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Avalanche Forecast

Archived

Dec 3rd, 2016–Dec 4th, 2016

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

Despite a favorable cooling trend with this storm, westerly winds will build fresh wind slab on lee aspects near and above treeline. Look for wind slab development further downslope than you might expect due to locally strong winds and persistent loading. 

Detailed Forecast

A strong frontal system will sweep through Mt. Hood late Saturday night through mid-day Sunday. This system should deliver a nice shot of snow to Mt. Hood with an overall cooling trend. 

Despite a favorable cooling trend with this storm, westerly winds will build fresh wind slab on lee aspects near and above treeline. Look for wind slab development further downslope than you might expect due to locally strong winds and persistent loading. Watch for firm wind transported or hollow sounding snow. 

Storm slab should be a lesser avalanche concern on Sunday. However, in wind protected areas that experience persistent and intense snowfall rates, locally sensitive storm slab may develop.   

In much of the lower part of the below treeline band there is still insufficient snow for avalanches, so watch for early season travel hazards such as terrain traps, rocks and open creeks. 

Snowpack Discussion

Weather and Snowpack

A front crossed the Cascades Friday with around 2 inches of new snow reported at NWAC stations on Mt. Hood through early Saturday morning. The new snow mixed with rain Friday up to at least 5500 feet and a breakable crust was reported on Saturday. Post-frontal showers were generally light on Saturday and moderate west winds continued throughout the day. With the mild snow levels seen so far this season, most avalanche problems have been confined to the most recent storm layers found in the upper snowpack.   

Recent Observations

NWAC pro observer Laura Green was at Mt Hood Meadows ski area on Thursday and reported significant wind transport but only minor small wind slab releases during ski area avalanche control work. 

Laura was touring in Iron Creek Saturday and found a generally stable snowpack with obvious signs of recent wind transport with scoured windward aspects as she traveled near treeline. The Meadows pro-patrol had no significant avalanche control results Saturday. 

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.