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Avalanche Forecast

Archived

Mar 24th, 2016–Mar 25th, 2016

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

Fresh wind slab will continue to be a primarily hazard on Friday in the Mt. Hood area. Loose snow avalanches will be likely on steeper slopes. Human triggered storm slabs are possible in less wind affected areas as new snow becomes more cohesive with daytime warming on Friday. 

Detailed Forecast

Orographic showers in cool NW flow aloft Thursday night will wind down Friday morning followed by with partial clearing in the afternoon. Moderate W-NW transport winds will also ease off late Thursday night and Friday morning. 

Fresh wind slab will continue to be a primarily hazard on Mt. Hood Friday. By avoiding obviously wind loaded slopes, you can enjoy some fine late season conditions. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.

We will flip which loose snow avalanche we identify as the primary problem with more sunshine expected Friday afternoon versus Thursday. Loose wet avalanches will be likely on steeper solar slopes by mid-day or afternoon. Loose dry avalanches should be small but likely on steeper non-solar slopes. Be aware of terrain traps where even a small loose avalanche could have unintended consequences in the wrong terrain.    

Human triggered storm slabs are possible as new snow becomes more cohesive with daytime warming on Friday. Watch for graupel layers received Thursday night to become potential bed surfaces for storm slabs. 

Recently formed cornices have grown large. If traveling along ridgelines, be aware that cornices break much further back than expected and you don't want to go for a ride with a chunk of cornice. Despite the cool weather, cornice releases can be unpredictable during the spring.

Snowpack Discussion

Weather and Snowpack

A low pressure system crossed the Northwest on Monday and Tuesday. NWAC stations at Mt. Hood accumulated a few inches above 4000 feet during this time. Light rain was seen on Wednesday afternoon in the below treeline band before a cold front swept through Wednesday night with moderate to strong westerly transport winds continuing through Thursday.

Post-frontal snow accumulations varied based on elevation, with only a few inches below 5000 feet and about 6-8 inches at the Mt. Hood Meadows and Timberline NWAC stations through Thursday afternoon. 

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

The Mt Hood Meadows pro-patrol on Tuesday morning 3/22 found wind loaded pockets of mostly small wind slab 12-18 inches deep in the above tree line which were stubborn to release by skis. On Tuesday afternoon a recent natural cornice release was seen at about 7000 feet on Marmot Ridge which released a small pocket of wind slab. By Wednesday morning, Meadows pro-patrol found dense wind slab above treeline and on lee slopes only affected by large explosives. Above treeline, satstrugi was widely sculpted into the snow surface with some icy surfaces on windward ridges.  

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.