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Avalanche Forecast

Archived

Mar 20th, 2016–Mar 21st, 2016

Alpine
Natural avalanches unlikely, human triggered possible.
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.

A mixture of typical spring avalanche problems exists with no one problem dominating. Evaluate the local snow and terrain carefully on Sunday.

Detailed Forecast

Cooler temperatures, cloudy and showery weather is expected Monday.  This weather should maintain areas of wet and weak snow below about 4500-5000 feet in the Below Treeline elevation band. Moderate to strong winds at the higher elevations may build new shallow wind slabs on some leeward exposed ridges, mainly NW-E facing.  Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.

Recently formed cornices have grown large and can become sensitive during mild weather, especially following nights with above freezing temperatures.  Avoid areas on ridges or summits where there may be a cornice and avoid slopes below cornices in the spring. Cooler temperatures Monday may help temporarily strengthen cornices Monday.

Snowpack Discussion

Weather and Snowpack

A combination of sun and mild temperatures for several days gave way to mild temperatures and light rain Sunday, March 20th. The recent weather over the past five days has allowed the roughly 4 feet of storm snow that accumulated from about March 9-15 to settle and stabilize. Wind, sun, warmth, freezing and now rain have drastically changed snow conditions over the past five days. Surface snow conditions have become highly variable, ranging from some well settled old snow on steep shaded slopes and a mix of surface crusts, wind buffed surfaces or shallow wet snow on other aspects.

Earlier avalanche problems of wind and storm slab have settled and stabilized over the past several days of mild weather. Sites on Mt Hood have seen the snowpack settle 10-15 inches over the past five days, since the strong storms ended March 15th. The stabilizing snowpack has been verified by numerous observers by snow safety personnel over the past few days.

The mid and lower snow pack at Mt Hood should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Strong easterly winds occurred since Thursday and may have transported any available surface snow to more westerly facing slopes, though any wind slabs formed have been gaining strength over the past several days.  

Recent Observations

On Thursday 3/17 the Meadows pro-patrol reported strong east winds but not much snow available for transport. Small loose wet avalanches and rollerballs were seen only on direct solar slopes.

The most recent reports from Mt Hood over the past few days indicate mostly stable surface crusts, shallow moist to wet snow and a variety of wind buffed surfaces all leading to generally favorable stability, if not the greatest ski conditions.  Some well settled storm snow from last week can still be found on steeper sun protected shaded slopes. 

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.