Avalanche Forecast
Regions: Olympics.
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Monitor your exposure to small avalanches where (1) solar radiation impacts 3” or more of recent snow or (2) where winds have blown snow to form a 6” wind slab. If you feel the warmth of the sunshine, move off of slopes greater than 35 degrees; approach areas with freshly drifted snow cautiously, steering around steep unsupported slopes.
Discussion
Snow and Avalanche Discussion
Expect a cool April day with mostly cloudy skies, light snow showers with some fresh accumulation. Monitor the recent snow and the solar radiation, particularly during time periods the snow lightens up because this is when the sun will initiate small wet avalanches.
Winter holds a thin veneer over a spring snowpack in the Olympics. Â
Reports from around the region have indicated that creeks are gushing and opening up with snow bridges collapsing. Cornices are sagging. Glide cracks are on all aspects, but particularly N and NE; they can act like a crevasse or release without warning. Use caution if you travel near these features. Expect low snow cover everywhere below 4500 ft. Keep in mind that smaller features may be covered up by recent snow.
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. Â Avalanche warnings will be issued as needed throughout the Spring if unusually dangerous avalanche conditions develop.
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.Â
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:
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Can you trigger avalanches due to new snow?
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If so, would they be storm slabs or wind slabs? And where?
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Can you trigger avalanches due to warming or rain?
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Will recent snow be warmed enough to result in loose wet avalanches?
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Will these avalanches be predictable point releases or more destructive wet slabs or gouging loose wet avalanches?
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What are the recent high and low temperatures and the forecasted temperatures during the time you’ll be in the mountains?
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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?
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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
Avalanche Problems
Loose Wet
We expect the sun to make some appearances on Sunday, but even if it doesn’t the sun can radiate through clouds to warm recent snow and create a “shed cycle” within the storm total expected 3-6” of recent snow. These generally small avalanches can be triggered by snow melting off of rocks and trees or if you give the snow a push. If you find the snow becoming sticky and wet, adjust your aspect and consider your exposure to steeper slopes 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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Wind Slabs
While moderate winds likely built wind slabs on Saturday, afternoon temperatures warmed to just above freezing at Hurricane Ridge Saturday afternoon, with the warm temperatures and light rain helping the wind slabs to heal. Thus, wind slabs will have to build afresh and they may not become large enough to be a problem. Approach steep unsupported slopes with wind-drifted snow cautiously, feeling for a firm or hollow sounding snow as a sign that wind slabs may be present. Be especially cognizant of the consequences of even a small avalanche around terrain traps like gullies, cliff bands or if you might be pushed into recently exposed ground hazards.
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