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Avalanche Forecast

Archived

Apr 20th, 2018–Apr 21st, 2018

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
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.

New and shallow wind slabs may form on lee slopes near and above treeline Saturday. If you see fresh cornices, drifted snow, or uneven snow surfaces, fresh wind slabs may have formed on nearby slopes. Older deeper wind slabs may still be found in isolated areas on steep northerly aspects at higher elevations. Watch for small loose wet avalanches on steep sunny slopes in the afternoon. 

Detailed Forecast

A frontal system passing through the Mt. Hood area early Saturday morning should bring much cooler temperatures, a fresh round of wind but only light amounts of new snow above 4000-5000 feet. More sun than cloud is expected in the afternoon as showers decrease. Snow levels will remain cool for April on Saturday. 

New and shallow wind slabs may form on lee slopes near and above treeline Saturday. If you see fresh cornices, drifted snow, or uneven snow surfaces, fresh wind slabs may have formed on nearby slopes. Wind slabs that formed on many slopes near and above treeline earlier in the week should be trending from stubborn to unreactive. Older deeper wind slabs may still be found in isolated areas on steep northerly aspects at higher elevations.  

Mid-day and afternoon sun breaks may allow small loose wet avalanches to release on steep sunny slopes. Look for signs of wet surface snow developing as you travel. New rollerballs, fresh fan shaped avalanche debris, and wet surface snow deeper than your ankle all indicate the potential for loose wet avalanches has developed on similar steep slopes. Avoid slopes with terrain traps where even a small loose wet avalanche could have unintended consequences. 

Cornices over the winter have grown very large. Do not linger below slopes overhung by large cornices or where a cornice fall may trigger larger deeper avalanches.

Snowpack Discussion

Warm and generally sunny weather affected the snow in the Mt Hood area Thursday and Friday. On slopes receiving sunshine and on lower elevation slopes, moist surface snow conditions developed. Cold drier surface snow may still be found on shaded slopes at higher elevations. The mild weather has allowed many of the recently formed wind slabs time to gain strength.

Significant snowfall accumulated in the Mt Hood area earlier in the week. Around 16-20 inches (40-50 cm) of snow fell Sunday (4/15) through Tuesday (4/17). Strong west-southwest winds during the storm transported the new snow forming wind slabs on many slopes at higher elevations.

While numerous layers can be found within the snowpack, there are no significant layers of concern at this time.

Observations

On Thurday, NWAC professional observer Laura Green traveled in the Mt Hood backcountry. Laura found wind slabs on NE-E aspects near treeline. These wind slabs were still reactive in some snowpack tests. On south-facing slopes, she observed large rollerballs up to 3 ft (90cm) in diameter.

On Wednesday, NWAC received reports through Mt Hood Meadows ski patrol of a skier triggered wind slab avalanche outside the ski area. The avalanche was triggered by a backcountry user and estimated to be about 2 feet deep.

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.