Avalanche Forecast
Regions: Mt Hood.
The avalanche danger will peak late in the day as a series of storms bring snow and wind to Mt. Hood. Wind slabs will grow larger and more dangerous throughout the day. Steer away from wind-affected slopes greater than 35 degrees.
Discussion
Snow and Avalanche Discussion
Mt. Hood could see an inch of water equivalent by the end of the day Friday and this will bring a drastic increase in slab avalanche danger. The new snow will force you to adjust your travel patterns that you’ve become used to during the recent spring diurnal cycle. It will also increase loose wet avalanche danger in steeper terrain below treeline.
Weather models are struggling with the shortwave features slated to arrive late Thursday night and late afternoon/evening on Friday. Thus, you will need to use your own observations to determine whether the avalanche conditions line up with the forecast. If they don’t, adjust your travel plans to choose more conservative and lower-elevation terrain.
Mt. Hood saw light rain and snow on Thursday with no snow at Mt. Hood base, but 3†of new at 6600 ft, drifting 4-5†in places. The new snow as bonding well and very stubborn to trigger. A larger storm system brought between 0.50" - 1.00" of precipitation to Mt. Hood Tuesday night and Wednesday. Unfortunately, most of this fell in liquid form below 6500'-6000'. Above this elevation, several inches of snow fell.
Snow cover below 4000' is disappearing quickly and south facing terrain near ridgelines are melting out as well. The mountains are still experiencing their spring transition. Creeks are opening up with snow bridges collapsing. Glide cracks are evident and may release in generally predictable locations, but without warning. Sagging cornices may fail when stressed by the warmth of the sunshine or the weight of a person. Use caution if you travel near these features.
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.
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?
Glide avalanches and holes opening up in rocky terrain near Mount Herman. Photo: Andrew Kiefer
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.
Avalanche Problems
Wind Slabs
An initial wave of precipitation will bring snow to areas above 6000 ft, with wind slabs building above this elevation. Wind slabs will increase throughout the day and peak in the afternoon as a second shortwave feature brings in moderate precipitation with moderate to strong winds in the afternoon. Wind slabs will build into the near treeline band and may become large at higher elevations. As the day wears on, choose more conservative terrain in wind-sheltered areas and avoid wind-loaded slopes greater than 35 degrees.
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: Likely
Expected Size: 1 - 1
Loose Wet
While we think that the snowpack has transitioned enough to generally handle 1” of rain in 24 hours, there may be isolated exceptions to that rule. If you encounter wet snow and sink in deeper than your ankle, choose terrain less than 35 degrees.
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: Possible
Expected Size: 1 - 1