Avalanche Forecast

Archived

Mar 30th, 2017–Mar 31st, 2017

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

The avalanche danger will be rated higher in the Mt. Hood area where stronger winds have built large wind slabs in the higher terrain. Loose wet avalanches will be increasingly likely Friday, where small wet snow avalanches may entrain deeper wet snow layers and become large. Use caution in steep sun exposed terrain during the warmest part of the day, avoiding steep slopes above terrain hazards such as trees or cliffs. Large recent wind deposited snow at higher elevations will maintain the low probability/high consequence potential for very large avalanches above treeline.

Detailed Forecast

High pressure is expected to rebuild over the area Thursday night through Friday. This will cause clearing overnight and allow for wet surface snow to begin refreezing and strengthening.

Partly to mostly sunny conditions Friday and warming temperatures should cause a gradual increase in danger from wet snow avalanches, especially on slopes receiving direct sun and during the warmest part of the day.

Recent shallow wind slabs should continue to settle and stabilize where formed on lee slopes, mainly above treeline and on NW-SE aspects.

The danger will be rated higher in the Mt Hood area than adjacent Cascade zones as winds have been significantly stronger, helping to maintain more widespread and potentially larger winds slabs above treeline.

The expected clearing and cooling overnight Thursday should cause a strengthening surface crust by Friday. However, if previously wet surface snow has not refrozen, be suspicious of loose wet avalanches that may begin small but entrain older snow and become dangerous and difficult to manage. Pay particular attention to steep solar facing slopes above terrain traps such as trees, cliffs or gullies where being caught and carried, even in a small slide, could prove consequential.

Recent cornices are very large. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes directly below large cornices. See a blog post regarding cornices here.

Snowpack Discussion

Weather and Snowpack

Let's just say it's been a wet and wild few weeks regarding weather and avalanches in the Cascades.

This past week has also been active weather-wise, with another strong front on Sunday, but water amounts/snowfall totals have been slightly lower relative to the extreme wetness of the past few weeks. In the last 5 days ending Monday morning NWAC stations at Mt Hood have picked up another 2-2.5 ft of snow.

A strong low pressure system brought rising snow levels and locally heavy precipitation Tuesday night through Wednesday along the west slopes of the Cascades including Mt. Hood. After receiving a few inches of snow Tuesday night, Mt. Hood saw periods of moderate to heavy rain above 7000 feet on Wednesday.

Cooling and showery weather Thursday allowed wet snow to begin refreezing with an additional 3-6 inches accumulating in most areas. The strong winds began diminishing Thursday, but continued to transport new and available snow to lee slopes, building fresh wind slabs, mainly above treeline.

Recent Observations

On Monday the Meadows patrol found touchy 6-12" but up to 12-18" wind slab on NE slopes near and above treeline. Surface loose wet snow was also becoming evident in the below treeline areas.

The Meadows patrol on Tuesday reported one explosive released 12-20 inch storm slab on an east slope at about 6000 ft. Patrollers also triggered a large cornice on approach. Surface snow was getting wet with some pinwheels in the afternoon.

The storm limited observations on Wednesday, but rain quickly penetrated the upper snowpack in the Mt. Hood Meadows base area by late morning.

Shallow loose wet avalanches were reported at Mt Hood Meadows Thursday afternoon, below treeline.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Loose Wet

Loose Wet avalanches are the 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. 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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.