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Avalanche Forecast

Archived

Mar 19th, 2016–Mar 20th, 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

Cloudy Sunday with periods of mostly light rain or snow showers and mild conditions are expected. Rain and snow amounts should be relatively light and not significantly affect the current danger.

Clouds should allow the temperatures to remain above freezing Saturday night and with the addition of light rain near and below treeline, small loose wet avalanches may be possible on Sunday. Loose wet avalanches may start small but could have the ability to entrain recent moist snow in mainly on steep terrain.

Moderate winds near ridgetop may combine with light precipitation to build new areas of shallow wind slab on lee aspects in the upper elevations above treeline. Any new wind slabs will mostly likely be found in NW-NE facing slopes. East winds over the past few days will also have built pockets of wind slab on westerly aspects. Hence the wind slab avalanche problem will be indicated on a variety of aspects near and above treeline.

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.

Snowpack Discussion

Weather and Snowpack

Storms moved across the Northwest at a nearly daily frequency the past few weeks with fluctuating but generally moderate snow levels. Two deep surface low-pressure systems moved across the Olympic Peninsula last week with the first passing last Thursday 3/10, and the next on Sunday, 3/13. These systems caused very stormy weather.  

Cool and showery weather Monday and Tuesday with  prolonged westerly winds deposited additional snow at lower snow levels.

Storm totals on Mt Hood were impressive with about 4 feet of new snow accumulating over about 4 days ending Wednesday morning 3/16 at the NWAC Mt. Hood Meadows and Timberline stations!  

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.  

Recent Observations

The Mt Hood Meadows pro-patrol on Sunday 3/13 reported stormy conditions with widespread wind and storm slab of 6-12 inches on all aspects in the area, easily triggered by ski cuts and running on a crust buried on Thursday 3/10.

A report for Newton Canyon via the NWAC observations page for Monday 3/14 indicated stormy weather and numerous ski triggered 3-10 inch storm slabs.

On Monday and again Tuesday 3/14-15, control performed by Mt. Hood Meadows pro-patrol produced extensive and sensitive wind and storm slab avalanches triggered by ski cuts in lower elevation slopes and explosives in wind loaded terrain. Avalanches were greatest on N-E facing terrain. Loose wet slides were releasing on solar aspects by Tuesday afternoon below treeline.

On Wednesday 3/16, the Meadows pro-patrol were still finding hard wind slab of up to a foot above treeline on the usual lee aspects with a rising hazard of loose wet avalanches in steeper terrain below treeline due to warming and sunbreaks.

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 with limited daytime softening. Settled storm snow can still be found on 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.