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Avalanche Forecast

Archived

Mar 30th, 2016–Mar 31st, 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.

Warmth and the late March sun will begin to shift the primary avalanche hazard to wet snow concerns Wednesday at Mt. Hood. High freezing levels, light winds and sunny skies over the next several days will increase the hazard, causing natural loose wet avalanches on solar aspects and increasing the odds for low likelihood/high consequence problems such as cornice failures. The loose wet potential will be greatest on steeper slopes below treeline.  

Detailed Forecast

On Wednesday, persistent E-NE winds near and above treeline will slowly ease during the day and freezing levels should rise to around 10000 feet. Warmth and the late March sun will begin to shift the avalanche hazard to wet snow concerns Wednesday on Mt. Hood. High freezing levels, light winds and sunny skies over the next several days will increase the hazard, causing natural loose wet avalanches on solar aspects and increasing the odds for low likelihood/high consequence problems such as cornice failures.

Due to the firm wind packed surfaces near and above treeline and continuing moderate E-NE winds in the morning, wet snow avalanches will be less likely than in the Washington Cascades.

The loose wet potential will be greatest on steeper slopes below treeline. Melt-freeze crusts can quickly break down with intense spring sunshine. In areas that have received more recent snowfall, small loose wet avalanches have the potential to entrain deeper layers. Be aware of terrain traps where even a small loose wet avalanche could have unintended consequences. Plan to avoid steep solar slopes by late morning/mid-day to minimize the problem.    

A mix of older wind slab on lee easterly aspects and newer wind slab on south through westerly aspects will make wind slab avalanches possible on a variety of aspects, primarily near and above treeline. Watch for cracking and firmer or chalky wind transported snow.   

Many areas have large cornices along ridge-lines so be cognizant of the hazard above. Unlike the loose wet problem, you may not have much notice that a cornice is about to fail. Cornices can break much further back on ridges than expected and releases can be unpredictable during the spring.

Snowpack Discussion

Weather and Snowpack

Snow accumulations Wednesday 3/23 to Friday 3/25 varied based on elevation, with only a few inches below 5000 feet and about 8-10 inches at the Mt. Hood Meadows and Timberline stations. This snow was generally well-bonded to a moist crust buried March 22nd.

A strong front crossed the Northwest Sunday morning followed by a large upper trough and a cool unstable air mass Sunday afternoon and evening. Mt. Hood stations accumulated 10-12" of new snow through Sunday evening. 

Skies were mostly sunny along the lower slopes Tuesday, but a warming trend was more muted at Mt. Hood due to cool and persistent NE winds. 

Frequent March storms have built large cornices along ridges in many areas.

The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

NWAC pro-observer Laura Green was out in Newton Canyon on Friday and found that winds had plastered the latest snow down with conviction, with moderate compression tests in the new snow and hard compression tests at the 3/22 interface. Limited rollerballs were seen on solar slopes. A size 2 natural avalanche from a potential cornice release was seen in White River Canyon.

Mt. Hood Meadows reported limited avalanche control results on Monday. NW winds Sunday night had transported snow to unusual aspects and into the below treeline band, but the new snow was generally bonded well in area. Another large natural avalanche was observed in White River Canyon above 8000 ft, likely releasing Sunday night.  A nice example of a reactive wind slab on a small slope was found in the Timberline area Monday. A 10" wind slab of denser snow released with a ski cut during warming Monday (photo below). 

On Tuesday, NE winds transported snow above treeline again to non-traditional aspects. Unlike areas further north, the warming trend on Mt. Hood was muted Tuesday and the snow surface remained firm near and above treeline with a mix of hard wind slab, sastrugi and wind scoured surfaces. Meadows pro-patrol noted their below treeline terrain softening on solar aspects with increasing loose wet potential in the afternoon.   

Photo 3-28-16 by Alex Laudon, Timberline 6200 ft SE aspect

Problems

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.

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.

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.