Avalanche Forecast
Issued: Apr 10th, 2019 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
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Be suspicious of steep slopes where you see evidence that the wind affected the snow surface, especially at higher elevations where slabs may be deeper and more destructive. You may see natural loose wet avalanches in terrain affected by the sun. Steer away from any slope greater than 35 degrees where you suspect wet surfaces snow developed.
Summary
Discussion
This weather pattern seems to keep the bulk of the precipitation south of the West-North zone Wednesday night and Thursday. Cool temperatures and modest precipitation totals should limit the development of most avalanche issues. The main source of uncertainty is the cloud forecast. Subtle shifts in either direction could impact the avalanche danger. We’re generally expecting filtered sunshine, at least in the morning, for the area. This could allow enough radiation to affect snow surface conditions.
Fluctuations in precipitation, temperature, and wind over the last several days created a wide range of snow surface conditions including wet heavy snow, breakable crust, firm surfaces, and even some soft powder. Be prepared for common spring hazards such as opening creeks, widening glide cracks, snow shedding from rocks, and sagging cornices.
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:
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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
Problems
Wind Slabs
At higher elevations, you’re likely to find a mix of shallow new winds slabs and deeper older wind slabs. Recent weather limited our ability to gather much information near and above treeline, so we’re not very confident about our assessment of this problem. Approach any wind affected slope with a high degree of suspicion. When you see textured snow surfaces, drifts, or sharp cornices, wind slabs could be nearby. Typically in weather patterns like this, we find that wind slabs are harder to trigger, which keeps us thinking about likely trigger points: convex rollovers, very steep slopes, areas near rocks, and unsupported terrain. Take time to identify where the wind slab may exist, and the features you want to avoid.
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
Expected Size
Loose Wet
There may be just enough effect from the sun tomorrow to see natural loose wet avalanches, especially on more sunny aspects at mid-elevations. You may not need blue sky for this to occur. Watch for clues that loose wet avalanches are likely such as rollerballs and wet heavy surface snow. Use caution if traveling on or under steep slopes where wet snow exists. If the skies do clear, you may see loose wet avalanche activity on a wider variety of aspects and elevations.
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: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Apr 11th, 2019 11:00AM
