closer to CONSIDERABLE on Saturday. In most other places, a refrozen surface and cloud cover will contribute to mostly firm or breakable crust conditions in the morning. In Icicle Creek on Tuesday, I found about 4" of new snow at 6,000ft, and 12" of new snow at 7,100ft since the storm cycle began on April 5. Recent snow was mostly well bonded to older crusts, but the old (April 5th) interface consisted of rounding faceted snow on steep north aspects above 7,000ft. Tests on a northeast aspect at 7,100ft showed some potential to propagate. This layer warrants consideration on steep, north facing slopes above 7,000ft such as the high peaks of the Stuart Range.

As you travel through the terrain, factor in a good margin for error as hard to predict events like cornice fall, glide avalanches, icefall, rockfall may occur sporadically. The more complex the terrain, the more likely it is you will see, or hear the mountains shedding their winter coat. Be prepared to evaluate quickly changing conditions.

Long shooting crack, northeast at 6,000ft near Gallagher Head Lake on Friday, April 12. Bryce Hill 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

Likelihood

likely

Expected Size

1 - 1

Pay attention to quickly changing conditions in the alpine, and expect wind slabs to be forming during the day. These may be fairly reactive, and are likely to be larger the higher up you go. Carefully evaluate the bonding with old snow layers. In areas closer to the crest like the Salmon la Sac, Chiwaukum Range, and the high Stuart Range, substantially more snow may fall. You also may find well over a foot of new snow since the storm cycle began on April 5.  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.

Loose Wet

An icon showing Loose Wet

Likelihood

possible

Expected Size

1 - 1

Wet snow and/or rain, along with radiation and daytime warming 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.

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