Avalanche Forecast
Regions: Mt Hood.
Very dangerous avalanche conditions continue to exist above treeline on Mt. Hood where winds continue to load start zones and very large slab avalanches may continue to release. Tuesday is a good day to keep things simple and avoid areas where these avalanches can start, stop or run. Lowering snow levels and continued winds and lighter snowfall is rebuilding wind slabs near and above treeline, while wet avalanches may be triggered on steep slopes at lower elevations.
Discussion
Snow and Avalanche Discussion
A widespread cycle of very large natural avalanches occurred on Mt. Hood during an atmospheric river event that brought literal buckets of water to Mt. Hood. Strong winds will continue to sustain high avalanche danger above treeline and with cooling temperatures, wind slab danger will increase into the near treeline band.
Very large crown (D3.5-D4) on the upper slopes of Mt. Hood above Mt. Hood Meadows resort. 04/08/2019. Photo Credit: Peter Moore.
These avalanches are deadly and they may travel tremendous distances from start zones well above treeline.
Over the last 4 days with waves of moisture dropping 5-8†of rain (much of it Sunday through Monday). Snow levels oscillated between 5000-7000' during the storm and are. After a deluge of rain Saturday night and Sunday morning with snow levels rising to 7000', several inches of wet snow have accumulated down to 5300' Sunday afternoon, then snow levels rose back to 7000 ft Sunday night, dropping down the mountain to below Mt. Hood Meadows base during the day. Winds were strong at times during the event. Snow will continue to accumulate to lowering elevations Monday and Monday night.
Moderate to heavy rain Sunday night and at least for part of Monday caused wet loose avalanches on steep slopes that see a transition from snow to rain.
Snowpack Discussion
April 3rd, 2019
Spring snowmelt
The snowpack in much of the Cascades has changed dramatically in the past two weeks. The weather has shifted solidly to spring-like patterns. The spring warm-up started in mid-March with a prolonged period of relatively clear skies and warm temperatures. Moving into April, we’re seeing periods of unsettled spring weather bringing rain to many low and mid-elevation slopes and snow to upper elevations.
Very bare southeast aspects of Rock Mtn/Nason Ridge. April 2nd. Photo: Josh Hirshberg
Since the peak height of snow in mid to late February, mountain weather stations in the 4,000-5,000ft range show an average of 27% decrease in height of snow. Looking at weather stations in nearly every zone, the percentage decrease ranged from 22-29%. This year's spring snowmelt is much earlier than normal. 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.
NWAC climatological snow depth data from April 1st. You can view it on our website here.
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A natural loose wet avalanche (D1), Lichtenberg Mtn, N, 4,850ft. 4/7/2019. Photo: Will Govus
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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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Is there any recent snow accumulation that could cause avalanches? If so, what kind of avalanches could you trigger? And where?
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What are the high and low temperatures of the past 24 hours as well as the forecasted temperatures during the time you’ll be in the mountains? Could these create weak, wet snow surfaces?
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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?
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 is creating some 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.
We are approaching the end of our daily avalanche forecasting season. The mountain weather forecast will continue into the spring, and the weather station data is available year round. Keep checking the forecast for conditions updates on the end of season information.
Glide avalanches and holes opening up in rocky terrain near Mount Herman. Photo: Andrew Kiefer
Avalanche Problems
Wind Slabs
Avoid all areas where avalanches can start, stop, and run from above treeline terrain on Mt. Hood. Strong winds and heavy snowfall produced very large, powerful and sensitive wind slabs which released naturally during the storm and we think will continue to do so for one more day. Strong westerly winds on Tuesday will continue to have plenty of snow (including fresh snow) to transport.
Before venturing into the storm and likely poor visibility, know your exposure to large avalanche paths (like the major canyons) with start zones located much higher on the mountain. Small to large wind slabs will build near treeline through the day on Monday. Slab avalanches can entrain wet snow at lower elevations and become even more powerful and destructive.
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: Very Likely
Expected Size: 1 - 2
Loose Wet
Wet snow underlies the new snow and we think that you may still be able to trigger the snow that has been recently saturated with rainfall for another day as the snowpack refreezes progressively from the top down. We think the temperatures and strong winds will transition the snowpack quickly above treeline, but below treeline, the healing process from all that water added will take time. If you sink up to your ankle or more deeply in the snow, avoid slopes steeper than 35 degrees and avoid areas where wet avalanches may entrain snow 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: Below Treeline.
Likelihood: Likely
Expected Size: 1 - 1