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Avalanche Forecast

Archived

Jan 20th, 2016–Jan 21st, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

A variety of avalanche problems on Thursday will present dangerous conditions in all elevation bands at Mt. Hood. If you venture out Thursday, use conservative decision making and terrain selection. 

Detailed Forecast

Mt. Hood should be in the warm sector Thursday with a frontal system stalled offshore. Precipitation amounts look rather paltry for this neck of the woods, but snow levels should begin to rise Wednesday night and continue on Thursday with sustained S-SW winds.  

Deep wind slab built Tuesday and Wednesday should most likely be found on lee north to southeast slopes near and above treeline, but watch for locally cross-loaded slopes. Wind slabs may have grown very large over the past few days... and although fairly unlikely to naturally release or to be responsive to human triggering, would lead to a large and deadly slide.  

Storm slabs may still be sensitive to human triggering on Thursday, so choose more moderate slopes if you find lingering storm snow instabilities. 

Loose wet avalanches will become likely due to light rain and rising freezing levels. Loose wet avalanches should be small and confined to steeper slopes near and below treeline. Be especially wary near terrain traps, where even a small avalanche could have unintended consequences. 

A variety of avalanche problems on Thursday will present dangerous conditions in all elevation bands at Mt. Hood. If you venture out Thursday, use conservative decision making and terrain selection relative to the avalanche problem. 

 

Snowpack Discussion

A continuous period of active weather for about the past week has brought several feet of snow to NWAC stations at Mt Hood. Fluctuating snow levels over this period has occasionally mixed rain up into the near treeline elevation band during this storm cycle.

Most recently, roughly 2 feet of new snow accumulated Tuesday and Tuesday night at Timberline and Meadows NWAC base stations with consistent westerly transport winds buffeting the near and above treeline that continued through Wednesday afternoon.  

Limited small wind slab avalanches to 14 inches on northeast slopes above tree line via avalanche control were reported by the Mt Hood Meadows pro patrol through Tuesday afternoon.

However, Meadows pro-patrol reported a much more active day on Wednesday with all the new snow. Storm and wind slab releases, mostly initiated with explosives but also sensitive to ski cutting, ranged from 1 to 3 feet, with the larger slabs on lee aspects near and above treeline. Wind affects were seen into the below treeline band, and also allowed hard slabs to form on lee slopes higher in the terrain. Debris from a natural avalanche was observed in White River Canyon at around 8000'. 

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.

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.

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.